Showing papers in "Pflügers Archiv: European Journal of Physiology in 1980"

Inward current activated during hyperpolarization in the rabbit sinoatrial node cell

Abstract: Inward current activated by hyperpolarization (ih) was dissected from the K-current by the difference in its activation voltage range and the selective blocking effect of Ba2+ on the K-current. The ih shows little specificity to any particular ion, and its reversal potential was -25 mV. The current system can be expressed well by Hodgkin-Huxley type kinetics. The time constant of ih ranged from 2-4 s at about -70 mV, but it became shorter at about -10 mV. The ih began to activate at -50 mV and fully saturated at about -100 mV. The fully activated current-voltage relation shows no rectifying property. Activation and deactivation time courses were fitted by a single exponential with the same time constant at a given membrane potential. Although ih plays only a small role during the normal action potential in the isolated preparation, it plays a significant role in keeping the pacemaker cell at a low membrane potential

Regional blood flow in genetically obese (ob/ob) mice. The importance of brown adipose tissue to the reduced energy expenditure on non-shivering thermogenesis.

Abstract: 1. Radioactively labelled microspheres have been used to measure changes in regional blood flow of lean and genetically obese (ob/ob) mice during the nonshivering thermogenesis (NST) induced by noradrenaline (NA). 2. Without NA the distribution of the cardiac output was similar in both phenotypes, with the muscular carcass and the splanchnic organs each taking approximately 30% of the output, the kidneys about 15%, the skin + white adipose tissue about 10%, and the heart 4%. Brown adipose tissue (BAT) received only 1–2% of the cardiac output. 3. The effect of NA on the total cardiac output was similar in lean and obese animals. During maximum NST the lean mice showed an increase in blood flow to most tissues, but BAT was particularly responsive with an almost 40-fold increase. The ob/ob mice responded to NA with similar changes in regional blood flow. However, in BAT the increase in flow was only half that of lean animals. 4. It is calculated from the changes in blood flow and from measurements of the arteriovenous difference in oxygen concentration across interscapular BAT that in lean mice BAT is directly responsible for at least 44% of the increase in metabolic rate effected by NA. 5. It is similarly calculated that the reduced NST and consequent lower energy expenditure of the obese mutant is entirely due to lower metabolic activity in its BAT.

Effects of vasopressin on water and NaCl transport across the in vitro perfused medullary thick ascending limb of Henle's loop of mouse, rat, and rabbit kidneys.

Abstract: Direct tubular effects of arginine vasopressin (AVP) on water and NaCl transport across the medullary thick ascending limb of Henle (MAL) were examined by the in vitro perfusion of isolated nephron fragments of mice, rats, and rabbits. Osmotic water permeability of the MAL of mice and rats was low and remained unchanged with 2 mU/ml AVP added to the bath. A dose-dependent increase in transepithelial electrical potential difference (PD) with AVP was observed in the mouse MAL when the ambient medium was isotonic. A similar result was also obtained when 2×10−4 mol/l dibutyryl adenosine 3′,5′-cyclic-monophosphate was added to the bath. In this preparation, AVP also caused an increase in the unidirectional Cl efflux from 323±45 to 398±61 pmoles·mm−1 ·min−1 (n=6,P<0.05). In contrast, under similar condition, we could not demonstrate any effect of AVP on PD, Cl efflux, or net Na flux in the rat MAL and on PD and Cl efflux in the rabbit MAL. Both PD and Cl efflux in the rat MAL were unaffected by AVP when the perfusate was made hypotonic. However, when the ambient medium was made hypertonic by adding NaCl and urea, a significant increase in PD was observed. In addition, we confirmed that AVP stimulated adenylate cyclase activity in the MAL as well as in the collecting tubule of mice and rats. We conclude that AVP stimulates Cl transport across the MAL of mice and rats by activating adenylate cyclase-cyclic AMP system. However, this effect of AVP may quantitatively vary among species.

Slow inward current and its role mediating the chronotropic effect of epinephrine in the rabbit sinoatrial node.

