Showing papers in "The Journal of General Physiology in 1975"

Access resistance of a small circular pore.

Abstract: Dear Sir: The access resistance of a small circular pore is of some importance in estimating the conductance of pores in biological membranes (Hille, 1968,1970). This resistance is usually approximated as the convergence resistance to a hemisphere of the same radius as the pore. The approximation assumes that the contribution of the hemisphere itself is negligible. It turns out that this is not true and that the resistance of the hemisphere is in fact of the same order of magnitude as the resistance of the material in the halfspace outside the hemisphere. We can estimate the resistance of the hemisphere by assuming field lines inside the hemisphere are straight and perpendicular to the mouth of the pore. With this approximation the resistance of the hemisphere is

Ionic selectivity, saturation, and block in sodium channels. A four-barrier model.

Abstract: Ionic fluxes in Na channels of myelinated axons show ionic competition, block, and deviations from simple flux independence. These phenomena are particularly evident when external Na+ ions are replaced by other permeant or impermeant ions. The observed currents require new flux equations not based on the concepts of free diffusion. A specific permeability model for the Na channel is developed from Eyring rate theory applied to a chain of saturable binding sites. There are four energy barriers in the pore and only one ion is allowed inside at a time. Deviations from independence arise from saturation. The model shows that ionic permeability ratios measured from zero-current potentials can differ from those measured from relative current amplitudes or conductances. The model can be fitted to experiments with various external sodium substitutes by varying only two parameters: For each ion the height of the major energy barrier (the selectivity filter) determines the biionic zero-current potential and the depth of the energy well (binding site) just external to that barrier then determines the current amplitudes. Voltage clamp measurements with myelinated nerve fibers are given showing numerous examples of deviations from independence in ionic fluxes. Strong blocks of ionic currents by guanidinium compounds and Tl+ ions are fitted by binding within the channel with apparent dissociation constants in the range 50-122 mM. A small block with high Na+ concentrations can be fitted by Na+ ion binding with a dissociation constant of 368 mM. The barrier model is given a molecular interpretation that includes stepwise dehydration of the permeating ion as it interacts with an ionized carboxylic acid.

Regulation of muscular contraction. Distribution of actin control and myosin control in the animal kingdom.

Abstract: The control systems regulating muscle contraction in approximately 100 organisms have been categorized. Both myosin control and actin control operate simultaneously in the majority of invertebrates tested. These include insects, chelicerates, most crustaceans, annelids, priapulids, nematodes, and some sipunculids. Single myosin control is present in the muscles of molluscs, brachiopods, echinoderms, echiuroids, and nemertine worms. Single actin control was found in the fast muscles of decapods, in mysidacea, in a single sipunculid species, and in vertebrate striated muscles. Classification is based on functional tests that include measurements of the calcium dependence of the actomyosin ATPase activity in the presence and the absence of purified rabbit actin and myosin. In addition, isolated thin filaments and myosins were also analyzed. Molluscs lack actin control since troponin is not present in sufficient quantities. Even though the functional tests indicate the complete lack of myosin control in vertebrate striated muscle, it is difficult to exclude unambiguously the in vivo existence of this regulation. Both control systems have been found in animals from phyla which evolved early. We cannot ascribe any simple correlation between ATPase activity, muscle structure, and regulatory mechanisms.

Multiple effects of calcium antagonists on plateau currents in cardiac Purkinje fibers.

Abstract: We studied the influence of Mn, La, and D600 on action potentials and plateau currents in cardiac Purkinje fibers. The Ca antagonists each abolished the second inward current, but they failed to act selectively. Voltage clamp experiments revealed two additional effects: decrease of slow outward current (iotachi) activation, and increase of net outward time-independent plateau current. These effects occurred at inhibitor concentrations used in earlier studies, and were essential to the reconstruction of observed Ca antagonist effects on electrical activity. The inhibitory influence of Mn, La, and D600 on iotachi suggested that iotachi activation might depend upon prior Ca entry. This hypothesis was not supported, however, when [Ca]omicron was varied: elevating [Ca]omicron enhanced Ca entry, but iotachi was nevertheless depressed. Thus, the results suggested instead that Ca antagonists and Ca ions have rather similar effects on iotachi, possibly mediated by changes in membrane surface charge.

Voltage clamp analysis of two inward current mechanisms in the egg cell membrane of a starfish.

