Showing papers in "The Journal of Physiology in 2002"
TL;DR: The water permeability of biological membranes has been a longstanding problem in physiology, but the proteins responsible for this remained unknown until discovery of the aquaporin 1 (AQP1) water channel protein.
Abstract: The water permeability of biological membranes has been a longstanding problem in physiology, but the proteins responsible for this remained unknown until discovery of the aquaporin 1 (AQP1) water channel protein. AQP1 is selectively permeated by water driven by osmotic gradients. The atomic structure of human AQP1 has recently been defined. Each subunit of the tetramer contains an individual aqueous pore that permits single-file passage of water molecules but interrupts the hydrogen bonding needed for passage of protons. At least 10 mammalian aquaporins have been identified, and these are selectively permeated by water (aquaporins) or water plus glycerol (aquaglyceroporins). The sites of expression coincide closely with the clinical phenotypes--ranging from congenital cataracts to nephrogenic diabetes insipidus. More than 200 members of the aquaporin family have been found in plants, microbials, invertebrates and vertebrates, and their importance to the physiology of these organisms is being uncovered.
1,033 citations
TL;DR: Findings support the view that LTP‐like mechanisms may underlie the cortical plasticity induced by IPAS, as motor evoked potentials induced by unconditioned, single TMS pulses increased after IPAS.
Abstract: Associative stimulation has been shown to enhance excitability in the human motor cortex (Stefan et al. 2000); however, little is known about the underlying mechanisms. An interventional paired associative stimulation (IPAS) was employed consisting of repetitive application of single afferent electric stimuli, delivered to the right median nerve, paired with single pulse transcranial magnetic stimulation (TMS) over the optimal site for activation of the abductor pollicis brevis muscle (APB) to generate approximately synchronous events in the primary motor cortex. Compared to baseline, motor evoked potentials (MEPs) induced by unconditioned, single TMS pulses increased after IPAS. By contrast, intracortical inhibition, assessed using (i) a suprathreshold test TMS pulse conditioned by a subthreshold TMS pulse delivered 3 ms before the test pulse, and (ii) a suprathreshold test TMS pulse conditioned by afferent median nerve stimulation delivered 25 ms before the TMS pulse, remained unchanged when assessed with appropriately matching test stimulus intensities. The increase of single-pulse TMS-evoked MEP amplitudes was blocked when IPAS was performed under the influence of dextromethorphan, an N-methyl-d-aspartate (NMDA) receptor antagonist known to block long-term potentiation (LTP). Further experiments employing the double-shock TMS protocol suggested that the afferent pulse, as one component of the IPAS protocol, induced disinhibition of the primary motor cortex at the time when the TMS pulse, as the other component of IPAS, was delivered. Together, these findings support the view that LTP-like mechanisms may underlie the cortical plasticity induced by IPAS.
623 citations
TL;DR: Findings suggest that SICI is mediated through a low‐threshold GABAA receptor‐dependent inhibitory pathway and summation of IPSP from S1 and EPSP fromS2 at the corticospinal neurone, whereas SICF originates through non‐synaptic facilitation at the initial axon segment of interneurones along a high‐th threshold excitatory pathway.
Abstract: Paired transcranial magnetic stimulation has greatly advanced our understanding of the mechanisms which control excitability in human motor cortex. While it is clear that paired-pulse excitability depends on the exact interstimulus interval (ISI) between the first (S1) and second stimulus (S2), relatively little is known about the effects of the intensities of S1 and S2, and the effects of manipulating neurotransmission through the GABA(A) receptor. When recording the motor evoked potential (MEP) from the resting abductor digiti minimi (ADM) muscle, using a fixed ISI of 1.5 ms, and expressing the interaction between S1 and S2 as MEP(S1+S2)/(MEP(S1) + MEP(S2)), then a systematic variation of the intensities of S1 and S2 revealed short-interval intracortical facilitation (SICF) if S1 and S2 were approximately equal to MEP threshold (RMT), or if S1 > RMT and S2 RMT. Contraction of the ADM left SICI unchanged but reduced SICF. The GABA(A) receptor agonist diazepam increased SICI and reduced SICF in the resting ADM while diazepam had no effect during ADM contraction. Surface EMG and single motor unit recordings revealed that during ADM contraction SICI onset was at the I3-wave latency of S2, whereas SICF typically "jumped up" by one I-wave and started with the I2-wave latency of S2. Findings suggest that SICI is mediated through a low-threshold GABA(A) receptor-dependent inhibitory pathway and summation of IPSP from S1 and EPSP from S2 at the corticospinal neurone. In contrast, SICF originates through non-synaptic facilitation at the initial axon segment of interneurones along a high-threshold excitatory pathway.
506 citations
TL;DR: The results suggest that each connected L4 spiny neurone produces a weak but reliable EPSP in the pyramidal cell, implying that transmission of signals to layer 2/3 is likely to have a high threshold requiring simultaneous activation of many L4 neurons, and postsynaptic glutamate receptors act as a gate for the lateral spread of excitation in layer 1/3.
Abstract: Whole-cell voltage recordings were obtained from 64 synaptically coupled excitatory layer 4 (L4) spiny neurones and L2/3 pyramidal cells in acute slices of the somatosensory cortex ('barrel' cortex) of 17- to 23-days-old rats. Single action potentials (APs) in the L4 spiny neurone evoked single unitary EPSPs in the L2/3 pyramidal cell with a peak amplitude of 0.7 +/- 0.6 mV. The average latency was 2.1 +/- 0.6 ms, the rise time was 0.8 +/- 0.3 ms and the decay time constant was 12.7 +/- 3.5 ms. The percentage of failures of an AP in a L4 spiny neurone to evoke a unitary EPSP in the L2/3 pyramidal cell was 4.9 +/- 8.8 % and the coefficient of variation (c.v.) of the unitary EPSP amplitude was 0.27 +/- 0.13. Both c.v. and percentage of failures decreased with increased average EPSP amplitude. Postsynaptic glutamate receptors (GluRs) in L2/3 pyramidal cells were of the N-methyl-D-aspartate (NMDA) receptor (NMDAR) and the non-NMDAR type. At -60 mV in the presence of extracellular Mg2+ (1 mM), 29 +/- 15 % of the EPSP voltage-time integral was blocked by NMDAR antagonists. In 0 Mg2+, the NMDAR/AMPAR ratio of the EPSC was 0.50 +/- 0.29, about half the value obtained for L4 spiny neurone connections. Burst stimulation of L4 spiny neurones showed that EPSPs in L2/3 pyramidal cells depressed over a wide range of frequencies (1-100 s(-1) ). However, at higher frequencies (30 s(-1)) EPSP summation overcame synaptic depression so that the summed EPSP was larger than the first EPSP amplitude in the train. The number of putative synaptic contacts established by the axonal collaterals of the L4 projection neurone with the target neurone in layer 2/3 varied between 4 and 5, with an average of 4.5 +/- 0.5 (n = 13 pairs). Synapses were established on basal dendrites of the pyramidal cell. Their mean geometric distance from the pyramidal cell soma was 67 +/- 34 microm (range, 16-196 microm). The results suggest that each connected L4 spiny neurone produces a weak but reliable EPSP in the pyramidal cell. Therefore transmission of signals to layer 2/3 is likely to have a high threshold requiring simultaneous activation of many L4 neurons, implying that L4 spiny neurone to L2/3 pyramidal cell synapses act as a gate for the lateral spread of excitation in layer 2/3.