Abstract: The ionic mechanism underlying the chronotropic effect of epinephrine on the rabbit sinoatrial (S-A) node has been studied. Epinephrine (5.5 X 10(-6) M) increased the spontaneous rate from 206 +/- 25 min-1 to 242 +/- 39 min-1. The effect of epinephrine was reproducible on repetitive applications. Voltage clamp experiments using the two microelectrode technique revealed the following changes in the membrane current: epinephrine (5.5 X 10(-7) M) increased the limiting conductance for the slow inward current (is) by approximately 30% and the potassium current (ik) by about 10%, keeping the kinetics of is and ik constant. From the holding potential of -70 mV the activation of is was observed on step depolarization positive to -60 or -55 mV in both control and epinephrine solution. The hyperpolarization-activated current (ih) was also increased by about 20% at -70 mV, and its time course was slightly accelerated. Participation of is for the chronotropic effect of epinephrine was strongly suggested by the findings that is was partially available positive to -60 mV and that epinephrine could not increase the slope of diastolic depolarization when is was blocked by D 600.

A new extraction of arginine vasopressin from blood: The use of octadecasilyl-silica

Abstract: Small columns packed with octadecasilylsilica were used to extract arginine-vasopressin in femtomole amounts from biological fluids for radioimmunoassay. This method is effective in isolating the peptide from substances in plasma and serum which interfere with its quantitation. Consistent and reproducible results and recoveries >80% were obtained with this procedure. High pressure liquid chromatography and serial dilution of extracted samples confirm the identity of the extracted product.

Potentiation of inhibition by general anaesthetics in neurones of the olfactory cortex in vitro

Abstract: Pentobarbitone, phenobarbitone, methohexitone, chloralose and alphaxalone produced 10-fold increases in the duration of an inhibitory post-synaptic conductance (i.p.s.c.) as recorded intracellularly from neurones of the guinea-pig olfactory cortex in vitro. Higher concentrations slightly depolarised these neurones and reduced their input resistances (Ri), presumably a spontaneous activation of the inhibitory conductance. The excitatory potentials were also depressed. Ketamine, halothane and urethane doubled the i.p.s.c. duration. Higher concentrations depressed synaptic activity and the action potential, as did lignocaine. Ketamine also increased Ri. These results confirm the idea that these compounds produced anaesthesia by prolonging inhibition (accompanied by a depression of the e.p.s.p. with some anaesthetics).

Static and dynamic discharge patterns of bursting cold fibers related to hypothetical receptor mechanisms.

Abstract: Static and dynamic discharge patterns of bursting cold fibers in the lingual nerve of the cat were analysed at temperatures between 10 and 40°C. The period of the static burst discharge depends only on temperature and is not affected by the number of spikes per burst. The termination of an intraburst sequence of impulses can be predicted by the duration of the intraburst intervals. Irregular impulse patterns at low static temperatures show a unimodal interval distribution, whereas the interval histograms from irregular spike sequences at high temperatures have multiple peaks with nearly equal distances. The dynamic responses to cooling steps of 5°C show a transient reduction of the burst period and a transient increase of the number of spikes per burst or the burst duration, respectively. In addition, the maximmum intraburst frequencies transiently increase while the frequency decay within the burst is continuously reduced. At low temperatures, the burst discharge is sometimes interrupted by a continuous impulse sequence of high frequency. A model of temperature transduction is discussed on the basis of membrane processes which are particularly described for molluscan neurons. The results support the hypothesis that the bursts are triggered by an endogenously oscillating receptor potential. It is assumed that at high temperatures the oscillation continues but certain cycles fail to trigger impulses. The results during dynamic cooling are interpreted as a flattening of the oscillation which is transiently superposed by a depolarizing shift of the potential waves.

Increase of adenosine content in cerebral cortex of the cat during bicuculline-induced seizure.

Abstract: In order to elucidate whether adenosine may be involved in the increase in cerebral blood flow (CBF) during functional hyperemia, cortical tissue levels of adenosine, inosine and hypoxanthine were measured in the cat following bicuculline (3 mg/kg)-induced seizure. In addition, the subcellular distribution of 5′-nucleotidase in the cortex was determined by histochemical techniques. Experiments were performed on anaesthetized and immobilized cats and tissue samples to be analyzed for the different purine compounds were obtained by freezing through the trepanized and non-trepanized skull. Control values for adenosine, inosine and hypoxanthine on the trepanized side were 1.31, 1.12 and 3.79 nmoles/g, respectively. The cortical content of adenosine increased to 3.85 nmoles/g, 15 s after beginning of seizure activity and remained elevated for 20 min. Inosine and hypoxanthine also increased, exhibiting highest values after 20 min. Values for adenosine and lactate were found to be generally higher when analysis was performed from tissue frozen through the intact skull bone. In all experiments the adenosine content correlated with lactate levels. From measurements of plasma concentrations of adenosine and inosine in the sinus sagittalis superior it is concluded that seizure is also accompanied by an enhanced release of adenosine from the brain. Localization of 5′-nucleotidase by electron-microscopic cytochemistry revealed that this enzyme is associated almost exclusively with plasma membranes of glial cell membranes including perivascular astrocytes. Thus high concentration of adenosine can be expected to accumulate locally in the brain cortex. In view of the well known increase in CBF during onset of seizure and the time course of adenosine formation, our findings support the view that adenosine may be involved in the initial phase of seizure-induced functional hyperemia of the brain.

