Abstract: The dynamic characteristics of the rat gracilis anticus muscle at 17.5°C have been determined by isotonic and isometric loading. For a fixed initial length these characteristics were represented either as a family of length-velocity phase trajectories at various isotonic afterloads or as a series of force-velocity curves at different lengths. An alternate method of viewing these data, the length-external load-velocity phase space, was also generated. When the muscle was allowed to shorten from different initial lengths, the velocity of shortening achieved at a given length was lower for longer initial lengths. The amount of departure was also dependent upon the isotonic load, the greater the load the greater the departure. The departures were not caused by changes in the elastic elements of the muscle or fatigue in the ordinary sense. When the behavior of the muscle was investigated at different frequencies of stimulation, the shortening velocity was a function of the number of stimulating pulses received by the muscle at a given frequency. The shortening velocity of the rat gracilis anticus muscle is, therefore, not only a function of load and length, but also of an additional variable related to the time elapsed from onset of stimulation.

Abstract: Calcium appears to be an essential participant in axon excitation processes. Many other polyvalent metal ions have calcium-like actions on axons. We have used the voltage-clamped lobster giant axon to test the effect of several of these cations on the position of the peak initial (sodium) and steady-state (potassium) conductance vs. voltage curves on the voltage axis as well as on the rate parameters for excitation processes. Among the alkaline earth metals, Mg+2 is a very poor substitute for Ca+2, while Ba+2 behaves like "high calcium" when substituted for Ca+2 on a mole-for-mole basis. The transition metal ions, Ni+2, Co+2, and Cd+2 also act like high calcium when substituted mole-for-mole. Among the trivalent ions, La+3 is a very effective Ca+2 replacement. Al+3 and Fe+3 are extremely active and seem to have some similar effects. Al+3 is effective at concentrations as low as 10-5 M. The data suggest that many of these ions may interact with the same cation-binding sites on the axon membrane, and that the relative effects on the membrane conductance and rate parameters depend on the relative binding constants of the ions. The total amount of Na+ transferred during a large depolarizing transient is nearly independent of the kind or amount of polyvalent ion applied.

Abstract: The present studies are concerned with a detailed examination of the apparent anomalous osmotic behavior of human red cells. Red cell water has been shown to behave simultaneously as solvent water for nonelectrolytes and nonsolvent water, in part, for electrolytes. The nonsolvent properties are based upon assumptions inherent in the conventional van't Hoff equation. However, calculations according to the van't Hoff equation give osmotic volumes considerably in excess of total cell water when the pH is lowered beyond the isoelectric point for hemoglobin; hence the van't Hoff equation is inapplicable for the measurement of the solvent properties of the red cell. Furthermore, in vitro measurements of osmotic and other properties of 3.7 millimolal solutions of hemoglobin have failed to reveal the presence of any salt exclusion. A new hypothesis has been developed from thermodynamic principles alone, which predicts that, at constant pH, the net charge on the hemoglobin molecule decreases with increased hemoglobin concentration. The existence of such cooperative interaction may be inferred from the effect of pH on the changes in hemoglobin net charge as the spacing between the molecules decreases. The resultant movement of counterions across the cell membrane causes the apparent anomalous osmotic behavior. Quantitative agreement has been found between the anion shift predicted by the equation and that observed in response to osmotic gradients. The proposed mechanism appears to be operative in a variety of tissues and could provide an electrical transducer for osmotic signals.

Abstract: Fluid balance at the capillary level has been simulated with an analogue computer program, based on experimental data on regional differences in capillary permeability, surface areas, and hydrostatic pressures. The program takes into account fluid and protein fluxes into and out of the interstitial space. Solutions are obtained for tissue hydrostatic pressure, tissue fluid osmotic pressure, interstitial space volume, and lymph flow. Simulation of a variety of physiological experiments and clinical disease states has yielded reasonable agreement between experimental data and data obtained by computer analysis. Dilution of the interstitial plasma protein pool with a consequent reduc6tion of its oncotic pressure appears to be a major factor, which prevents edema unless plasma oncotic pressures are reduced by 10–15 mm Hg or, alternatively, venous pressures are elevated by a similar amount. The computer analysis in all instances yields positive values for tissue pressure, in agreement with experimental data obtained by needle puncture. The negative tissue pressures observed in subcutaneous capsules can be reproduced in the computer program, if the interface between the capsule and the surrounding interstitial space is assumed to have the properties of a semipermeable membrane.