Abstract: Ionic mechanisms of excitation were studied in the immature egg cell membrane of a starfish, Mediaster aequalis, by analyzing membrane currents during voltage clamp. The cell membrane shows two different inward current mechanisms. One is activated at a membrane potential of -55 approximately -50 mV and the other at -7 approximately -6 mV. They are referred to as channels I and II, respectively. A similar difference is also found in the membrane potential of half inactivation. Currents of the two channels can, therefore, be separated by selective inactivation. The currents of both channels depend on Ca++ (Sr++ or Ba++) but only the current of channel I depends on Na+. The time-course of current differs significantly between the two channels when compared at the same membrane potential. The relationship between the membrane current and the concentration of the permeant ions is also different between the two channels. The result suggests that channel II is a more saturable system. The sensitivity of the current to blocking cations such as Co++ or Mg++ is substantially greater in channel II than in channel I. Currents of both channels depend on the external pH with an apparent pK of 5.6. They are insensitive to 3 muM tetrodotoxin (TTX) but are eliminated totally by 7.3 mM procaine. The properties of channel II are similar to those of the Ca channel found in various adult tissues. The properties of channel I differ, however, from those of either the typical Ca or Na channels. Although the current of the channel depends on the external Na the amplitude of the Na current decreases not only with the Na concentration but also with the Ca concentration. No selectivity is found among Li+, Na+, Rb+, and Cs+. The experimental result suggests that Na+ does not carry current but modifies the current carried by Ca in channel I.

Sizes of components in frog skeletal muscle measured by methods of stereology.

Abstract: Stereological techniques of point and intersection counting were used to measure morphological parameters from light and electron micrographs of frog skeletal muscle. Results for sartorius muscle are as follows: myofibrils comprise 83% of fiber volume; their surface to volume ratio is 3.8 mum-1. Mitochondria comprise 1.6% of fiber volume. Transverse tubules comprise 0.32% of fiber volume, and their surface area per volume of fiber is 0.22 mum-1. Terminal cisternae of the sarcoplasmic reticulum comprise 4.1% of fiber volume; their surface area per volume of fiber is 0.54 mum-1. Longitudinal sarcoplasmic reticullum comprises 5.0% of fiber volume, and its surface area per volume of fiber is 1.48 mum-1. Longitudinal bridges between terminal cisternae on either side of a Z disk were observed infrequently; they make up only 0.035% of fiber volume and their surface area per volume of fiber is 0.009 mum-1. T-SR junction occurs over 67% of the surface of transverse tubules and over 27% of the surface of terminal cisternae. The surface to volume ratio of the caveolae is 48 mum-1; caveolae may increase the sarcolemmal surface area by 47%. Essentially the same results were obtained from semitendinosus fibers.

Absorption Spectra and Linear Dichroism of Some Amphibian Photoreceptors

Abstract: Absorption spectra and linear dichroism of dark-adapted, isolated photoreceptors of mudpuppies, larval and adult tiger salamanders, and tropical toads were measured microspectrophotometrically. Spectral half-band width, dichroic ratio, and transverse specific density were determined using averaged polarized absorptance spectra and photomicrographs of seven types of rod outer segments. Two classes of cells were found, one with higher specific density and dichroic ratio, associable with the presence of rhodopsins, the other, lower in both quantities, associable with porphyropsins. Relationships were derived to calculate the product of molar concentration and extinction coefficient (CEmax) from specific density and dichroic ratio. By utilizing the hypothesis of invariance of oscillator strengths and measured half-band widths, Emax values were independently determined, permitting the calculation of C. The pigment concentration for all cells tested was about 3.5 mM. The broadness of green rod pigment spectra is correlated with reduced molar absorptivity and reduced cellular specific density. Estimation of physiological spectral sensitivities is discussed. Based on dichroic ratio considerations, a model is proposed for the orientation of retinals in situ which could account for the apparent degree of alignment of transition moments. In the chosen orientation, the ring portion of conjugation becomes primarily responsible for axial extinction. Reduced dichroism of dehydroretinal-bearing cells can thus result from the extended ring conjugation of chromophores. Some inferences derivable from the model are discussed.

Oxygen diffusion in biological and artificial membranes determined by the fluorochrome pyrene.