Pathways mediating optokinetic responses of vestibular nucleus neurons in the rat.

Abstract: 1. The effects of various brain lesions on the responses of vestibular nuclear neurons (Vn) of the horizontal semicircular canal system to optokinetic stimulations were studied to elucidate the optokinetic path from the retina to the vestibular nuclei. A previous study performed in intact rats served as a control [2]. 2. It was shown that the pretectal region including the n. of the optic tract is the first central relay in the optokinetic path; it receives its functionally effective input from the contralateral eye. Unilateral lesions of this area rendered all Vn responses unidirectional when tested with binocular stimulation. Lesions of other visual centers such as the superior colliculi or visual cortices had no influence on the optokinetic response properties of Vn. 3. The area of the n. reticularis tegmenti pontis (NRTP) proved to be an important link between pretectum and vestibular nuclei: Unilateral lesions produced effects similar to those described for pretectal lesions. Pretectal axons to NRTP descend lateral to the MLF and tectospinal tract. 4. It was demonstrated that the vestibular commissure plays the crucial role in mediating the mirror image optokinetic effects to Vn on the opposite side and assures the bidirectionality of the responses to binocular stimulation. 5. Cerebellectomy did not significantly affect the Vn responses to the optokinetic stimuli presented in this study. 6. Electrical stimulation of the pretectum excited type II and inhibited type I Vn ipsilaterally and had the opposite effect on Vn located on the opposite side. NRTP stimulation excited type II and inhibited type I ipsilaterally; latency analysis of these effects suggested that the pretectal stimuli excited ipsilateral NRTP neurons which, in turn, excited ipsilateral type II Vn. Ipsilateral type I inhibition as well as the concurrent contralateral type II inhibition and type I excitation are produced by the inhibitory action of type II on type I and the commissural system. 7. Systemic application of picrotoxin abolished all optokinetic responses of Vn except the type II activation. This finding further supports the hypothesis described above. 8. Unilateral pretectal or NRTP lesions abolished OKN to surround motion in the direction of the lesion.

On the mechanism of slow calcium channel block in heart.

Abstract: 1. D600 (methoxyverapamil, 2×10−6 M) has little activity when it is applied during a rest interval. Its ability to depress Isi and contraction is much reduced when the muscle is rested. 2. Upon each activation the blockade of the slow channel increases until the use-dependent increase of blockade equals the diastolic removal, thus resulting in a frequency-dependent steady-state blockade. 3. Removal of block during a variable rest interval at various holding potentials is the faster the more negative the holding potential and gets slower in the more positive potential range. This process can be described by two voltage-dependent time constants (τ1, τ2).

Firing characteristics of neurons mediating optokinetic responses to rat's vestibular neurons.

Abstract: 1) The responses of single units in the pretectum (Pt) and in the n reticularis tegmenti pontis (NRTP) to constant velocity horizontal rotation (025–60 deg/s) of a large-field visual pattern were studied in immobilized, non-anesthetized DA-HAN rats In addition, responses of Pt and NRTP neurons to pure vestibular stimuli (rotation in the dark) were studied 2) Pt neurons showed seven response types to optokinetic stimulation (Table 1) The most frequent response (48%) consisted of a very rapid increase in firing to steady state on temporonasal motion stimulation of the contralateral eye; nasotemporal stimuli yielded no change in resting rate as did stimulation of the ipsilateral eye The response maximum occurred at a retinal slip velocity of 1 deg/s None of the Pt units tested responded to pure vestibular stimuli 3) NRTP neurons — as Pt units — most frequently (43%) increased their discharge rate on temporonasal stimulation of the contralateral eye and maintained a constant resting rate during nasotemporal motion Peak response amplitudes also occurred with retinal slip velocities of 1 deg/s Contrary to the fast time-to-peak of the responses of Pt neurons NRTP units showed a slow rise in frequency of firing to peak response levels 4) NRTP neurons responded to pure vestibular stimuli (horizontal angular acceleration in the dark) The vestibular responses were synergistic with those evoked in the same neurons by optokinetic stimuli Thus, the most frequently encountered type of optokinetic response (s above) showed a type II vestibular response 5) Comparison of OKN and Vn optokinetic responses with those of Pt and NRTP suggests that the unidirectional-selective Pt and NRTP neurons are important links in the central optokinetic path In addition, the NRTP may represent the site at which the retinal slip signal and the eye velocity signal converge This convergence has been postulated in models of the system [12]