Abstract: For some years, work in this laboratory has been directed toward elucidation of the mechanism of selectivity of high-potassium (HK) and low-potassium (LK) sheep red cell membranes for Na + and K + (I-4). One recent approach has been to compare the ionic permeability of the intact ceil membranes with thin artificial bilayer membranes prepared from lipids extracted from these cells (5). It was found that the thin lipid membranes have ionic permeability properties which are vastly different from those of intact HK and LK sheep red cell membranes. For example, the PC electrical resistance of the bilayers is about 2 X 10 s ohm cm 2, while that of the red cell membranes is estimated to be from 10 to 100 ohm cm 2. Furthermore, the bilayers are more permeable to both Na + and K + than they are to Cl-, while the ratio of Clto K + or Na + permeability of red cells is of the order of 106 (6). Finally, the thin lipid membranes do not distinguish between Na + and K +, and the ionic transport properties of such membranes prepared from HE and LK sheep red cell lipids are identical. The intact cell membranes do distinguish between these alkali metal ions, and, furthermore, the Na + and K + transport properties of the two genetic types of sheep red cells are significantly different. Apparently, the thin lipid membranes lack essential components which are responsible for the distinctive cation permeability properties of the intact cell membranes. In the course of continuing attempts to identify such components, we became aware of reports of the effect of the macrocyclic depsipeptide antibiotic, valinomycin, on the ionic permeabil i ty of lecithin (7) and mixed brain lipid (8) bilayer membranes. This compound had previously been reported to stimulate the respiration of mitochondria in the presence of K + but not in the presence of Na + (9). Valinomycin was found to produce specific permeabili ty of intact H K and L K sheep red cell membranes, as well as thin artificial membranes prepared from lipids extracted from these cells, to K + but not to Na + ( 10, 11). In the presence of K + ( 10 t M) and valinomycin (5 X 10 -~ M), the electrical resistance of the lipid bilayer membranes is about 103 ohm cm ~, which is in the range of biological membranes. The significance of these observations derives from the unusual structure of valino-

Abstract: Xylocaine and its derivatives act specifically at the neuromuscular junction within the concentration range 0.05 to 2.0 mM. The charged form is the active form of the drugs. There is no correlation between "local anesthetic" activity and effect at the junction. Like d-tubocurarine, these drugs have little or no effect on quantum content, acetylcholinesterase activity, or the passive impedance of the muscle fiber. Yet they produce end plate potentials characterized by a brief, early component and a late, greatly prolonged component, as does procaine. Analysis of these changes in time course suggests that the drugs have little or no effect before receptors are activated by acetylcholine, but cause a decreased and often greatly prolonged response. Clear structure-activity relations indicate that the receptor to which the drugs bind to produce the prolonged response can be the receptor for acetylcholine. Comparison of the effects of the drugs on the end plate potential and on the response to iontophoretically applied acetylcholine also shows that the effects of Xylocaine depend on the time course of receptor activation and are quite different from the effects of d-tubocurarine.

Abstract: The voltage clamp technique (Cole, 1949; Hodgkin, Huxley, and Katz, 1952) has been applied successfully to the analysis of ionic currents in a wide variety of excitable ceils. I t has proved difficult to apply this technique to striated muscle, both because it is hard to isolate a small area of membrane electrically and because the fiber contracts when depolarized. Takeuchi and Takeuchi (1959) applied voltage feedback at the end plate region of striated muscle fibers. Their method of controlling the voltage at a point produced an elegant method for measuring the currents of the muscle membrane at the end plate. Previous attempts to control the voltage of an area of membrane have been made by Adrian and Freygang (1962 b) and by Frankenhaeuser, Lindley and Smith (1965). In the present experiments (Adrian, Chandler, and Hodgkin, 1966) we have tried to control the voltage over a short length of a striated muscle fiber, by a technique which uses an internal microelectrode to deliver current and two additional microelectrodes to measure membrane current and voltage. The three electrodes are inserted near the pelvic end of a muscle fiber, in a frog sartorius muscle. Fig. 1 shows the positions of the microelectrodes and the arrangement of the feedback amplifier. The current through electrode 3 is controlled by feedback to give steady voltages or steps of voltage at electrode 1. The membrane current density at 1 is estimated by the difference in potential between electrodes 1 and 2. As a simple approximation, this potential difference can be considered to be produced by a longitudinal current flowing along the internal resistance between 1 and 2. The same current crosses the membrane between the end of the fiber and a point 3l/2 from the end of the fiber. On this basis the membrane current per unit length would be given by