471 citations
TL;DR: The authors' measurements suggest that the triceps surae muscles maintain balance via a spring‐like element which is itself too compliant to guarantee stability, suggesting that the brain cannot set ankle stiffness and then ignore the control task because additional modulation of torque is required to maintain balance.
Abstract: During quiet standing the human ‘inverted pendulum’ sways irregularly. In previous work where subjects balanced a real inverted pendulum, we investigated what contribution the intrinsic mechanical ankle stiffness makes to achieve stability. Using the results of a plausible model, we suggested that intrinsic ankle stiffness is inadequate for providing stability. Here, using a piezo-electric translator we applied small, unobtrusive mechanical perturbations to the foot while the subject was standing freely. These short duration perturbations had a similar size and velocity to movements which occur naturally during quiet standing, and they produced no evidence of any stretch reflex response in soleus, or gastrocnemius. Direct measurement confirms our earlier conclusion; intrinsic ankle stiffness is not quite sufficient to stabilise the body or pendulum. On average the directly determined intrinsic stiffness is 91 ± 23 % (mean ± s.d.) of that necessary to provide minimal stabilisation. The stiffness was substantially constant, increasing only slightly with ankle torque. This stiffness cannot be neurally regulated in quiet standing. Thus we attribute this stiffness to the foot, Achilles’ tendon and aponeurosis rather than the activated calf muscle fibres. Our measurements suggest that the triceps surae muscles maintain balance via a spring-like element which is itself too compliant to guarantee stability. The implication is that the brain cannot set ankle stiffness and then ignore the control task because additional modulation of torque is required to maintain balance. We suggest that the triceps surae muscles maintain balance by predictively controlling the proximal offset of the spring-like element in a ballistic-like manner.
449 citations
TL;DR: It is suggested that both interhemispheric inhibition and long interval intracortical inhibition are predominately mediated by low threshold cortical neurons and may share common inhibitory mechanisms.
Abstract: Transcranial magnetic stimulation can be used to non-invasively study inhibitory processes in the human motor cortex. Interhemispheric inhibition can be measured by applying a conditioning stimulus to the motor cortex resulting in inhibition of the contralateral motor cortex. Transcranial magnetic stimulation can also be used to demonstrate ipsilateral cortico-cortical inhibition in the motor cortex. At least two different ipsilateral cortico-cortical inhibitory processes have been identified: short interval intracortical inhibition and long interval intracortical inhibition. However, the relationship between interhemispheric inhibition and ipsilateral cortico-cortical inhibition remains unclear. This study examined the relationship between interhemispheric inhibition, short interval intracortical inhibition and long interval intracortical inhibition. First, the effect of test stimulus intensity on each inhibitory process was studied. Second, the effects of interhemispheric inhibition on short interval intracortical inhibition and long interval intracortical inhibition on interhemispheric inhibition were examined. Motor evoked potentials were recorded from the right first dorsal interosseous muscle in 11 right-handed healthy volunteers. For interhemispheric inhibition, conditioning stimuli were applied to the right motor cortex and test stimuli to the left motor cortex. For short interval intracortical inhibition and long interval intracortical inhibition, both conditioning stimuli and test stimuli were applied to the left motor cortex. With increasing test stimulus intensities, long interval intracortical inhibition and interhemispheric inhibition decreased, while short interval intracortical inhibition increased. Moreover, short interval intracortical inhibition was significantly reduced in the presence of interhemispheric inhibition. Interhemispheric inhibition was significantly reduced in the presence of long interval intracortical inhibition when matched for test motor evoked potential amplitude but the difference was not significant when matched for test pulse intensity. These findings suggest that both interhemispheric inhibition and long interval intracortical inhibition are predominately mediated by low threshold cortical neurons and may share common inhibitory mechanisms. In contrast, the mechanisms mediating short interval intracortical inhibition are probably different from those mediating long interval intracortical inhibition and interhemispheric inhibition although these systems appear to interact.
431 citations
TL;DR: A novel function for M/KCNQ channels in the brain is suggested: to facilitate neuronal resonance and network oscillations in cortical neurons, thus providing a basis for an oscillation‐based neural code.
Abstract: Coherent network oscillations in the brain are correlated with different behavioural states. Intrinsic resonance properties of neurons provide a basis for such oscillations. In the hippocampus, CA1 pyramidal neurons show resonance at theta (theta) frequencies (2-7 Hz). To study the mechanisms underlying theta-resonance, we performed whole-cell recordings from CA1 pyramidal cells (n = 73) in rat hippocampal slices. Oscillating current injections at different frequencies (ZAP protocol), revealed clear resonance with peak impedance at 2-5 Hz at approximately 33 degrees C (increasing to approximately 7 Hz at approximately 38 degrees C). The theta-resonance showed a U-shaped voltage dependence, being strong at subthreshold, depolarized (approximately -60 mV) and hyperpolarized (approximately -80 mV) potentials, but weaker near the resting potential (-72 mV). Voltage clamp experiments revealed three non-inactivating currents operating in the subthreshold voltage range: (1) M-current (I(M)), which activated positive to -65 mV and was blocked by the M/KCNQ channel blocker XE991 (10 microM); (2) h-current (I(h)), which activated negative to -65 mV and was blocked by the h/HCN channel blocker ZD7288 (10 microM); and (3) a persistent Na(+) current (I(NaP)), which activated positive to -65 mV and was blocked by tetrodotoxin (TTX, 1 microM). In current clamp, XE991 or TTX suppressed the resonance at depolarized, but not hyperpolarized membrane potentials, whereas ZD7288 abolished the resonance only at hyperpolarized potentials. We conclude that these cells show two forms of theta-resonance: "M-resonance" generated by the M-current and persistent Na(+) current in depolarized cells, and "H-resonance" generated by the h-current in hyperpolarized cells. Computer simulations supported this interpretation. These results suggest a novel function for M/KCNQ channels in the brain: to facilitate neuronal resonance and network oscillations in cortical neurons, thus providing a basis for an oscillation-based neural code.
427 citations
TL;DR: It is suggested that skin receptors in the foot sole behave differently from those receptors found on the glabrous skin of the hand, which may reflect the role of foot sole skin receptorsIn standing balance and movement control.
Abstract: To document the activity of cutaneous mechanoreceptors in the glabrous skin of the foot sole, tungsten microelectrodes were inserted through the popliteal fossa and into the tibial nerve of thirteen healthy human subjects. A total of 104 cutaneous mechanoreceptors were identified in the glabrous skin of the foot. This sample consisted of 15 slow adapting type I (14 %), 16 slow adapting type II (15 %), 59 fast adapting type I (57 %), and 14 fast adapting type II units (14 %). The location of the receptors and the outline of the receptive fields were determined by using nylon monofilaments perpendicularly applied against the surface of the skin. This revealed that the receptors were widely distributed without an accumulation of receptors in the toes. There were also larger receptive fields predominantly isolated on the plantar surface of the metatarsal-tarsal region of the foot sole. Furthermore, with the foot in an unloaded position, there was no background discharge activity in any of the cutaneous receptors in the absence of intentionally applied stimulation. These findings suggest that skin receptors in the foot sole behave differently from those receptors found on the glabrous skin of the hand. This may reflect the role of foot sole skin receptors in standing balance and movement control.