Extracellular K+ concentration and K+ balance of the gastrocnemius muscle of the dog during exercise.

Abstract: Exercise-induced changes of the extracellular and the venous K+ concentrations ([K+]e, [K+]ven) were measured continuously in autoperfused dog gastrocnemii using PVC liquid membrane electrodes. O2 consumption ( $$\dot V$$ O2), blood flow and performance of the muscles were also determined. The muscles were stimulated indirectly and isotonic tetanic contractions (0.2 s) were produced every 0.7 s for more than 45 min.

Pulmonary arterial hypertension in male and female chickens at 3300 m.

Abstract: Two-hundred one-day-old male (M) and female (F) chickens were exposed to 3300m (HA). Two-hundred control chickens were raised at sea level (SL). Chickens from both HA and SL were studied each week from the 3rd to the 7th week of age. Pulmonary arterial pressure (Ppa) was measured under local anesthesia in conscious animals. Hb, Hct and red blood cell counts (E) and the weights of the right ventricle (RV), left ventricle (LV) and septum (S) were obtained. HA chickens, both males and females, reached lower body weights than SL chickens.Ppa were 16 and 43 mm Hg (2.13 and 5.73 kPa) while RV/(RV+LV+S) values were 0.195 and 0.347 in SL and HA chickens, respectively. RV was 90% greater and LV 20% smaller in the HA chickens. S was not affected by altitude. Hb, Hct and E were higher in the HA birds. Sixteen HA M and 1 HA F died with signs of right heart insufficiency (RHI), however, M did not show greater values ofPpa or RV/(RV+LV+S) than F at SL or at HA. This indicates that the higher incidence of RHI in the M cannot be attributed to a higher responsiveness of their pulmonary vasculature to hypoxia.

On the kinetics of the potassium channel activated by acetylcholine in the S-A node of the rabbit heart.

Abstract: 1. Voltage-clamp experiments were conducted on small specimens of rabbit sinoatrial node. In the same preparation the dose-response curve of the potassium current induced by application of different concentrations of acetylcholine (ACh), the time constant of relaxation and the current fluctuations were measured. From these measurements the apparent dissociation constant and the rate constants for the opening and closing of the ACh-activated potassium channel were estimated. 2. In the presence of neostigmine a measurable response was recorded at around 10−8 M ACh, the saturation was reached at 10−4 M, and the half saturation was attained at around 10−6 M. 3. The time constant of relaxation at −35 mV decreased from 100 ms at 10−8 M to 45 ms at 10−4 M ACh. 4. The variance of the fluctuations of the ACh-activated current increased with increasing ACh concentration to a peak value of around 10−5 M. 5. From the above 3 kinds of measurements, opening and closing rate constants of about 12 s−1 and 10 s−1, respectively, and a dissociation constant of 1.7 μM were calculated. 6. The Katz-Miledi model was considered to be appropriate to describe the reaction of ACh with the muscarinic receptor in the S-A node. 7. The current on ionophoretic application of ACh was computed using the rate constants and taking into account diffusion in the S-A node in which the density of receptors is low. The computed response had a similar time course to the recorded current.