Abstract: The duration of ciliary reversal of Paramecium caudatum in response to changes in external ionic factors was determined with various ionic compositions of both equilibration and stimulation media. The reversal response was found to occur when calcium ions bound by an inferred cellular cation exchange system were liberated in exchange for externally applied cations other than calcium. Factors which affect the duration of the response were (a) initial amount of calcium bound by the cation exchange system, (b) final amount of calcium bound by the system after equilibration with the stimulation medium, and (c) concentration of calcium ions in the stimulation medium. An empirical equation is presented which relates the duration of the response to these three factors. On the basis of these and previously published data, the following hypothesis is proposed for the mechanism underlying ciliary reversal in response to cationic stimulation: Ca(++) liberated from the cellular cation exchange system activates a contractile system which is energized by ATP. Contraction of this component results in the reversal of effective beat direction of cilia by a mechanism not yet understood. The duration of reversal in live paramecia is related to the time course of bound calcium release.

Abstract: Optically black, thin lipid membranes prepared from sheep erythrocyte lipids have a high dc resistance (Rm ≅ 108 ohm-cm2) when the bathing solutions contain NaCl or KCl. The ionic transference numbers (Ti) indicate that these membranes are cation-selective (TNa ≅ 0.85; TCl ≅ 0.15). These electrical properties are independent of the cholesterol content of the lipid solutions from which the membranes are formed. Nystatin, and probably amphotericin B, are cyclic polyene antibiotics containing ≈36 ring atoms and a free amino and carboxyl group. When the lipid solutions used to form membranes contained equimolar amounts of cholesterol and phospholipid, these antibiotics reduced Rm to ≈102 ohm-cm2; concomitantly, TCl became ≅0.92. The slope of the line relating log Rm and log antibiotic concentration was ≅4.5. Neither nystatin (2 x 10-5 M) nor amphotericin B (2 x 10-7 M) had any effect on membrane stability. The antibiotics had no effect on Rm or membrane permselectivity when the lipids used to form membranes were cholesterol-depleted. Filipin (10-5 M), an uncharged polyene with 28 ring atoms, produced striking membrane instability, but did not affect Rm or membrane ionic selectivity. These data suggest that amphotericin B or nystatin may interact with membrane-bound sterols to produce multimolecular complexes which greatly enhance the permeability of such membranes for anions (Cl-, acetate), and, to a lesser degree, cations (Na+, K+, Li+).

Abstract: Sodium pentobarbital and sodium thiopental decrease both the peak initial (Na) and late steady-state (K) currents and reduce the maximum sodium and potassium conductance increases in voltage-clamped lobster giant axons. These barbiturates also slow the rate at which the sodium conductance turns on, and shift the normalized sodium conductance vs. voltage curves in the direction of depolarization along the voltage axis. Since pentobarbital (pKa = 8.0) blocks the action potential more effectively at pH 8.5 than at pH 6.7, the anionic form of the drug appears to be active. The data suggest that these drugs affect the axon membrane directly, rather than secondarily through effects on intermediary metabolism. It is suggested that penetration of the lipid layer of the membrane by the nonpolar portion of the barbiturate molecules may cause the decrease in membrane conductances, while electrostatic interactions involving the anionic group on the barbiturate, divalent cations, and "fixed charges" in the membrane could account for the slowing of the rate of sodium conductance turn-on and the shift of the normalized conductance curves along the voltage axis.

Abstract: Skeletal muscle myofibrils, in the presence of 2 mM MgCl2 at pH 7.0, were found to have two classes of calcium-binding sites with apparent affinity constants of 2.1 x 106 M-1 (class 1) and ∼3 x 104 M-1 (class 2), respectively. At free calcium concentrations essential for the activation of myofibrillar contraction (∼10-6 M) there would be significant calcium binding only to the class 1 sites. These sites could bind about 1.3 µmoles of calcium per g protein. Extraction of myosin from the myofibrils did not alter their calcium-binding parameters. Myosin A, under identical experimental conditions, had little affinity for calcium. The class 1 sites are, therefore, presumed to be located in the I filaments. The class 1 sites could only be detected in F actin and myosin B preparations which were contaminated with the tropomyosin-troponin complex. Tropomyosin bound very little calcium. Troponin, which in conjunction with tropomyosin confers calcium sensitivity on actomyosin systems, could bind 22 µmoles of calcium per g protein with an apparent affinity constant of 2.4 x 106 M-1. In view of the identical affinity constants of the myofibrils and troponin and the much greater number of calcium-binding sites on troponin it is suggested that calcium activates myofibrillar contraction by binding to the troponin molecule.