403 citations
TL;DR: It is concluded that somatosensory stimulation elicited a focal increase in corticomotoneuronal excitability that outlasts the stimulation period and probably occurs at cortical sites.
Abstract: In humans, somatosensory stimulation results in increased corticomotoneuronal excitability to the stimulated body parts. The purpose of this study was to investigate the underlying mechanisms. We recorded motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) from abductor pollicis brevis (APB), first dorsal interosseous (FDI), and abductor digiti minimi (ADM) muscles. MEP amplitudes, recruitment curves (RC), intracortical inhibition (ICI), intracortical facilitation (ICF), resting (rMT) and active motor thresholds (aMT) were recorded before and after a 2-h period of ulnar nerve electrical stimulation at the wrist. Somatosensory input was monitored by recording somatosensory evoked potentials. To differentiate excitability changes at cortical vs. subcortical sites, we recorded supramaximal peripheral M-responses and MEPs to brainstem electrical stimulation (BES). In order to investigate the involvement of GABAergic mechanisms, we studied the influence of lorazepam (LZ) (a GABAA receptor agonist) relative to that of dextromethorphan (DM) (an NMDA receptor antagonist) and placebo in a double-blind design. We found that somatosensory stimulation increased MEP amplitudes to TMS only in the ADM, confirming a previous report. This effect was blocked by LZ but not by either DM or placebo and lasted between 8 and 20 min in the absence of (i) changes in MEPs elicited by BES, (ii) amplitudes of early somatosensory-evoked potentials or (iii) M-responses. We conclude that somatosensory stimulation elicited a focal increase in corticomotoneuronal excitability that outlasts the stimulation period and probably occurs at cortical sites. The antagonistic effect of LZ supports the hypothesis of GABAergic involvement as an operating mechanism.
389 citations
TL;DR: The τ values of the fundamental component of [PCr] and V̇O2 dynamics cohere to within 10 %, during both the on‐ and off‐transients to a constant‐load work rate of both moderate‐ and high‐intensity exercise.
Abstract: The on- and off-transient (ie phase II) responses of pulmonary oxygen uptake (V(O(2))) to moderate-intensity exercise (ie below the lactate threshold, theta;(L)) in humans has been shown to conform to both mono-exponentiality and 'on-off' symmetry, consistent with a system manifesting linear control dynamics However above theta;(L) the V(O(2)) kinetics have been shown to be more complex: during high-intensity exercise neither mono-exponentiality nor 'on-off' symmetry have been shown to appropriately characterise the V(O(2)) response Muscle [phosphocreatine] ([PCr]) responses to exercise, however, have been proposed to be dynamically linear with respect to work rate, and to demonstrate 'on-off' symmetry at all work intenisties We were therefore interested in examining the kinetic characteristics of the V(O(2)) and [PCr] responses to moderate- and high-intensity knee-extensor exercise in order to improve our understanding of the factors involved in the putative phosphate-linked control of muscle oxygen consumption We estimated the dynamics of intramuscular [PCr] simultaneously with those of V(O(2)) in nine healthy males who performed repeated bouts of both moderate- and high-intensity square-wave, knee-extension exercise for 6 min, inside a whole-body magnetic resonance spectroscopy (MRS) system A transmit-receive surface coil placed under the right quadriceps muscle allowed estimation of intramuscular [PCr]; V(O(2)) was measured breath-by-breath using a custom-designed turbine and a mass spectrometer system For moderate exercise, the kinetics were well described by a simple mono-exponential function (following a short cardiodynamic phase for V(O(2))), with time constants (tau) averaging: tauV(O(2))(,on) 35 +/- 14 s (+/- SD), tau[PCr](on) 33 +/- 12 s, tauV(O(2))(,off) 50 +/- 13 s and tau[PCr](off) 51 +/- 13 s The kinetics for both V(O(2)) and [PCr] were more complex for high-intensity exercise The fundamental phase expressing average tau values of tauV(O(2))(,on) 39 +/- 4 s, tau[PCr](on) 38 +/- 11 s, tauV(O(2))(,off) 51 +/- 6 s and tau[PCr](off) 47 +/- 11 s An associated slow component was expressed in the on-transient only for both V(O(2)) and [PCr], and averaged 153 +/- 54 and 139 +/- 91 % of the fundamental amplitudes for V(O(2)) and [PCr], respectively In conclusion, the tau values of the fundamental component of [PCr] and V(O(2)) dynamics cohere to within 10 %, during both the on- and off-transients to a constant-load work rate of both moderate- and high-intensity exercise On average, approximately 90 % of the magnitude of the V(O(2)) slow component during high-intensity exercise is reflected within the exercising muscle by its [PCr] response
378 citations
TL;DR: It is concluded that the spinal SDH comprises many types of neurones whose morphological characteristics are associated with specific functional features implying diversity in functional organization of the SDH and in its role as a major synaptic termination for thin primary afferent fibres.
Abstract: Relationships between the morphology of individual neurones of the spinal superficial dorsal horn (SDH), laminae I and II, and their electrophysiological properties were studied in spinal cord slices prepared from anaesthetized, free-ranging hamsters. Tight-seal, whole-cell recordings were made with pipette microelectrodes filled with biocytin to establish electrophysiological characteristics and to label the studied neurones. Neurones were categorized according to location and size of the somata, the dendritic and axonal pattern of arborization, spontaneous synaptic potentials, evoked postsynaptic currents, pattern of discharge to depolarizing pulses and current-voltage relationships. Data were obtained for 170 neurones; 13 of these had somata in lamina I and 157 in lamina II. Stimulation of the segmental dorsal root evoked a prompt excitatory response in almost every neurone sampled (161/166) with nearly 3/4 displaying putative monosynaptic EPSCs. The majority of neurones (133/170) fitted one of several distinctive morphological categories. To a considerable extent, neurones with a common morphological configuration and neurite disposition shared electrophysiological characteristics. Five of the 13 lamina I neurones were relatively large with extensive dendritic arborization in the horizontal dimension and a prominent axon directed ventrally and contralaterally. These presumptive ventrolateral projection neurones differed structurally and electrophysiologically from the other lamina I neurones, which had ipsilateral, locally arborizing axons and/or branches entering the dorsal lateral funiculus. One hundred and twenty lamina II neurones fitted one of five morphological categories: islet, central, medial-lateral, radial or vertical. Central cells were further divided into three groups on functional features. We conclude that the spinal SDH comprises many types of neurones whose morphological characteristics are associated with specific functional features implying diversity in functional organization of the SDH and in its role as a major synaptic termination for thin primary afferent fibres.
TL;DR: It is concluded that PSPs in ensembles of barrel cells represent dynamically the deflection of a single whisker with high temporal and spatial acuity, initially by the excitation in a single PW‐barrel followed by multi-barrel excitation.