Abstract: Quenching of pyrene fluorescence by oxygen was used to determine oxygen diffusion coefficients in phospholipid dispersions and erythrocyte plasma membranes. The fluorescence intensity and lifetime of pyrene in both artificial and natural membranes decreases about 80% in the presence of 1 atm O2, while the fluorescence excitation and emission spectra and the absorption spectrum are unaltered. Assuming the oxygen partition coefficient between membrane and aqueous phase to be 4.4, the diffusion coefficients for oxygen at 37 degrees C are 1.51 X 10(-5) cm2/s in dimyristoyl lecithin vesicles, 9.32 X 10(-6) cm2/s in dipalmitoyl lecithin vesicles, and 7.27 X 10(-6) cm2/s in erythrocyte plasma membranes. The heats of activation for oxygen diffusion are low (less than 3 kcal/degree-mol). A dramatic increase in the diffusion constant occurs at the phase transition of dimyristoyl and dipalmitoyl lecithin, which may result from an increase in either the oxygen diffusion coefficient, partition coefficient, or both. The significance of the change in oxygen diffusion below and above the phase transition for biological membranes is discussed.

Activities of potassium and sodium ions in rabbit heart muscle.

Abstract: Activities (a) of intracellular K and Na in rabbit ventricular papillary muslces were determined with cation-selectivve glass microelectrodes and concentrations (C) were estimated with flame photometry. The CK and aK of the muscles were 134.9 +/- 3.1 mM (mean value +/- SE) and 82.6 mM, respectively, at 25 degrees C. The corresponding CNa and aNa were 32.7 +/- 2.7 and 5.7, respectively. The apparent intracellular activity coefficients for K (gammaK) and Na (gammaNa) were 0.612 and 0.175, respectively. Similar results were obtained at 35 +/- 1 degree C. gammaK was substantially lower than the activity coefficient (0.745) of extracellular fluid (Tyrode's solution), which might be expected on the basis of a different intracellular ionic strength. gammaNa was much lower than that of extracellular fluid, and suggest that much of the Na was compartmentalized or sequestered. For external K concentrations greater than 5 mM, the resting membrane potentials agreed well with the potential differences calculated from the K activity gradients across the cell membrane as a potassium electrode. These results emphasize that potassium equilibrium potentials in heart muscle should be calculated by activities rather than concentrations.

Intracellular recordings from gecko photoreceptors during light and dark adaptation.

Abstract: Intracellular recordings were obtained from rods in the Gekko gekko retina and the adaptation characteristics of their responses studied during light and dark adaptation. Steady background illumination induced graded and sustained hyperpolarizing potentials and compressed the incremental voltage range of the receptor. Steady backgrounds also shifted the receptor's voltage-intensity curve along the intensity axis, and bright backgrounds lowered the saturation potential of the receptor. Increment thresholds of single receptors followed Weber's law over a range of about 3.5 log units and then saturated. Most of the receptor sensitivity change in light derived from the shift of the voltage-intensity curve, only little from the voltage compression. Treatment of the eyecup with sodium aspartate at concentrations sufficient to eliminate the beta-wave of the electroretinogram (ERG) abolished initial transients in the receptor response, possibly indicating the removal of horizontal cell feedback. Aspartate treatment, however, did not significantly alter the adaptation characteristics of receptor responses, indicating that they derive from processes intrinsic to the receptors. Dark adaptation after a strongly adapting stimulus was similarly associated with temporary elevation of membrane potential, initial lowering of the saturation potential, and shift of the voltage-intensity curve. Under all conditions of adaptation studied, small amplitude responses were linear with light intensity. Further, there was no unique relation between sensitivity and membrane potential suggesting that receptor sensitivity is controlled at least in part by a step of visual transduction preceding the generation of membrane voltage change.

Pores formed in lipid bilayer membranes by nystatin, Differences in its one-sided and two-sided action.

Abstract: Nystatin and amphotericin B induce a cation-selective conductance when added to one side of a lipid bilayer membrane and an anion-selective conductance when added to both sides. The concentrations of antibiotic required for the one-sided action are comparable to those employed on plasma membranes and are considerably larger than those required for the two-sided action. We propose that the two-sided effect results from the formation of aqueous pores formed by the hydrogen bonding in the middle of the bilayer of two "half pores," whereas the one-sided effect results from the half pores alone. We discuss, in terms of the flexibility of bilayer structure and its thickness, how it is possible to have conducting half pores and "complete pores" in the same membrane. The role of sterol (cholesterol and ergosterol) in pore formation is also examined.