Abstract: The voltage dependence of the voltage clamp responses of myelinated nerve fibers depends on the concentration of divalent cations and of hydrogen ions in the bathing medium. In general, increases of the [Ca], [Ni], or [H] increase the depolarization needed to elicit a given response of the nerve. An e -fold increase of the [Ca] produces the following shifts of the voltage dependence of the parameters in the Hodgkin-Huxley model: m ∞, 8.7 mv; h ∞, 6.5 mv; τ n , 0.0 mv. The same increase of the [H], if done below pH 5.5, produces the following shifts: m ∞, 13.5 mv; h ∞, 13.5 mv; τ n , 13.5 mv; and if done above pH 5.5: m ∞, 1.3 mv; h ∞, 1.3 mv; τ n , 4.0 mv. The voltage shifts are proportional to the logarithm of the concentration of the divalent ions and of the hydrogen ion. The observed voltage shifts are interpreted as evidence for negative fixed charges near the sodium and potassium channels. The charged groups are assumed to comprise several types, of varying affinity for divalent and hydrogen ions. The charges near the sodium channels differ from those near the potassium channels. As the pH is lowered below pH 6, the maximum sodium conductance decreases quickly and reversibly in a manner that suggests that the protonation of an acidic group with a pK a of 5.2 blocks individual sodium channels.

Abstract: The mechanical responses (active and resting tension, dP/dt, TPT) and ionic exchange characteristics (Ca++, K+, Na+) which follow upon a variation in temperature, rate, and [K+]0 were studied in the rabbit papillary muscle and arterially perfused rabbit interventricular setpum. Abrupt changes in temperature provided a means of separating the contributions of rate of development (intensity) of active state and duration of active state to total active tension development (approximated by isometric tension). Threefold changes in duration of active state with proportional changes in active tension can be induced without evidence for alteration of Ca++, K+, or Na+ exchange. Abrupt cooling produced a moderate (∼15%) increase of dP/dt which suggests an augmentation of active state intensity. Evidence is presented to suggest that this increase of dP/dt is based upon an increase in membrane Ca++ concentration which occurs secondary to inhibition of active Na+ transport. The alterations in ionic exchange and active state produced by variation of temperature are discussed in terms of a five-component control system.

Abstract: Calcium retained at binding sites of the sarcoplasmic reticulum membranes isolated from rabbit skeletal muscle requires 10-5 - 10-4 M ATP to exchange with 45Ca added to the medium. The ATP requirement for Ca exchangeability was observed with respect to the "intrinsic" Ca of the reticulum membranes and the fraction of Ca that is "actively" bound in the presence of ATP. Furthermore, a concentration of free Ca in the medium higher than 10-8 M is required for ATP to promote Ca exchangeability. This exchangeability is not influenced by caffeine, quinine, procaine, and tetracaine, and Ca that is either nonexchangeable (in the absence of ATP) or exchangeable (in the presence of ATP) is released by 1–5 mM quinine or tetracaine, but neither caffeine (6 mM) nor procaine (2–5 mM) has this effect. Quinine or tetracaine also releases Ca and Mg bound passively to the reticulum membranes. A possible role of ATP in maintaining the integrity of cellular membranes is discussed, and the effects of caffeine, quinine, and of local anesthetics on the binding of Ca by the isolated reticulum are related to the effects of these agents on 45Ca fluxes and on the twitch output observed in whole muscles.

Abstract: A kinetic scheme postulating the rapid formation of a partially active acetylcholine-receptor-drug complex from Xylocaine (or a derivative) and the active acetylcholine-receptor complex can account for the effects of Xylocaine and its derivatives at the neuromuscular junction. Transmembrane currents generated by an analogue computer programmed according to the scheme can exactly match end plate currents produced by nerve stimulation in the presence of the drugs. The scheme also accounts for the qualitatively different effects of the drugs on the end plate potential and on responses to iontophoretically applied acetylcholine. The analysis presented is consistent with very rapid reactions between acetylcholine and receptors, characterized by rate coefficients in the range 104 to 106 sec-1. It is based on the hypothesis that the activation of receptors by acetylcholine changes the structure of the receptors and thus their affinity for Xylocaine. The analysis does not require pharmacological separability of sodium and potassium conductances during the end plate current.