Abstract: The elaborate morphology of cortical neurons has been established for a long time (Ramon y Cajal, 1893), yet the functional significance of the differences in the architecture of the dendritic and axonal arbors of cells located in the same or different cortical layers is still unclear. Layer 4 of rodent somatosensory cortex is divided cytoarchitectonically into barrels with a high density of neurons, and septa between barrels with a lower density (Woolsey & Van der Loos, 1970). Barrel cells are targeted by thalamic inputs from the ventral posterior medial nucleus (VPM; for review see Diamond, 1995) while septum cells are innervated by thalamic afferents projecting from the posterior medial nucleus (PoM; for review see Kim & Ebner, 1999). A functional equivalent of the cytoarchitectonically defined barrels are the barrel-columns, ensembles of cells in the different cortical layers which share functional properties such as a response preference for the deflection of a particular whisker. The receptive fields (RFs) of barrel-column cells are characterised by a dominant input from a principal whisker (PW) and weaker inputs from surround whiskers (SuW). In L4 the barrel-columns correspond in their dimensions roughly to barrels (Welker, 1976). Barrel borders can be visualised simultaneously with the dendritic morphology of individual cells (Ito, 1992), and both the laminar location of a cell's soma and the spread of dendrites and axon collaterals can be determined relative to the barrel borders. Thus possible anatomical determinants of RF structure, such as the geometry of the dendritic and axonal arbor can be delineated.
Spiny stellate cells are confined to the borders of barrels and their dendritic arbor is asymmetric (Woolsey et al. 1975; Simons & Woolsey 1984; Feldmeyer et al. 1999; Lubke et al. 2000). They relay thalamic output to other cortical layers via axon collaterals projecting to L2/3 and to L5 or L6. Although most anatomical studies on L4 neurons have focused on spiny stellate cells, pyramidal neurons have also been described in somatosensory (Lorente de No, 1922; Elston et al. 1997; Lubke et al. 2000) and in visual cortex (Martin & Whitteridge, 1984). In the somatosensory cortex neurons in layer 4 are selective in their responses to the direction of whisker deflection and they respond with short latency. Their RF structure is somewhat controversial, however. Intracellular recordings with microelectrodes (Carvel & Simons, 1988) and more recently, whole-cell voltage recordings have demonstrated afferent inputs from several whiskers and large subthreshold RFs (Moore & Nelson, 1998; Zhu & Connors, 1999). Most (Simons, 1995) but not all (Armstrong-James, 1995) extracellular unit-recording studies report small, often single-whisker RFs. In addition multielectrode unit recordings indicate that RF properties are time dependent (Petersen & Diamond, 2000).
We report in vivo whole-cell voltage recordings combined with morphological reconstruction of the recorded neurons and determination of their columnar position. The aim was to establish firstly the dependency of RF structure on the geometry of dendritic and axonal arborisation of the different classes of neurons to identify possible constraints of RF structure given by cell morphology. Secondly we wanted to determine possible relationships between a cell's location and morphology and the time dependent structure of sub- and suprathreshold RFs. Such relations are essential to elucidate how different tactile object cues are represented at the input (PSPs) and the output stage (APs) of specific ensembles of cells in the cortical input layer.
TL;DR: The findings question the assumption that sensitivity to diazoxide and 5‐HD implies involvement of mitochondrial KATP channels and propose that pharmacological preconditioning may be reelated to partial inhibition of respiratory chain complexes.
Abstract: Diazoxide and 5-hydroxydecanoate (5-HD; C10:0) are reputed to target specifically mitochondrial ATP-sensitive K+ (KATP) channels. Here we describe KATP channel-independent targets of diazoxide and 5-HD in the heart. Using submitochondrial particles isolated from pig heart, we found that diazoxide (10-100 μm) dose-dependently decreased succinate oxidation without affecting NADH oxidation. Pinacidil, a non-selective KATP channel opener, did not inhibit succinate oxidation. However, it selectively inhibited NADH oxidation. These direct inhibitory effects of diazoxide and pinacidil cannot be explained by activation of mitochondrial KATP channels. Furthermore, application of either diazoxide (100 μm) or pinacidil (100 μm) did not decrease mitochondrial membrane potential, assessed using TMRE (tetramethylrhodamine ethyl ester), in isolated guinea-pig ventricular myocytes. We also tested whether 5-HD, a medium-chain fatty acid derivative which blocks diazoxide-induced cardioprotection, was ‘activated’ via acyl-CoA synthetase (EC 6.2.1.3), an enzyme present both on the outer mitochondrial membrane and in the matrix. Using analytical HPLC and electrospray ionisation mass spectrometry, we showed that 5-HD-CoA (5-hydroxydecanoyl-CoA) is indeed synthesized from 5-HD and CoA via acyl-CoA synthetase. Thus, 5-HD-CoA may be the active form of 5-HD, serving as substrate for (or inhibiting) acyl-CoA dehydrogenase (β-oxidation) and/or exerting some other cellular action. In conclusion, we have identified KATP channel-independent targets of 5-HD, diazoxide and pinacidil. Our findings question the assumption that sensitivity to diazoxide and 5-HD implies involvement of mitochondrial KATP channels. We propose that pharmacological preconditioning may be reelated to partial inhibition of respiratory chain complexes.
TL;DR: The complex stochastic relations between and variabilities in these rhythms indicate no single mechanism is responsible, and both experimental design and model construction will have to be more trenchant to move beyond qualitative suggestions of determinants to quantitative elucidation of critical physical mechanisms.
Abstract: Research into cardiovascular variabilities intersects both human physiology and quantitative modelling. This is because respiratory and Mayer wave (or 10 s) cardiovascular oscillations represent the integrated control of a system through both autonomic branches by systemic haemodynamic changes within a fluid-filled, physical system. However, our current precise measurement of short-term cardiovascular fluctuations does not necessarily mean we have an adequate understanding of them. Empirical observation suggests that both respiratory and Mayer wave fluctuations derive from mutable autonomic and haemodynamic inputs. Evidence strongly suggests that respiratory sinus arrhythmia both contributes to and buffers respiratory arterial pressure fluctuations. Moreover, even though virtual abolition of all R-R interval variability by cholinergic blockade suggests that parasympathetic stimulation is essential for expression of these variabilities, respiratory sinus arrhythmia does not always reflect a purely vagal phenomenon. The arterial baroreflex has been cited as the mechanism for both respiratory and Mayer wave frequency fluctuations. However, data suggest that both cardiac vagal and vascular sympathetic fluctuations at these frequencies are independent of baroreflex mechanisms and, in fact, contribute to pressure fluctuations. Results from cardiovascular modelling can suggest possible sources for these rhythms. For example, modelling originally suggested low frequency cardiovascular rhythms derived from intrinsic delays in baroreceptor control, and experimental evidence subsequently corroborated this possibility. However, the complex stochastic relations between and variabilities in these rhythms indicate no single mechanism is responsible. If future study of cardiovascular variabilities is to move beyond qualitative suggestions of determinants to quantitative elucidation of critical physical mechanisms, both experimental design and model construction will have to be more trenchant.
TL;DR: An artificial task to illuminate the mechanisms underlying the sways and to account for changes in their size using the ankle musculature showed that balance was achieved by the constant repetition of a neurally generated ballistic‐like biphasic pattern of torque which can control both position and sway size.