Sodium chloride transport by rabbit gallbladder. Direct evidence for a coupled NaCl influx process.

Abstract: The results of the present study that NaCl transport by in vitro rabbit gallbladder must be a consequence of a neutral coupled carrier-mediated mechanism that ultimately results in the active absorption of both ions; pure electrical coupling between the movements of Na and Cl can be excluded on the grounds of electrphysiologic considerations. Studies on the unidirectional influxes of Na and Cl have localized the site of this coupled mechanism to the mucosal membranes. Studies on the intracellular ion concentrations and the intracellular electrical potential are consistent with the notion that (a) the coupled NaCl influx process results in the movement of Cl from the mucosal solution into the cell against an apparent electrochemical potential difference; (b) the energy for the uphill movement of Cl is derived from the Na gradient across the mucosal membrane which is maintained by an active Na extrusion mechanism located at the basolateral membranes; and (c) Cl exit from the cell across the basolateral membranes is directed down an electrochemical potential gradient and may be diffusional. Finally, as for the case of rabbit ileum, the coupled NaCl influx process is inhibited by elevated intracellular levels of cyclic 3',5'-adenosine monophosphate. A working model for transcellular and paracellular NaCl transport by in vitro rabbit gallbladder is proposed.

Slow PIII component of the carp electroretinogram.

Abstract: The slow PIII component of the electroretinogram (ERG) was studied in the isolated, aspartate-treated carp retina. Although the latter is richly populated with cones, slow PIII appeared to reflect almost exclusively the activity of rods; e.g. the spectral sensitivity of the potential paralleled closely the rod pigment curve, its operating range (i.e. the V-log I curve) was limited to 3 log units above absolute threshold, and raising background intensities to photopic levels produced saturation of the increment threshold function without evidence of a cone-mediated segment. Only after bleaching away a significant fraction of the porphyropsin was it possible to unmask a small photopic contribution to slow PIII, as evidenced by a displacement in the action spectrum to longer wavelengths. The spatial distribution of the slow PIII voltage within the retina (Faber, D.S. 1969. Ph.D. Thesis. State University of New York. Buffalo, N.Y.; Witkovsky, P.J. Nelson, and H. Ripps. 1973. J. Gen Physiol. 61:401) and its ability to survive aspartate treatment indicate that this potential arises in the Muller (glial) fiber. Additional support for this conclusion is provided by the slow rise time (several seconds) and long temporal integration (up to 40s) of the response. In many respects the properties of slow PIII resemble those of the c-wave, a pigment epithelial response also subserved by rod activity. On the other hand, the receptoral (fast PIII) and the b-wave components of the ERG behave quite differently. Unlike slow PIII, response saturation could not be induced, since both potentials are subserved by cones when the stimulus conditions exceed the limits of the scotopic range. Receptors appear to govern light adaptation at photopic background levels; both fast PIII and b-wave manifest identical incremental threshold values over this range of intensities. However, under scotopic conditions, the sensitivity of the b-wave is affected by luminous backgrounds too weak to alter fast PIII threshold, indicating a postreceptoral stage of adaptation.

Characterization of the effects of Mg2+ on Ca2+- and Sr2+-activated tension generation of skinned skeletal muscle fibers.

Abstract: Changes in [Mg2+] in a millimolar range have a significant inverse effect on the Ca2+- (or Sr2+)activated tension generation of skeletal muscle fibers. Single frog (Rana pipiens) semitendinosus muscle fibers were "skinned" (sarcolemma removed) and contracted isometrically in bathing solutions of varying [Ca2+] or [Sr2+] and [Mg2+] but a constant pH, [MgATP2-], [K+], [CP2-], [CPK], and ionic strength. Ca2+- (or Sr2+-)activated steady-state tensions were recorded for three [Mg2+]'s: 5 X 10(-5)M, 1 X 10(-3) M, and 2 X 10(-3) M; and these tensions were expressed as the percentages of maximum tension generation of the fibers for the same [Mg2+]. Maximum tension was not affected by [Mg2+] within Ca2+-activating or Sr2+-activating sets of solutions; however, the submaximum Ca2+-(or Sr2+)activated tension is strongly affected in an inverse fashion by increasing [Mg2+]. Mg2+ behaves as a competitive inhibitor of Ca2+ and also affects the degree of cooperativity in the system. At [Mg2+] = 5 X 10(-5)M the shape of tension versus [Ca2+] (or [Sr2+]) curve showed evidence of cooperativity of Ca2+ (or Sr2+) binding or activation of the contractile system. As [Mg2+] increased, the apparent affinity for Ca2+ or Sr2+ and cooperativity of the contractile system declined. The effect on cooperativity suggests that as [Mg2+] decreases a threshold for Ca2+ activation appears.