The role of beta-1-adrenoceptors in the renin release response to graded renal sympathetic nerve stimulation

Abstract: The contribution of beta-adrenoceptor activation to renin release was examined in anaesthetized dogs using renal nerve stimulation (RNS) at different discharge rates in the presence of i.v. beta-adrenoceptor blockade. The animals were divided into 2 groups, which received either low or high level of RNS, defined by the frequency of stimulation producing decrease in renal blood flow of 5 and 50%, respectively. Low level RNS increased renin release tenfold. The renin release response was almost abolished by 0.5 mg/kg of metoprolol ordl-propranolol but unaffected by 0.5 mg/kg ofd-propranolol. The increase in renin release to high level RNS was equally reduced by 33% by 0.5 mg/kg and 2.0 mg/kg of metoprolol.dl-propranolol, 0.5 mg/kg, reduced the renin release response to about the same extent, 44%, while 2.0 mg/kg reduced it somewhat more, 59%. This was probably due to its membrane stabilizing properties asd-propranolol, 2.0 mg/kg and lidocaine 2.0 mg/kg+0.1 mgxkg−1xmin−1, also reduced the renin release response. These data suggest that the renin release response to low level RNS is almost completely mediated by beta-adrenoceptors which are of the beta-1 subtype. High level RNS results in a renin release, which is only partly mediated by beta-1-adrenoceptors. The remainder is apparently related to other mechanisms activated by high level RNS and is probably a consequence of the associated renal vasoconstriction.

Protein filtration and secretion at human body fluid barriers

Abstract: The permeability properties of several bloodbody fluid barriers are evaluated by means of 1) barrier lines (protein concentration ratios versus hydrodynamic radii) and 2) two-dimensional protein maps. Under steady state equilibrium conditions the barrier is sufficiently characterized by a restriction parameter (serum/body fluid concentration ratio of an equivalent protein) and a selectivity coefficient (slope of the barrier line). It is possible to define an equivalent protein which depends upon the selectivity of the filtration process. Its serum/body fluid concentration ratio indicates the amount of serum proteins in “pure” filtrates. The steady state barrier line can be used to quantitate passively filtered and actively secreted protein portions. This approach is exemplified by the immunoglobulins of various body fluids, but should be applicable for any protein that fulfills defined requirements. The individual protein maps reflect the proportional contributions of serum filtration and local protein synthesis to the formation of a body fluid. The protein composition of serum filtrates depends primarily on the barrier selectivity irrespective of the total protein content. The selectivity of any filtration barrier is strongly influenced by the hydraulic conductivity of the barrier sites and the fluid dynamics of the secondary compartment. An attempt is made to formulate an universal filtration concept that is valid for all barriers from the highly selective kidney glomerulus to the poorly selective blood-CSF barrier.

Effect of reduced red cell "deformability" on flow velocity in capillaries of rat mesentery

Abstract: A graded reduction of “deformability” of red blood cells (RBC's) of rats was obtained by treatment with the SH-oxidizing agent, diamide. Rigidified RBC's were injected into rats by isovolemic exchange against 60% of the native RBC's and RBC flow velocities in capillaries of rat mesentery measured. At normal mean arterial pressure RBC flow velocity decreases by 29% in rats receiving cells rigidified with 0.5 mmol·l−1 diamide. Surprisingly a further rigidification of erythrocytes by 1.5 mmol·l−1 diamide results in a decrease of flow by only 15%. During hypotension RBC flow velocities dropped precipitously to 8±15% for the 0.5 nmol·l−1 and to 2±6% for the 1.5 mmol·l−1 diamide group compared to velocities during normotension. By microscopy we observed a stop of flow in many vessels. This result outlines the importance of a normal red cell “deformability” for the maintenance of sufficient perfusion of the microcirculation, in particular at low blood pressure gradients.

Optokinetic responses of vestibular nucleus neurons in the rat.

Abstract: 1. Vestibular nucleus neurons of the brown rat (DA-HAN) responding to horizontal angular acceleration in the dark (type I and II neurons) have been studied during horizontal optokinetic stimulation in the time and frequency domain. For recording animals were nonanesthetized and paralyzed. 2. All type I and type II neurons studied responded in a direction-selective fashion to rotation of large-field visual pattern. With both eyes open, type I (type II) neurons increased (decreased) their discharge on optokinetic stimuli directed away from the recording side and decreased (increased) firing on rotation towards the recording side. Covering one eye, abolished the inhibition of type II and excitation of type I on the ipsilateral side and removed type II excitation and type I inhibition on the opposite side. 3. The missing responses of vestibular units to nasotemporal stimulation in monocular condition were paralleled by the absence of OKN on stimulation in the same direction. 4. Response maxima (± Δf) of vestibular units occurred at stimulus velocities of 1 deg/s (here equal retinal slip velocity). Below and above this velocity a sharp response decline occurred. The mean firing increases were larger than the decreases. There were no significant differences in mean ± Δf values between monocular and binocular conditions. 5. Frequency domain data show that response phase was in phase with surround velocity only at very low frequencies. With higher frequencies a progressive phase lag was noted; similarly the sensitivity decreased with increasing frequency.