Abstract: In standing, there are small sways of the body. Our interest is to use an artificial task to illuminate the mechanisms underlying the sways and to account for changes in their size. Using the ankle musculature, subjects balanced a large inverted pendulum. The equilibrium of the pendulum is unstable and quasi-regular sway was observed like that in quiet standing. By giving full attention to minimising sway subjects could systematically reduce pendulum movement. The pendulum position, the torque generated at each ankle and the soleus and tibialis anterior EMGs were recorded. Explanations about how the human inverted pendulum is balanced usually ignore the fact that balance is maintained over a range of angles and not just at one angle. Any resting equilibrium position of the pendulum is unstable and in practice temporary; movement to a different resting equilibrium position can only be accomplished by a biphasic ‘throw and catch’ pattern of torque and not by an elastic mechanism. Results showed that balance was achieved by the constant repetition of a neurally generated ballistic-like biphasic pattern of torque which can control both position and sway size. A decomposition technique revealed that there was a substantial contribution to changes in torque from intrinsic mechanical ankle stiffness; however, by itself this was insufficient to maintain balance or to control position. Minimisation of sway size was caused by improvement in the accuracy of the anticipatory torque impulses. We hypothesise that examination of centre of mass and centre of pressure data for quiet standing will duplicate these results.
TL;DR: It is suggested that the resistance training increased the stiffness of tendon structures as well as muscle strength and size, and the stretching training affected the viscosity of tendon structure but not the elasticity.
Abstract: The present study examined whether resistance and stretching training programmes altered the viscoelastic properties of human tendon structures in vivo. Eight subjects completed 8 weeks (4 days per week) of resistance training which consisted of unilateral plantar flexion at 70 % of one repetition maximum with 10 repetitions per set (5 sets per day). They performed resistance training (RT) on one side and resistance training and static stretching training (RST; 10 min per day, 7 days per week) on the other side. Before and after training, the elongation of the tendon structures in the medial gastrocnemius muscle was directly measured using ultrasonography, while the subjects performed ramp isometric plantar flexion up to the voluntary maximum, followed by a ramp relaxation. The relationship between estimated muscle force (Fm) and tendon elongation (L) was fitted to a linear regression, the slope of which was defined as stiffness. The hysteresis was calculated as the ratio of the area within the Fm-L loop to the area beneath the load portion of the curve. The stiffness increased significantly by 18.8 ± 10.4 % for RT and 15.3 ± 9.3 % for RST. There was no significant difference in the relative increase of stiffness between RT and RST. The hysteresis, on the other hand, decreased 17 ± 20 % for RST, but was unchanged for RT. These results suggested that the resistance training increased the stiffness of tendon structures as well as muscle strength and size, and the stretching training affected the viscosity of tendon structures but not the elasticity.
TL;DR: It is concluded that increased trunk roll stiffness is a key biomechanical change with age and interferes with early compensatory trunk movements and leads to trunk displacements in the direction of the impending fall.
Abstract: We investigated the effects of ageing on balance corrections induced by sudden stance perturbations in different directions. Effects were examined in biomechanical and electromyographic (EMG) recordings from a total of 36 healthy subjects divided equally into three age groups (20-34, 35-55 and 60-75 years old). Perturbations consisted of six combinations of support-surface roll (laterally) and pitch (forward-backward) each with 7.5 deg amplitude (2 pure pitch, and 4 roll and pitch) delivered randomly. To reduce stimulus predictability further and to investigate scaling effects, perturbations were at either 30 or 60 deg s(-1). In the legs, trunk and arms we observed age-related changes in balance corrections. The changes that appeared in the lower leg responses included smaller stretch reflexes in soleus and larger reflexes in tibialis anterior of the elderly compared with the young. For all perturbation directions, onsets of balance correcting responses in these ankle muscles were delayed by 20-30 ms and initially had smaller amplitudes (between 120-220 ms) in the elderly. This reduced early activity was compensated by increased lower leg activity after 240 ms. These EMG changes were paralleled by comparable differences in ankle torque responses, which were initially (after 160 ms) smaller in the elderly, but subsequently greater (after 280 ms). Findings in the middle-aged group were generally intermediate between the young and the elderly groups. Comparable results were obtained for the two different stimulus velocities. Stimulus-induced trunk roll, but not trunk pitch, changed dramatically with increasing age. Young subjects responded with early large roll movements of the trunk in the opposite direction to platform roll. A similarly directed but reduced amplitude of trunk roll was observed in the middle-aged. The elderly had very little initial roll modulation and also had smaller stretch reflexes in paraspinals. Balance-correcting responses (over 120-220 ms) in gluteus medius and paraspinals were equally well tuned to roll in the elderly, as in the young, but were reduced in amplitude. Onset latencies were delayed with age in gluteus medius muscles. Following the onset of trunk and hip balance corrections, trunk roll was in the same direction as support-surface motion for all age groups and resulted in overall trunk roll towards the fall side in the elderly, but not in the young. Protective arm movements also changed with age. Initial arm roll movements were largest in the young, smaller in the middle aged, and smallest in the elderly. Initial arm roll movements were in the same direction as initial trunk motion in the young and middle aged. Thus initial roll arm movements in the elderly were directed oppositely to those in the young. Initial pitch motion of the arms was similar across age groups. Subsequent arm movements were related to the amplitude of deltoid muscle responses which commenced at 100 ms in the young and 20-30 ms later in the elderly. These deltoid muscle responses preceded additional arm roll motion which left the arms directed 'downhill' (in the direction of the fall) in the elderly, but 'uphill' (to counterbalance motion of the pelvis) in the young. We conclude that increased trunk roll stiffness is a key biomechanical change with age. This interferes with early compensatory trunk movements and leads to trunk displacements in the direction of the impending fall. The reversal of protective arm movements in the elderly may reflect an adaptive strategy to cushion the fall. The uniform delay and amplitude reduction of balance-correcting responses across many segments (legs, hips and arms) suggests a neurally based alteration in processing times and response modulation with age. Interestingly, the elderly compensated for these 'early abnormalities' with enlarged later responses in the legs, but no similar adaptation was noted in the arms and trunk. These changes with age provide an insight into possible mechanisms underlying falls in the elderly.
TL;DR: The present results indicate that the average brain temperature is at least 0.2 °C higher than that of the body core during exercise with or without hyperthermia.
Abstract: Brain temperature appears to be an important factor affecting motor activity, but it is not known to what extent brain temperature increases during prolonged exercise in humans. Cerebral heat exchange was therefore evaluated in seven males during exercise with and without hyperthermia. Middle cerebral artery mean blood velocity (MCA Vmean) was continuously monitored while global cerebral blood flow (CBF) and cerebral energy turnover were determined at the end of the two exercise trials in three subjects. The arterial to venous temperature difference across the brain (v-aDtemp) was determined via thermocouples placed in the internal jugular vein and in the aorta. The jugular venous blood temperature was always higher than that of the arterial blood, demonstrating that heat was released via the CBF during the normothermic as well as the hyperthermic exercise condition. However, heat removal via the jugular venous blood was 30 ± 6 % lower during hyperthermia compared to the control trial. The reduced heat removal from the brain was mainly a result of a 20 ± 6 % lower CBF (22 ± 9 % reduction in MCA Vmean), because the v-aDtemp was not significantly different in the hyperthermic (0.20 ± 0.05 °C) compared to the control trial (0.22 ± 0.05 °C). During hyperthermia, the impaired heat removal via the blood was combined with a 7 ± 2 % higher heat production in the brain and heat was consequently stored in the brain at a rate of 0.20 ± 0.06 J g−1 min−1. The present results indicate that the average brain temperature is at least 0.2 °C higher than that of the body core during exercise with or without hyperthermia.