Models for coupling of salt and water transport; Proximal tubular reabsorption in Necturus kidney.

Abstract: Models for coupling of salt and water transport are developed with two important assumptions appropriate for leaky epithelia. (a) The tight junction is permeable to both sale and water. (b) Active Na transport into the lateral speces is assumed to occur uniformly along the length of the channel. The proposed models deal specifically with the intraepithelial mechanism of proximal tubular resbsorption in the Necturus kidney although they have implications for epithelial transport in the gallbladder and small intestine as well. The first model (continuous version) is similar to the standing gradient model devised by Diamond and Bossert but used different boundary conditions. In contrast to Diamond and Bossert's model, the predicted concentration profiles are relatively flat with no sizable gradients along the interspace. The second model (compartment version) expands Curran's model of epithelial salt and water transport by including additional compartments and considering both electrical and chemical driving forces for individual Na and Cl ions as well as hydraulic and osmotic driving forces for water. In both models, ion and water fluxes are investigated as a function of the transport parameters. The behavior of the models is consistent with previously suggested mechanisms for the control of net transport, particularly during saline diuresis. Under all conditions the predicted ratio of net solute to solvent flux, or emergent concentration, deviates from exact isotonicity (except when the basement membrane has an appreciable salt reflection coefficient). However, the degree of hypertonicity may be small enough to be experimentally indistinguishable from isotonic transport.

Effects of intracellular injection of calcium buffers on light adaptation in Limulus ventral photoreceptors.

Abstract: The calcium sequestering agent, EGTA, was injected into Limulus ventral photoreceptors. Before injection, the inward membrane current induced by a long stimulus had a large initial transient which declined to a smaller plateau. Iontophoretic injection of EGTA tended to prevent the decline from transient to plateau. Before injection the plateau response was a nonlinear function of light intensity. After EGTA injection the response-intensity curves tended to become linear. Before injection, bright lights lowered the sensitivity as determined with subsequent test flashes. EGTA injection decreased the light-induced changes in sensitivity. Ca-EGTA buffers having different levels of free calcium were pressure-injected into ventral photoreceptors; the higher the level of free calcium, the lower the sensitivity measured after injection. The effects of inotophoretic injection of EGTA were not mimicked by injection or similar amounts of sulfate and the effects of pressure injection of EGTA buffer solutions were not mimicked by injection of similar volumes of pH buffer or mannitol. The data are consistent with the hypothesis that light adaptation is mediated by a rise of the intracellular free calcium concentration.

Variation of muscle stiffness with force at increasing speeds of shortening.

Abstract: Single frog skeletal muscle fibers were attached to a servo motor and force transducer by knotting the tendons to pieces of wire at the fiver insertions. Small amplitude, high frequency sinusoidal length changes were then applied during tetani while fibers contracted both isometrically and isotonically at various constant velocities. The amlitude of the resulting force oscillation provides a relative measure of muscle stiffness. It is shown from an analysis of the transient force responses observed after sudden changes in muscle length applied both at full and reduced overlap and during the rising phase of short tetani that these responses can be explained on the basis of varying numbers of cross bridges attached at the time of the length step. Therefore, the stiffness measured by the high frequency legth oscillation method is taken to be directly proportional to the number of cross bridges attached to thin filament sittes. It is found that muscle stiffness measured in this way falls with increasing shortening velocity, but not as rapidly as the force. The results suggest that at the maximum velocity of shortening, when the external force is zero, muscle stiffness is still substantial. The findings are interpreted in terms of a specific model for muscle contraction in which the maximum velocity of shortening under zero external load arises when a force balance is attained between attached cross bridges somr interpretations of these results are also discussed.

Sensitivity of calcium efflux from squid axons to changes in membrane potential.