Abstract: Net uptake of potassium by low K, high Na cells of Neurospora at pH 58 is accompanied by net extrusion of sodium and hydrogen ions The amount of potassium taken up by the cells is matched by the sum of sodium and hydrogen ions lost, under a variety of conditions: prolonged preincubation, partial respiratory inhibition (DNP), and lowered [K]o All three fluxes are exponential with time and obey Michaelis kinetics as functions of [K]o The Vmax for net potassium uptake, 227 mmoles/kg cell water/min, is very close to that for K/K exchange reported previously (20 mmoles/kg cell water/min) However, the apparent Km for net potassium uptake, 118 mM [K]o, is an order of magnitude larger than the value (1 mM) for K/K exchange It is suggested that a single transport system handles both net K uptake and K/K exchange, but that the affinity of the external site for potassium is influenced by the species of ion being extruded

Abstract: Membrane potentials were recorded in isolated segments of interscapular brown adipose tissue from rats. After equilibration at 29°C in Krebs-Ringer bicarbonate buffer a mean value of -51 ± 4 mv (SD) was found. This level could be maintained for up to 5 hr. The mean effective membrane resistance was 1.35 ± 0.45 megohm. The membrane potential was a function of the extracellular potassium concentration. Ouabain (10-6-10-3 M) and incubation in K-free buffer produced progressive depolarization. Epinephrine and norepinephrine in concentrations as low as 10-8 g/ml produced a prompt depolarization. Cooling of the tissue and lowering of the oxygen tension caused a marked and reversible decrease in the membrane potential. In tissue obtained from cold-adapted rats, the membrane potential was considerably diminished. 6Assuming that the membrane potential is some function of the Na permeability of the plasma membrane it is suggested that an increase in the rate of active Na-K transport and ensuing ADP formation might contribute to the increase in respiration seen during exposure to thermogenic stimuli.

Abstract: Quinidine potentiates twitch tension and (at higher concentrations) causes contracture of skeletal muscle whereas the same drug reduces tension development of cardiac muscle. To gain insight into the possible differences in the excitation-contraction coupling mechanism of the two types of muscle the effect of quinidine on calcium accumulation by isolated sarcoplasmic reticulum from skeletal and cardiac muscle was investigated. In a medium containing ATP, Mg++, oxalate, and 45Ca, pharmacologically active concentrations of the drug inhibited calcium accumulation by both skeletal and cardiac sarcoplasmic reticulum. The inhibition of the rates of calcium, uptake by the skeletal muscle preparation ranged from 11% with 10-4M quinidine to 90% with 10-3 M quinidine. With the cardiac muscle preparation the inhibition ranged from 16% with 3 x 10-6 M quinidine to 100% with 10-3 M quinidine. With both preparations the inhibition of calcium transport was accompanied by an inhibition of the Ca++-activated ATPase activity of the sarcoplasmic reticulum. The effect of quinidine on the skeletal sarcoplasmic reticulum supports the hypothesis that this compound produces twitch potentiation and contracture by interfering with intracellular calcium, sequestration. Its effect on cardiac sarcoplasmic reticulum. has been interpreted in terms of the hypothesis that cardiac contractility is a function of the amount of calcium released from the sarcoplasmic reticulum which is in turn dependent upon the absolute calcium content of the reticulum. Hence, following inhibition of calcium transport there would be less calcium available for coupling.

Abstract: Mediated (nonactive) transport of glucose in mammalian cells is characterized by saturation kinetics, stereospecificity, sensitivity to inhibition by phlorizin and certain sulfhydryl-blocking agents, a temperature coefficient of about 2, an inability to utilize metabolic energy, and countertransport. Countertransport can be explained by the development of carrier gradients in the cell membrane and provides the best evidence for carrier mobility. Efforts to identify and isolate chemical components of the transport system, have not been successful. Transport among different types of mammalian cells shows a wide range of activities (Vmax values differ by three or more orders of magnitude) and different sensitivities to hormones. Glucose enters the liver cell by mediated transport, as shown by a difference in the penetration rates of D- and L-glucose and sensitivity to phlorizin. The activity of the system is one of the highest known. Transport in muscle is the most important rate-controlling step for glucose utilization and is strongly accelerated by hypoxia, work, and insulin. The effect of work or insulin is strongly inhibited by metabolism, of fatty acids. Insulin also stimulates glucose transport in adipose tissue. Using isolated fat cells, it could be shown that insulin is rapidly bound to sites on the cell surface. The effect is lost within a few minutes after the exogenous hormone is removed. The bound insulin is not released as such, but is metabolized to unknown products. Binding is prevented by preexposure of cells to maleimide, which presumably blocks certain sulfhydryl groups at or near the insulin-binding site. Pretreatment with insulin protects against maleimide. Digestion of the cell with trypsin eliminates the acceleration of glucose transport and the inhibition of lipolysis by insulin. The glucose transport and adenyl cyclase systems are not grossly affected by trypsin, indicating that the insulin effector system is a separate entity.