Mechanism of action of aldosterone on active sodium transport across toad skin.

Abstract: Epithelium of the abdominal skin of the toad,Bufo marinus, has been studied by microelectrode impalement. Using an electrical equivalent circuit model, effective EMF's and specific conductances of the apical and basolateral membrane could be calculated. The skin was divided into 2 fragments for incubation in the presence, or not, of aldosterone (≥0.1 μM). After incubation overnight, sodium transport by the hormone-treated piece was increased 2.7-fold on average, compared to the untreated control. Concomitantly, conductance of the apical border increased more than 3-fold. Furthermore, mean conductance and electromotive force at the basolateral border increased by 80% and by 10%, respectively. Whether the latter changes merely represent delayed adaptation to increased apical conductance, cannot be settled from the data available.

Potassium current during the pacemaker depolarization in rabbit sinoatrial node cell.

Abstract: Voltage clamp experiments were carried out on the rabbit sinoatrial (S-A) node. The delayed outward current in the voltage range between −60 mV and −22 mV almost disappeared in the presence of 5 mM Ba2+. The slow inward current and the hyperpolarization-activated current remained unaffected. In the absence of the time-dependent potassium current the S-A node cell generated spontaneous action potentials, provided that the membrane was hyperpolarized by constant outward current. Therefore it seems unlikely that the potassium current plays an essential role in generating the pacemaker potential in the S-A node. The time course of the potassium current (iK) during the cardiac cycle was calculated using equations simulating the kinetics ofiK. According to this computation, the change ofiK.in the S-A node is small during pacemaker depolarization. It is proposed that the gradual decay of potassium conductance is less important for the development of the pacemaker potential than the contribution of the slow inward current.

Effects of potential blood substitutes (Perfluorochemicals) on rat liver and spleen

Abstract: The effect of an emulsion of perfluorochemicals (PFC) (7 parts perfluorodecalin and 3 parts perfluorotripropylamine, 4.4 g PFC/kg body weight) on organ function was determined. Whereas maximal storage of PFC was reached in the spleen as early as 12 h after PFC administration, the liver attained a maximal PFC content only after 2 days. The increase in weight also differed: a maximum occurred in the spleen on the 4th day, in the liver on the 8th day. Indocyanine green (ICG) clearance showed a small decrease, statistically significant after 12 and 24h. Colloidal carbon clearance, used as a measure of the function of the reticuloendothelial system (RES) decreased instantly after PFC to less than half the control value; after full recovery a second decrease was seen which lasted till the 4th day after PFC. Pretreatment with C 48/80 or with increasing doses ofE. coli endotoxin could largely obviate the depressive effect of PFC-loading on carbon clearance. Serum transaminases increased to about twice the control levels but were normal by the 2nd day, and thereafter. Alkaline phosphatase showed a 2.5 fold increase but returned to control level after the 2nd day. It is concluded that while a severe disturbance of liver function did not occur, the reduction in the capacity of the RES can become a serious factor in the defence against a simultaneously appearing infection if not compensated by activating the RES.

Role of calcium in the regulation of acetylcholine receptor synthese in cultured muscle cells

Abstract: Embroyonic muscles differentiated in vitro were used to study the effects of intracellular Ca2+ ([Ca2+1]i) variations on the amount of acetylcholine receptors ([AChR]) in the cell membrane. 2. Increased Ca2+ concentration in the growth medium ([Ca2+]o) caused a marked elevation of AChR levels, apparently through de novo synthesis. 3. Agents known to increase [Ca2+]i and its accumulation in the sarcoplasmic reticulum (SR), such as ionophore A23187, sodium dantrolene (DaNa), or high [Mg2+]o all enhanced alpha-bungarotoxin (alpha-BGT) binding after 48 h of treatment. 4. Electrical stimulation or caffeine, both affectors of SR calcium release, brought about a decrease in [AChR] probably by suppressing its synthesis. 5. The effects of simultaneous treatment with two AChR-inducing agents, namely, high [Ca2+]o in the presence of tetrodotoxin (TTX) or high [Mg2+]o were not additive, thus suggesting action via a common saturable mediator. 6. Intermediate AChR levels obtained following simultaneous treatments with opposing effects, e.g., electrical stimulation in the presence of high [Ca2+]o or DaNa, suggest contradictory actions on a common mediator. 7. All these observations indicate a strong correlation between SR calcium levels and [AChR] on myotubes; while calcium accumulation in the Sr was followed by increased AChR synthesis, calcium release was accompanied by suppression of receptor synthesis.