Abstract: Squid giant axons were internally dialyzed with a medium free of metabolic substrates but containing 45Ca buffered with EGTA to concentrations of free Ca++ in the range 0.01-230 muM. At (Ca)i of 1.0 muM OR GREATER, Ca efflux was in the range of 1-3 pmol/cm2 s, was dependent on (Na)o and (Ca)o, and was sensitive to membrane potential. At lower (Ca)i, the sensitivity of Ca efflux to membrane potential was greater. Hyperpolarization of the membrane increased, and depolarization decreased Ca efflux over the range of potentials studied (-20 to -100 mV). The maximum sensitivity of Ca efflux to membrane potential was of the order of an e-fold increase in Ca efflux for a 25-mV increase in Em; this sensitivity of Ca efflux to membrane potential was lost if (Na)o was removed and was greatly reduced when (Ca)i was increased to 230 muM.

Human platelet secretion and aggregation induced by calcium ionophores. Inhibition by PGE1 and dibutyryl cyclic AMP.

Abstract: Ca2+, Mg2+-ionophores X537A and A23,187 (10(-7)-10(-6) M) induced the release of adenine nucleotides adenosine diphosphate (ADP, adenosine triphosphate (ATP), serotonin, beta-glucuronidase, Ca2+, and Mg2+ from washed human platelets. Enzymes present in the cytoplasm or mitochondria, and Zn2+ were not released. The rate of ATP and Ca2+ release measured by firefly lantern extract and murexide dye, respectively, was equivalent to that produced by the physiological stimulant thrombin. Ionophore-induced release of ADP, and serotonin was substantially (approximately 60%) but not completely inhibited by EGTA, EDTA, and high extracellular Mg2+, without significant reduction of Ca2+ release. The ionophore-induced release reaction is therefore partly dependent upon uptake of extracellular Ca2+ (demonstrated using 45Ca), but also occurs to a significant extent due to release into the cytoplasm of intracellular Ca2+. The ionophore-induced release reaction and aggregation of platelets could be blocked by prostaglandin E1 (PGE1) or dibutyryl cyclic AMP. The effects of PGE1, and N6, O2-dibutyryl adenosine 3':5'-cyclic monophosphoric acid (dibutyryl cAMP) were synergistically potentiated by the phosphodiesterase inhibitor theophylline. It is proposed that Ca2+ is the physiological trigger for platelet secretion and aggregation and that its intracellular effects are strongly modulated by adenosine 3':5'-cyclic monophosphoric acid (cyclic AMP).

Retinal mechanisms of visual adaptation in the skate.

Abstract: Electrical potentials were recorded from different levels within the skate retina. Comparing the adaptive properties of the various responses revealed that the isolated receptor potential and the S-potential always exhibited similar changes in sensitivity, and that the b-wave and ganglion-cell thresholds acted in concert. However, the two sets of responses behaved differently under certain conditions. For example, a dimly iluminated background that had no measurable effect on the senitivities of either of the distal responses, raised significantly the thresholds of both the b-wave and the ganglion cell responses. In addition, the rate of recovery during the early, "neural" phase of dark adaptation was significantly faster for the receptor and S-potentials than for the b-wave or ganglion cell discharge. These results indicate that there is an adaptive ("network") mechanism in the retina which can influence significantly b-wave and gaglion cell activity and which behaves independently of the receptors and horizontal cells. We conclude that visual adaptation in the skate retina is regulated by a combination of receptoral and network mechanisms.

Effect of cross-reinnervation on physiological parameters and on properties of myosin and sarcoplasmic reticulum of fast and slow muscles of the rabbit.

Abstract: Cross-reinnvervation of fast (extensor digitorum longus) and slow (soleus) twitch muscles of the rabbit showed essentially complete fast to slow and slow to fast conversion, respectively, 11-12 mo after surgery with respect to a number of physiological parameters including intrinsic shortening, velocity, and isometric twitch time to peak. There was pronounced bu incomplete biochemical conversion as judged by Ca2+ uptake by sarcoplasmic reticulum, myosin ATPase, alkali lability, and light chain complement. The question of trophic substances of neural origin is discussed in light of the fact that chronic stimulation for 15 wk of a fast muscle produces complete biochemical and physiological conversion to the slow type.

Diminution and enlargement of the mosquito rhabdom in light and darkness.

Abstract: The rhabdoms of the larval ocelli of the mosquito Aedes aegypti undergo morphological light and dark adaptation over periods of hours. The rhabdom enlarges during dark adaptation and grows smaller during light adaptation. Diminution is exponential, enlargement linear, and rates of change are proportional to log light intensity. Rhabdoms maintained at a constant intensity level off at a constant volume proportional to log intensity. We argue that changes in rhabdom volume after changes in light intensity reflect an influence of light on the turnover of photoreceptro membrane, and that the volumes at which rhabdoms level off represent equilibria between opposed processes of membrane loss and renewal.