TL;DR: ERG components of proximal retinal origin that are more sensitive to test flashes and adapting backgrounds than PII provide the ‘threshold’ negative and positive (b‐wave) responses of the mouse dark‐adapted ERG.
Abstract: The most sensitive response in the dark-adapted electroretinogram (ERG), the scotopic threshold response (STR) which originates from the proximal retina, has been identified in several mammals including humans, but previously not in the mouse. The current study established the presence and assessed the nature of the mouse STR. ERGs were recorded from adult wild-type C57/BL6 mice anaesthetized with ketamine (70 mg kg−1) and xylazine (7 mg kg−1). Recordings were between DTL fibres placed under contact lenses on the two eyes. Monocular test stimuli were brief flashes (λmax 462 nm; -6.1 to +1.8 log scotopic Troland seconds(sc td s)) under fully dark-adapted conditions and in the presence of steady adapting backgrounds (-3.2 to -1.7 log sc td). For the weakest test stimuli, ERGs consisted of a slow negative potential maximal ≈200 ms after the flash, with a small positive potential preceding it. The negative wave resembled the STR of other species. As intensity was increased, the negative potential saturated but the positive potential (maximal ≈110 ms) continued to grow as the b-wave. For stimuli that saturated the b-wave, the a-wave emerged. For stimulus strengths up to those at which the a-wave emerged, ERG amplitudes measured at fixed times after the flash (110 and 200 ms) were fitted with a model assuming an initially linear rise of response amplitude with intensity, followed by saturation of five components of declining sensitivity: a negative STR (nSTR), a positive STR (pSTR), a positive scotopic response (pSR), PII (the bipolar cell component) and PIII (the photoreceptor component). The nSTR and pSTR were approximately 3 times more sensitive than the pSR, which was approximately 7 times more sensitive than PII. The sensitive positive components dominated the b-wave up to > 5 % of its saturated amplitude. Pharmacological agents that suppress proximal retinal activity (e.g. GABA) minimized the pSTR, nSTR and pSR, essentially isolating PII which rose linearly with intensity before showing hyperbolic saturation. The nSTR, pSTR and pSR were desensitized by weaker backgrounds than those desensitizing PII. In conclusion, ERG components of proximal retinal origin that are more sensitive to test flashes and adapting backgrounds than PII provide the ‘threshold’ negative and positive (b-wave) responses of the mouse dark-adapted ERG. These results support the use of the mouse ERG in studies of proximal retinal function.
TL;DR: Imposed flow inhibited the active lymph pump in both mesenteric lymphatics and in the thoracic duct, and inhibition of the NO synthase by NG‐monomethyl‐l‐arginine attenuated but did not completely abolish the effects of flow.
Abstract: There are only a few reports of the influence of imposed flow on an active lymph pump under conditions of controlled intraluminal pressure. Thus, the mechanisms are not clearly defined. Rat mesenteric lymphatics and thoracic ducts were isolated, cannulated and pressurized. Input and output pressures were adjusted to impose various flows. Lymphatic systolic and diastolic diameters were measured and used to determine contraction frequency and pump flow indices. Imposed flow inhibited the active lymph pump in both mesenteric lymphatics and in the thoracic duct. The active pump of the thoracic duct appeared more sensitive to flow than did the active pump of the mesenteric lymphatics. Imposed flow reduced the frequency and amplitude of the contractions and accordingly the active pump flow. Flow-induced inhibition of the active lymph pump followed two temporal patterns. The first pattern was a rapidly developing inhibition of contraction frequency. Upon imposition of flow, the contraction frequency immediately fell and then partially recovered over time during continued flow. This effect was dependent on the magnitude of imposed flow, but did not depend on the direction of flow. The effect also depended upon the rate of change in the direction of flow. The second pattern was a slowly developing reduction of the amplitude of the lymphatic contractions, which increased over time during continued flow. The inhibition of contraction amplitude was dependent on the direction of the imposed flow, but independent of the magnitude of flow. Nitric oxide was partly but not completely responsible for the influence of flow on the mesenteric lymph pump. Exposure to NO mimicked the effects of flow, and inhibition of the NO synthase by N (G)-monomethyl-L-arginine attenuated but did not completely abolish the effects of flow.
TL;DR: In this paper, the basic framework for physiologic gene regulation was selected during an era of obligatory physical activity, as the survival of our Late Palaeolithic (50 000-10 000 BC) ancestors depended on hunting and gathering.
Abstract: The current human genome was moulded and refined through generations of time. We propose that the basic framework for physiologic gene regulation was selected during an era of obligatory physical activity, as the survival of our Late Palaeolithic (50 000–10 000 BC) ancestors depended on hunting and gathering. A sedentary lifestyle in such an environment probably meant elimination of that individual organism. The phenotype of the present day Homo sapiens genome is much different from that of our ancient ancestors, primarily as a consequence of expressing evolutionarily programmed Late Palaeolithic genes in an environment that is predominantly sedentary. In this sense, our current genome is maladapted, resulting in abnormal gene expression, which in turn frequently manifests itself as clinically overt disease. We speculate that some of these genes still play a role in survival by causing premature death from chronic diseases produced by physical inactivity. We also contend that the current scientific evidence supports the notion that disruptions in cellular homeostasis are diminished in magnitude in physically active individuals compared with sedentary individuals due to the natural selection of gene expression that supports the physically active lifestyle displayed by our ancestors. We speculate that genes evolved with the expectation of requiring a certain threshold of physical activity for normal physiologic gene expression, and thus habitual exercise in sedentary cultures restores perturbed homeostatic mechanisms towards the normal physiological range of the Palaeolithic Homo sapiens. This hypothesis allows us to ask the question of whether normal physiological values change as a result of becoming sedentary. In summary, in sedentary cultures, daily physical activity normalizes gene expression towards patterns established to maintain the survival in the Late Palaeolithic era.
TL;DR: It was concluded that during CM muscle fibres optimally work almost isometrically, by leaving the task of storing and releasing elastic energy for enhancing exercise performance to the tendon.
Abstract: Six men performed a single ankle plantar flexion exercise in the supine position with the maximal effort with counter movement (CM, plantar flexion preceded by dorsiflexion) and without counter movement (NoCM, plantar flexion only) produced by a sliding table that controlled applied load to the ankle (40 % of the maximal voluntary force). The reaction force at the foot and ankle joint angle were measured using a force plate and a goniometer, respectively. From real-time ultrasonography of the gastrocnemius medialis muscle during the movement, the fascicle length was determined. The estimated peak force, average power, and work at the Achilles’ tendon during the plantar flexion phase in CM were significantly greater than those in NoCM. In CM, in the dorsiflexion phase, fascicle length initially increased with little electromyographic activity, then remained constant while the whole muscle-tendon unit lengthened, before decreasing in the final plantar flexion phase. In NoCM, fascicle length decreased throughout the movement and the fascicle length at the onset of movement was longer than that of the corresponding phase in CM. It was concluded that during CM muscle fibres optimally work almost isometrically, by leaving the task of storing and releasing elastic energy for enhancing exercise performance to the tendon.
TL;DR: The results suggest that resistance training changes the functional properties of spinal cord circuitry in humans, but does not substantially affect the organisation of the motor cortex.