Discharge properties of afferent fibres of the goldfish semicircular canal with high frequency stimulation.

Abstract: The horizontal semicircular canals of goldfish were sinusoidally stimulated between 0.07 and 63 Hz (about 3 decades). Single afferent fibre recordings showed sinusoidal modulation of discharges. Above 4 Hz the discharges became phase-locked to the stimulus. With increasing frequency the number of spikes per period decreased so that finally only one spike per period remained. At 63 Hz a stimulus of as little as 0.005o was sufficient to drive the units to far above their spontaneous activity. As cupular deflection is less than the angle of body movement, the cupular deflection threshold for modulation of afferent discharges must be much less than 0.005o. Transfer functions of afferent activity were determined. The simple pendulum model does not fit the data. Additional introduction of a third time constant and the low pass properties of the receptor cell membrane, the synaptic delay and the leaky integrator of the post synaptic afferent terminal improve the fit.

Efferent activity in the goldfish vestibular nerve and its influence on afferent activity

Abstract: Out of 326 fibres in the horizontal semicircular canal branch of the goldfish vestibular nerve, 7 fibres could be identified as efferents. They showed irregular spontaneous activity and responded to rotatory stimuli with double frequency. Additionally in the central stump of the dissected nerve, efferent fibres were found, the spontaneous and stimulus modulated activity of which could not be differentiated from afferents. Efferents could be driven by a number of stimuli (vestibular, visual, somatosensory). Disruption of the efferent influence upon the receptors by dissection of the nerve or by pharmacological means (Gallamine) led to an increase of spontaneous afferent activity by 50%, showing that there is tonic efferent inhibition. Transfer functions of afferents were not changed after release from efferent influence. Electrical stimulation of efferents in 41% of the fibres led to an increase of afferent activity instead of the expected inhibition, which was seen in another 32%.

Spike potentials and membrane properties of dorsal root ganglion cells in pigeons.

Abstract: Active and passive membrane properties of dorsal root ganglion (DRG)-cells from the intact superfused ganglion of pigeons have been compared with the conduction velocity of their centrifugal axons. About two thirds of the neurones were associated with myelinated axons and classified as A-cells; the remainder were associated with unmyelinated axons and classified as C-cells. Slowly conducting group III A-cells (5–25 m·s−1) constituted half of the A-cell population. With exception of spike duration, spike parameters and membrane properties did not differ among the A-cells. Spike duration increased with decreasing conduction velocity demonstrating a small plateau (“hump”) during the fall time in group III neurones. This hump was more distinct in C-cells, resulting in a 2–5 times longer duration of action potentials. Amplitude and duration of afterhyperpolarization (AHP) of C-cells was 2–3 times that of A-cells. Administration of 10 mM CoCl2 decreased the rate of rise and the overshoot but increased the rate of fall of the action potential in C-cells and group III A-cells, largely abolishing the hump. It is suggested that the hump of the spike potential is largely produced by a Cacurrent and that the resultant increase of intracellular Ca might produce the larger AHP in C-cells, secondary to an increase in K-conductance.

Secretin and VIP-stimulated adenylate cyclase from rat heart. I. General properties and structural requirements for enzyme activation

Abstract: Membrane adenylate cyclase from rat heart was activated by the two gut peptides secretin and vasoactive intestinal peptide (VIP), glucagon, and the β-adrenergic drug isoproterenol, in the presence of guanosine 5′-triphosphate (GTP). With all the stimuli tested, the optimal magnesium concentration was 5 mM, i.e. in excess over the 0.5 mM ATP substrate concentration and 0.01 mM GTP used as cofactor. Under these conditions, half-maximal adenylate cyclase activation with glucagon, secretin, and VIP was achieved at concentrations of 0.5, 0.5 and 1.0 μM, respectively. Data obtained with the secretin (7–27) fragment, a secretin antagonist, indicate that secretin and VIP acted on the same binding sites, which differed from glucagon binding sites. Structural requirements for secretin activation of cardiac adenylate cyclase were evaluated by comparing the potency and efficacy of parent peptides and synthetic analogs. The gastric inhibitory peptide GIP was inactive. When using 13 mono-or bi-substituted analogs, it appeared that amino acids in positions 1, 2, 3, 4 and 6 were of major importance while those in position 5 and 11 played a relatively minor role.