Slow inward current and contraction of sheep cardiac Purkinje fibers.

Abstract: A "slow" inward current (Is) has been identified in ventricular muscle and Purkinje fibers of several mammalian species. The two-microelectrode voltage clamp technique is used to examine some of the relationships between Is and contraction of the sheep cardiac Purkinje fiber. "Tails" of inward current occurring on repolarization and extrapolation of Is recovery each show that the Is system may not inactivate completely during prolonged depolarization. The rate of recovery of Is after a depolarization is slow, and when a train of 300-ms clamps (frequency 1 s-1) is begun after a rest, Is is larger for the first clamp than it is for succeedings clamps. For the first clamp after a rest, the thresholds for Is and tension are the same and there is a direct correlation between peak tension and peak Is for clamp voltages between threshold and minus 40 mV. After a clamp, however, the ability to contract recovers much more slowly than does Is. Therefore, since Is may occur under certain conditions without tension, the realtionship between Is and tension must be indirect. Calcium entering the cell via this current may replenish or augment an intracellular calcium pool.

Ultraviolet and green receptors in principal eyes of jumping spiders.

Abstract: Spectral sensitivities of cells in principal eyes of the jumping spider Phidippus reqius were measured using techniques of intracellular recording. Three types of cells were found. UV cells had peak sensitivities at 370 nm and were over 4 log units less sensitive at wavelengths longer than 460 nm. Green-sensitive cells had spectral sensitivities which were well fit by nomogram curves peaking at 532 nm. UV-green cells had dual peaks of sensitivity at about 370 and 525 nm, but the ratios of UV-to-green sensitivities varied over a 40: 1 range from cell to cell. Moreover, responses of UV-green cells to flashes of UV light were slower than to flashes of green light. Segregation of receptor types into the known layers of receptors in these eyes could not be shown. It is concluded that jumping spiders have the potential for dichromatic color vision.

Electrophysiological properties of the membrane and acetylcholine receptor in developing rat and chick myotubes.

Abstract: Membrane properties of rat and chick myotubes in various stages of development were studied. Resting membrane potentials (Em) increased from -8 to -55 mV in both rat and chick as the myotubes developed from myoblasts to large multinucleated fibers. In the rat myotubes, this increase was not accompanied by significant changes in specific membrane resistivity or changes in Na+ and K+ ion distribution. Nor have we observed a significant electrogenic component to the resting Em of mature rat myotubues under normal circumstances. A progressive increase in the passive permeability of the membrane to K+ relative to Na+ ions has been observed which can account for the changes in Em with development. In contrast to the changes in the ionic selectivity of the membrane, we have found that the ionic selectivity of the ACh receptor of rat and chick myotubes remains constant during the same period of myotube development.

Threshold phenomena in chemoreception and taxis in slime mold Physarum polycephalum.

Abstract: The plasmodium of Physarum polycephalum reacts to various kinds of chemicals substances and moves towards or away from them. Threshold concentration of recognition of chemicals was examined in terms of membrane potential and of the averaged motive force of tactic movement by using a double-chamber method, i.e., a single plasmodium was placed between two compartments through a narrow ditch, and differences in membrane potential and in pressure between two compartments were measured. Results are summarized as follows: (a) By increasing the concentration of various substances in one compartment, the membrane potential started to change at a certain threshold concentration, C-th, for each chemical. Chemotactic movement of the plasmodium took place at the same threshold concentration. These results held both for attractants (glucose, galactose, phosphates, pyrophosphates, ATP, c-AMP, etc) and for repellents (various inorganic salts, sucrose, fructose, etc.). (b) The threshold concentration, Cth, for inorganic salts decreased remarkably with increase of the valences of cations, zeta, and was proportional to Z-6, I.E., THE Shultze-Hardy rule known in the field of colloid chemistry was found to be applicable. (c) The plasmodium distinguished the species of monovalent cations in the following order: H(Li(K(Na(Rb(Cs(NH-4 Plots of log Cth against the lyotropic number of anion fell on different straight lines for each monovalent cation species. (d) Plots of log Cth, against the reciprocal of the absolute tempe lines were almost the same and gave a value of 12 kcal/mol for the enthalpy change. These results suggest that the recognition of chemical substances appears as the result of a structural change of the membrane at the threshold point, and that the change in membrane structure is transmitted simultaneously to the motile system of the plasmodium.