Abstract: Although it has long been supposed that resistance training causes adaptive changes in the CNS, the sites and nature of these adaptations have not previously been identified. In order to determine whether the neural adaptations to resistance training occur to a greater extent at cortical or subcortical sites in the CNS, we compared the effects of resistance training on the electromyographic (EMG) responses to transcranial magnetic (TMS) and electrical (TES) stimulation. Motor evoked potentials (MEPs) were recorded from the first dorsal interosseous muscle of 16 individuals before and after 4 weeks of resistance training for the index finger abductors (n = 8), or training involving finger abduction-adduction without external resistance (n = 8). TMS was delivered at rest at intensities from 5 % below the passive threshold to the maximal output of the stimulator. TMS and TES were also delivered at the active threshold intensity while the participants exerted torques ranging from 5 to 60 % of their maximum voluntary contraction (MVC) torque. The average latency of MEPs elicited by TES was significantly shorter than that of TMS MEPs (TES latency = 21.5 +/- 1.4 ms; TMS latency = 23.4 +/- 1.4 ms; P < 0.05), which indicates that the site of activation differed between the two forms of stimulation. Training resulted in a significant increase in MVC torque for the resistance-training group, but not the control group. There were no statistically significant changes in the corticospinal properties measured at rest for either group. For the active trials involving both TMS and TES, however, the slope of the relationship between MEP size and the torque exerted was significantly lower after training for the resistance-training group (P < 0.05). Thus, for a specific level of muscle activity, the magnitude of the EMG responses to both forms of transcranial stimulation were smaller following resistance training. These results suggest that resistance training changes the functional properties of spinal cord circuitry in humans, but does not substantially affect the organisation of the motor cortex.
TL;DR: The role of intracellular Ca2+ (Ca2+i) in triggering early afterdepolarization (EADs), the origins of EADs and the mechanisms underlying Torsade de Pointes (TdP) were investigated in a model of long QT syndrome as discussed by the authors.
Abstract: The role of intracellular Ca2+ (Ca2+i) in triggering early afterdepolarizations (EADs), the origins of EADs and the mechanisms underlying Torsade de Pointes (TdP) were investigated in a model of long QT syndrome (Type 2). Perfused rabbit hearts were stained with RH327 and Rhod-2/AM to simultaneously map membrane potential (V(m)) and Ca2+i with two photodiode arrays. The I(Kr) blocker E4031 (0.5 microM) together with 50 % reduction of [K+]o and [Mg2+]o elicited long action potentials (APs), V(m) oscillations on AP plateaux (EADs) then ventricular tachycardia (VT). Cryoablation of both ventricular chambers eliminated Purkinje fibres as sources of EADs. E4031 prolonged APs (0.28 to 2.3 s), reversed repolarization sequences (baseapex) and enhanced repolarization gradients (30 to 230 ms, n = 12) indicating a heterogeneous distribution of I(Kr). At low [K+]o and [Mg2+]o, E4031 elicited spontaneous Ca2+iand V(m) spikes or EADs (3.5 +/- 1.9 Hz) during the AP plateau (n = 6). EADs fired 'out-of-phase' from several sites, propagated, collided then evolved to TdP. Phase maps (Ca2+ivs. V(m)) had counterclockwise trajectories shaped like a 'boomerang' during an AP and like ellipses during EADs, with V(m) preceding Ca2+iby 9.2 +/- 1.4 (n = 6) and 7.2 +/- 0.6 ms (n = 5/6), respectively. After cryoablation, EADs from surviving epicardium (~1 mm) fired at the same frequency (3.4 +/- 0.35 Hz, n = 6) as controls. At the origins of EADs, Ca2+ipreceded V(m) and phase maps traced clockwise ellipses. Away from EAD origins, V(m) coincided with or preceded Ca2+i. In conclusion, overload elicits EADs originating from either ventricular or Purkinje fibres and 'out-of-phase' EAD activity from multiple sites generates TdP, evident in pseudo-ECGs.
TL;DR: It is concluded that the particular spatial profile of cellular Ca2+ signals is a major, previously unrecognised, determinant for arrhythmogenic potency and that the InsP3 signalling cassette might therefore be a promising new target for understanding and managing atrial arrhythmia.
Abstract: Various cardio-active stimuli, including endothelin-1 (ET-1), exhibit potent arrhythmogenicity, but the underlying cellular mechanisms of their actions are largely unclear. We used isolated rat atrial myocytes and related changes in their subcellular Ca(2+) signalling to the ability of various stimuli to induce diastolic, premature extra Ca(2+) transients (ECTs). For this, we recorded global and spatially resolved Ca(2+) signals in indo-1- and fluo-4-loaded atrial myocytes during electrical pacing. ET-1 exhibited a higher arrhythmogenicity (arrhythmogenic index; ratio of number of ECTs over fold-increase in Ca(2+) response, 8.60; n = 8 cells) when compared with concentrations of cardiac glycosides (arrhythmogenic index, 4.10; n = 8 cells) or the beta-adrenergic agonist isoproterenol (arrhythmogenic index, 0.11; n = 6 cells) that gave similar increases in the global Ca(2+) responses. Seventy-five percent of the ET-1-induced arrhythmogenic Ca(2+) transients were accompanied by premature action potentials, while for digoxin this proportion was 25 %. The beta-adrenergic agonist failed to elicit a significant number of ECTs. Direct activation of inositol 1,4,5-trisphosphate (InsP(3)) receptors with a membrane-permeable InsP(3) ester (InsP(3) BM) mimicked the effect of ET-1 (arrhythmogenic index, 14.70; n = 6 cells). Inhibition of InsP(3) receptors using 2 microM 2-aminoethoxydiphenyl borate, which did not display any effects on Ca(2+) signalling under control conditions, specifically suppressed the arrhythmogenic action of ET-1 and InsP(3) BM. Immunocytochemistry indicated a co-localisation of peripheral, junctional ryanodine receptors with InsP(3)Rs. Thus, the pronounced arrhythmogenic potency of ET-1 is due to the spatially specific recruitment of Ca(2+) sparks by subsarcolemmal InsP(3)Rs. Summation of such sparks efficiently generates delayed after depolarisations that trigger premature action potentials. We conclude that the particular spatial profile of cellular Ca(2+) signals is a major, previously unrecognised, determinant for arrhythmogenic potency and that the InsP(3) signalling cassette might therefore be a promising new target for understanding and managing atrial arrhythmia.
TL;DR: In isolated mitochondria, 5HD was rapidly converted to 5HD‐CoA by mitochondrial fatty acyl CoA synthetase and acted as a weak substrate or inhibitor of respiration depending on the conditions employed and highlighted the dangers of using 5HD and diazoxide as specific modulators of mitoKATP channels in the heart.