Abstract: Richards and his group showed, about 40 yr ago, that the blood is filtered in the glomerulus. The largest portion of the water and solutes of the filtered fluid is absorbed across the proximal tubular wall, toward the peritubular blood capillaries (see 17). Thus, proximal tubular absorption is a process of paramount importance in the maintenance of homeostasis (1, 16). I shall review here studies on the way in which this absorptive process is accomplished and refer to new experiments on the nature of the sodium pump in kidney proximal tubular cells.

Abstract: Responses of the labellar sugar receptor of the fleshfly, Boettcherisca peregrina, were studied over a wide range of concentrations of several sugars (sucrose, maltose, glucose, fructose, and mannose) in single solutions and in mixtures. The results suggest (a) that the receptor sites are not completely differentiated for glucose and for fructose combination, (b) that the receptor site is composed of two subunits. Such suggestions are based on the classical model, where the response is proportional to the number of the sites, two subunits of each site being simultaneously occupied with one molecule of disaccharides or two molecules of monosaccharides. It is shown, however, that an allosteric model gives a somewhat better interpretation of the experimental results.

Abstract: The exchange of substances between higher organisms and the environment takes place at the level of epithelial membranes constituted by one or more layers of cells. Experimental models (17, 31, 50) and theoretical analysis (13, 45) generally assume that any molecule going across an epithelial membrane must traverse at least two plasma membranes: one to get into the epithelial cells and another to get out at the opposite side of the cell. Models constructed on this basis have been helpful in understanding how the intestine absorbs amino acids, how urine is formed in the kidney, or how the gall bladder concentrates the bile. However, it is increasingly difficult to reconcile the predictions of those models with the information obtained in the last few years. At the center of the problem is the remarkable ability of the epithelial membranes to transport Na + (Huf, reference 25) and the coupling of this phenomenon to the movement of almost any substance capable of undergoing net transfer in absence of a difference of electrochemical potential gradient (7, 10, 48). Studies carried out in our laboratory on the movement and distribution of Na in the frog skin led us to develop a working model, and it is our purpose to present here some of its features. Our discussion will be divided into two parts. In the first one we will analyze some of the properties of the transcellular models and show that they are at variance with certain experimental observations and even teleological expectancies. In the second we will present our model, discuss part of the experimental justification and try to explain from our point of view some very well-known phenomena that are hardly understood on the basis of the transcellular models.

Abstract: The relationship between cellular calcium (Ca) stores and isometric contractile force was investigated in isolated, gas-perfused cat hearts. The hearts were preperfused for approximately 5 min with substrate-free Krebs solution modified to contain 0, 1.25, 2.5, 5.0 or 10.0 mEq/liter Ca, gas-perfused for up to 120 min, and then perfused with zero-Ca Krebs solution. Contractile force and [Ca] in the effluent fluid were measured. Our results indicate that: (a) The washout of Ca was characteristic of a three compartment system, (b) A Ca compartment directly associated with muscle contraction was identified by correlation of the rate of Ca washout with the rate of decay of contractility (r = 0.79). (c) Ca content in the compartment was correlated with contractile force (r = 0.77). (d) Contractile force and the Ca content of the compartment which was correlated with contractility approached a steady state during 120 min of gas perfusion.

Abstract: The paucimolecular unit membrane model of the structure of the plasma membrane is critically reviewed in relation to current knowledge of the chemical and enzymatic composition of isolated plasma membranes, the properties of phospholipids, the chemistry of fixation for electron microscopy, the conformation of membrane proteins, the nature of the lipid-protein bonds in membranes, and possible mechanisms of transmembrane transport and membrane biosynthesis. It is concluded that the classical models, although not disproven, are not well supported by, and are difficult to reconcile with, the data now available. On the other hand, although a model based on lipoprotein subunits is, from a biochemical perspective, an attractive alternative, it too is far from proven. Many of the questions may be resolved by studies of membrane function and membrane biosynthesis rather than by a direct attack on membrane structure.