Chronic hypoxia does not affect guinea pig skeletal muscle capillarity.

Abstract: The soleus and gastrocnemius muscles of chronically hypoxic guinea pigs were analyzed for capillary supply and myoglobin concentration. Weanling male guinea pigs were exposed to a simulated altitude of 5,100 m and an average ambient temperature of 22°C for 2, 4, 6, 10 and 14 weeks (range of BW 244–965 g). The soleus and gastrocnemius-plantaris muscles of one leg were analyzed for myoglobin concentration while the soleus and medial head of the gastrocnemius of the contralateral leg were cut at the midpoint, frozen and sectioned in a cryostat. The myosin ATPase method was used to visualize fibers and capillaries. Values of muscle weight, fiber cross sectional area, capillary density, capillary to fiber ratio and the number of capillaries around the fiber were compared to the values of the same parameters from normoxic guinea pigs selected to match the average body weights of the hypoxic animals. The growth rates of the two groups were not different. No significant differences in the regression lines for the normoxic and hypoxic animals were found so that when the data were combined no significant differences in the normoxic lines were introduced by adding the values of the hypoxic animals. The myoglobin values were significantly higher only in the hypoxic soleus after 14 weeks of exposure.

Evidence of altered calcium accumulation and calcium binding by the membranes of adipocytes in spontaneously hypertensive rats

Abstract: Calcium accumulation and calcium binding (ATP-dependent and ATP-independent calcium uptake) by the fragmented plasma membrane, the cytoplasmic reticulum and the mitochondria of isolated adipocytes obtained from spontaneously hypertensive (SHR, Kyoto-Wistar) and normotensive Wistar and Kyoto-Wistar rats were studied by means of isotopic (45Ca) exchange in vitro. The value of Ca accumulation in the cytoplasmic reticulum fraction of adipocytes obtained from SHR was found decreased, while in the mitochondrial fraction it was considerably greater as compared to those in both normotensive control groups. Ca binding (ATP-independent) by the plasma membrane fraction of hypertensive rats was less than that of the normotensive rats. There was no difference between the groups studied in calcium binding ability for the cytoplasmic reticulum fraction of adipocytes. These results seem to indicate that the adipose cells of hypertensive rats have an alteration of the membrane mechanism maintaining intracellular calcium distribution.

Aldosterone in vitro restores nephron Na-K-ATPase of distal segments from adrenalectomized rabbits.

Abstract: Aldosterone, when added in vitro to defined nephron segments, dissected from adrenalectomized rabbits, restores the depressed Na+−K+-ATPase activity in the thick ascending limb of the loop of Henle and in the cortical collecting tubule within 1 h of incubation. The direct effect is specific and, as demonstrated for the cortical collecting tubule, dose-dependent. The time course of aldosterone-mediated Na+−K+-ATPase activation is compatible with the conjecture of changes in the enzyme lipid environment.

Unidirectional influx of phosphate across the mucosal membrane of rabbit small intestine.

Abstract: The influx of phosphate across the mucosal border of different regions of rabbit small intestine was investigated using the technique of Schultz et al., J. Gen. Physiol.50, 1241–1260 (1967). In the duodenum, the phosphate influx consisted of two components: 1. a saturable part, inhibited competitively by the presence of arsenate in the mucosal solution and strongly dependent on the mucosal Na concentration, and 2. a Na-independent part, linearly related to the mucosal phosphate concentration. In the jejunum and the ileum, the phosphate influx was a linear function of the mucosal phosphate concentration. In these regions arsenate had no effect on the influx, supporting the idea of a diffusional transport. HgCl2 (0.5 mM) reduced the phosphate influx in the duodenum, at 140 mM Na, to the levels under Na-free conditions. The Na-independent influx was only slightly decreased by HgCl2, suggesting that this agent affects mainly the Na-dependent phosphate influx. In the ileum HgCl2 decreased the influx by about the same amount as under Na-free conditions in the duodenum. Thus, in the rabbit, the duodenum only appears to have a Na-dependent, carrier mediated phosphate transport mechanism at the mucosal membrane. In the jejunum and the ileum the phosphate uptake seems to be by simple diffusion.