Abstract: Studies with different ATP-sensitive potassium (KATP) channel openers and blockers have implicated opening of mitochondrial KATP (mitoKATP) channels in ischaemic preconditioning (IPC). It would be predicted that this should increase mitochondrial matrix volume and hence respiratory chain activity. Here we confirm this directly using mitochondria rapidly isolated from Langendorffperfused hearts. Pre-ischaemic matrix volumes for control and IPC hearts (expressed in ml per mg protein ± S.E.M., n = 6), determined with 3 H2O and [ 14 C]sucrose, were 0.67 ± 0.02 and 0.83 ± 0.04 (P < 0.01), respectively, increasing to 1.01 ± 0.05 and 1.18 ± 0.02 following 30 min ischaemia (P < 0.01) and to 1.21 ± 0.13 and 1.26 ± 0.25 after 30 min reperfusion. Rates of ADP-stimulated (State 3) and uncoupled 2-oxoglutarate and succinate oxidation increased in parallel with matrix volume until maximum rates were reached at volumes of 1.1 ml ml _1 or greater. The mitoKATP channel opener, diazoxide (50 mM), caused a similar increase in matrix volume, but with inhibition rather than activation of succinate and 2-oxoglutarate oxidation. Direct addition of diazoxide (50 mM) to isolated mitochondria also inhibited State 3 succinate and 2-oxoglutarate oxidation by 30 %, but not that of palmitoyl carnitine. Unexpectedly, treatment of hearts with the mitoKATP channel blocker 5-hydroxydecanoate (5HD) at 100 or 300 mM, also increased mitochondrial volume and inhibited respiration. In isolated mitochondria, 5HD was rapidly converted to 5HD–CoA by mitochondrial fatty acyl CoA synthetase and acted as a weak substrate or inhibitor of respiration depending on the conditions employed. These data highlight the dangers of using 5HD and diazoxide as specific modulators of mitoKATP channels in the heart.
TL;DR: The results suggest that the differential expression of calpain 2 and 3 mRNA levels may be important in the process of exercise‐induced muscle damage, whereas expression of α‐sarcoglycan, desmin, αB‐crystallin and Hsp27 may be essentially involved in the subsequent remodelling of myofibrillar structure.
Abstract: The molecular events by which eccentric muscle contractions induce muscle damage and remodelling remain largely unknown. We assessed whether eccentric exercise modulates the expression of proteinases (calpains 1, 2 and 3, proteasome, cathepsin B+L), muscle structural proteins (α-sarcoglycan and desmin), and the expression of the heat shock proteins Hsp27 and αB-crystallin. Vastus lateralis muscle biopsies from twelve healthy male volunteers were obtained before, immediately after, and 1 and 14 days after a 30 min downhill treadmill running exercise. Eccentric exercise induced muscle damage as evidenced by the analysis of muscle pain and weakness, creatine kinase serum activity, myoglobinaemia and ultrastructural analysis of muscle biopsies. The calpain 3 mRNA level was decreased immediately after exercise whereas calpain 2 mRNA level was increased at day 1. Both mRNA levels returned to control values by day 14. By contrast, cathepsin B+L and proteasome enzyme activities were increased at day 14. The α-sarcoglycan protein level was decreased immediately after exercise and at day 1, whereas the desmin level peaked at day 14. αB-crystallin and Hsp27 protein levels were increased at days 1 and 14. Our results suggest that the differential expression of calpain 2 and 3 mRNA levels may be important in the process of exercise-induced muscle damage, whereas expression of α-sarcoglycan, desmin, αB-crystallin and Hsp27 may be essentially involved in the subsequent remodelling of myofibrillar structure. This remodelling response may limit the extent of muscle damage upon a subsequent mechanical stress.
TL;DR: The Na+‐ and Ca2+‐permeable conductance might be involved in haemolytic diseases induced by elevated oxidative stress, such as glucose‐6‐phosphate dehydrogenase deficiency.
Abstract: The permeability properties of the red blood cell (RBC) membrane govern its acid-base status and directly control the transport of carbon dioxide through the blood In addition to this respiratory function, several transport pathways are involved in the regulation of cell volume and intracellular ion homeostasis, but little is known about the ion channels of human RBCs
By using single channel patch-clamp techniques, different types of cation channels have been described: the Gardos K+ channel, which is a Ca2+-activated IK channel, is thought to be involved in the regulatory volume decrease (RVD) of RBCs (Grygorczyk & Schwarz, 1983; Bennekou & Christophersen, 1990; Christophersen, 1991; Pellegrino et al 1998) The activity of a non-selective cation (NSC) channel has also been described, but the physiological relevance of this channel is not known (Christophersen & Bennekou, 1991; Bennekou, 1993; Kaestner et al 1999; Kaestner et al 2000) Recently, Huber et al (2001), using the whole-cell patch-clamp configuration, described a non-selective cation conductance regulated by cell volume and by internal Cl− concentration In addition, human RBCs infected with Plasmodium falciparum showed increased permeabilities to mono- or divalent cations (Kirk & Horner, 1995) and Desai et al (1996) have reported a Ca2+-permeable cation channel in these cells
In many diseases such as glucose-6-phosphate dehydrogenase deficiency (Mavelli et al 1984; Turrini et al 1985), human RBCs have to deal with elevated oxidative stress In vitro oxidation of fresh RBCs has been shown to induce a complete change in the electrophoresis pattern, especially that of membrane proteins (Koster & Slee, 1983; Ingrosso et al 2000)
In order to study the involvement of ion conductances in oxidation-induced haemolysis, we performed whole-cell recordings in human RBCs exposed to elevated oxidative stress
TL;DR: Low muscle glycogen content enhances the transcriptional activation of some metabolic genes in response to exercise, raising the possibility that signalling mechanisms sensitive to glycogencontent and/or FFA availability may be linked to the transcriptionAL control of exercise‐responsive genes.
Abstract: Transcription of metabolic genes is transiently induced during recovery from exercise in skeletal muscle of humans. To determine whether pre-exercise muscle glycogen content influences the magnitude and/or duration of this adaptive response, six male subjects performed one-legged cycling exercise to lower muscle glycogen content in one leg and then, the following day, completed 2.5 h low intensity two-legged cycling exercise. Nuclei and mRNA were isolated from biopsies obtained from the vastus lateralis muscle of the control and reduced glycogen (pre-exercise glycogen = 609 +/- 47 and 337 +/- 33 mmol kg(-1) dry weight, respectively) legs before and after 0, 2 and 5 h of recovery. Exercise induced a significant (P 6-fold) than in the control (< 3-fold) trial. Induction of PDK4 and UCP3 mRNA in response to exercise was also significantly higher in the low glycogen (11.4- and 3.5-fold, respectively) than in the control (5.0- and 1.7-fold, respectively) trial. These data indicate that low muscle glycogen content enhances the transcriptional activation of some metabolic genes in response to exercise, raising the possibility that signalling mechanisms sensitive to glycogen content and/or FFA availability may be linked to the transcriptional control of exercise-responsive genes.
TL;DR: It is highlighted that titin is a versatile and adjustable spring with a range of important functions in passive and contracting myocardium, and novel ligands have been identified that link titin to membrane channels, protein turnover and gene expression.
Abstract: The giant elastic protein titin contains a molecular spring segment that underlies the majority of myocardial passive stiffness. The mechanical characteristics of this spring may be tuned to match changing mechanical demands placed on muscle, using mechanisms that operate on different time scales and that include post-transcriptional and post-translational processes. Recent work also suggests that titin performs roles that go beyond passive stiffness generation. In contracting myocardium, titin may modulate actomyosin interaction by a titin-based alteration of the distance between myosin heads and actin. Furthermore, novel ligands have been identified that link titin to membrane channels, protein turnover and gene expression. This review highlights that titin is a versatile and adjustable spring with a range of important functions in passive and contracting myocardium.