Light-induced changes of sensitivity in Limulus ventral photoreceptors.

Abstract: The responses of Limulus ventral photoreceptors to brief test flashes and to longer adapting lights were measured under voltage clamp conditions. When the cell was dark adapted, there was a range of energy of the test flashes over which the peak amplitude of the responses (light-induced currents) was directly proportional to the flash energy. This was also true when test flashes were superposed on adapting stimuli but the proportionality constant (termed peak currently/photon) was reduced. The peak current/photon was attenuated more by brighter adapting stimuli than by less bright adapting stimuli. The peak current/photon is a measure of the sensitivity of the conductance-increase mechanism underlying the light response of the photo-receptor. The response elicited by an adapting stimulus had a large initial transient which declined to a smaller plateau. The peak current/photon decreased sharply during the declining phase of the transient and was relatively stable during the plateau. This indicates that the onset of light adaptation is delayed with respect to the onset of the response to the adapting stimulus. If the adaptational state just before the onset of each of a series of adapting stimuli was constant, the amplitude of the transient was a nearly linear function of intensity. When the total intensity was rapidly doubled (or halved) during a plateau response, the total current approximately doubled (or halved). We argue that the transition from transient to plateau, light-elicited changes of threshold, and the nonlinear function relating the plateau response to stimulus intensity all reflect changes of the responsiveness of the conductance-increase mechanism.

Olfactory receptor responses to sex pheromone components in the redbanded leafroller moth.

Abstract: Electrical responses of single olfactory receptor neurons of the male redbanded leafroller moth were elicited by each of the principle components of the sex pheromone and six other behaviorally active compounds. Response frequencies to equal intensities of each of these compounds and changes in response frequency with increasing amounts of any one compound, varied from receptor to receptor. These differences in response characteristics appear to be due to factors intrinsic to the olfactory recptor neuron and not to factors external to it. The encoding of odor quality by these receptor neurons cannot be in the simple presence or absence of activity in any one of them. Rather, odor quality may be encoded by the pattern of activity which invariably arises across an ensemble of receptor neurons, each having its own distribution of sensitivities.

Role of calcium in volume regulation by dog red blood cells.

Abstract: Dog red blood cells (RBC) are shown to regulate their volume in anisosmotic media. Extrusion of water from osmotically swollen cells requires external calcium and is associated with net outward sodium movement. Accumulation of water by osmotically shrunken cells is not calcium dependent and is associated with net sodium uptake. Net movements of calcium are influenced by several variables including cell volume, pH, medium sodium concentration, and cellular sodium concentration. Osmotic swelling of cells increases calcium permeability, and this effect is diminished at acid pH. Net calcium flux in either direction between cells and medium is facilitated when the sodium concentrations is low in the compartment from which calcium moves and/or high in the compartment to which calcium moves. The hypothesis is advanced that energy for active sodium extrusion in dog RBC comes from passive, inward flow of calcium through a countertransport mechanism.

Conductance increases produced by bath application of cholinergic agonists to Electrophorus electroplaques.

Abstract: When solutions containing agonists are applied to the innervated face of an Electrophorus electroplaque, the membrane's conductance increases. The agonist-induced conductance is increased at more negative membrane po- tentials. The "instantaneous" current-voltage curve for agonist-induced cur- rents is linear and shows a reversal potential near zero mV; chord conductances, calculated on the basis of this reversal potential, change e-fold for every 62-mV change in potential when the conductance is small. Conductance depends non- linearly on small agonist concentrations; at all potentials, the dose-response curve has a Hill coefficient of 1.45 for decamethonium (Deca) and 1.90 for carbamylcholine (Carb). With agonist concentrations greater than 10 -'a M Carb or 10 -5 M Deca, the conductance rises to a peak 0.5-1.5 rain after in- troduction of agonist, then declines with time; this effect resembles the "de- sensitization" reported for myoneural junctions. Elapid a-toxin, tubocurarine, and desensitization reduce the conductance without changing the effects of potential; the apparent dissociation constant for tubocurarine is 2 X 10 -7 M. By contrast, procaine effects a greater fractional inhibition of the conductance at high negative potentials.