Abstract: "Low sodium" muscles were prepared which contained around 5 mmoles/kg fiber of intracellular sodium. "High sodium" muscles containing between 15 and 30 mmoles/kg fiber of intracellular sodium were also prepared. In low sodium muscles application of 10-5 M strophanthidin reduced potassium influx by about 5%. Potassium efflux was unaffected by strophanthidin under these conditions. In high sodium muscles, 10-5 M strophanthidin reduced potassium influx by 45% and increased potassium efflux by 70%, on the average. In low sodium muscles sodium efflux was reduced by 25% during application of 10-5 M strophanthidin while in high sodium muscles similarly treated, sodium efflux was reduced by about 60%. Low sodium muscles showed a large reduction in sodium efflux when sodium ions in the Ringer solution were replaced by lithium ions. The average reduction in sodium efflux was 4.5-fold. Of the amount of sodium efflux remaining in lithium. Ringer's solution, 40% could be inhibited by application of 10-5 M strophanthidin. The total sodium efflux from low sodium muscles exposed to Ringer's solution in which lithium had been substituted for sodium ions for a period of 1 hr can be fractionated as 78% Na-for-Na interchange, 10% strophanthidin-sensitive sodium pump, and 12% residual sodium efflux. It is concluded that large strophanthidin-sensitive components of sodium and potassium flux can be expected only at elevated sodium concentrations within the muscle cells.

Abstract: The total osmotic flow of water across cell membranes generally exceeds diffusional flow measured with labeled water. The ratio of osmotic to diffusional flow has been widely used as a basis for the calculation of the radius of pores in the membrane, assuming Poiseuille flow of water through the pores. An important assumption underlying this calculation is that both osmotic and diffusional flow are rate-limited by the same barrier in the membrane. Studies employing a complex synthetic membrane show, however, that osmotic flow can be limited by one barrier (thin, dense barrier), and the rate of diffusion of isotopic water by a second (thick, porous) barrier in series with the first. Calculation of a pore radius is meaningless under these conditions, greatly overestimating the size of the pores determining osmotic flow. On the basis of these results, the estimation of pore radius in biological membranes is reassessed. It is proposed that vasopressin acts by greatly increasing the rate of diffusion of water across an outer barrier of the membrane, with little or no accompanying increase in pore size.

Abstract: Rana pipiens eggs dividing normally in diluted Ringer's solution show an increase in transmembrane potential inside negative, a decrease in resistance, and no change in total surface membrane capacitance at the appearance of a division furrow. Furrows of eggs in solutions with the tonicity of full Ringer develop partially, then regress so that the surface is again spherical. The potential and resistance changes are greater and substantial increases in capacitance occur when furrowing is so inhibited. It is proposed that the electrical changes at division are due to the introduction of new plasma membrane, between the blastomeres, having selective permeability to K and a low resistance compared to the outer spherical membrane. A narrow gap between blastomeres limits current flow through new membrane during normal division. A direct exposure of new membrane to the bathing medium when furrowing is disrupted results in larger changes in potential and resistance and permits the capacitance of new membrane to be detected.

Abstract: Sartorius muscle cells from the frog were stored in a K-free Ringer solution at 3°C until their average sodium contents rose to around 23 mM/kg fiber (about 40 mM/liter fiber water). Such muscles, when placed in Ringer's solution containing 60 mM LiCl and 50 mM NaCl at 20°C, extruded 9.8 mM/kg of sodium and gained an equivalent quantity of lithium in a 2 hr period. The presence of 10-5 M strophanthidin in the 60 mM LiCl/50 mM NaCl Ringer solution prevented the net extrusion of sodium from the muscles. Lithium ions were found to enter muscles with a lowered internal sodium concentration at a rate about half that for entry into sodium-enriched muscles. When sodium-enriched muscles labeled with radioactive sodium ions were transferred from Ringer's solution to a sodium-free lithium-substituted Ringer solution, an increase in the rate of tracer sodium output was observed. When the lithium-substituted Ringer solution contained 10-5 M strophanthidin, a large decrease in the rate of tracer sodium output was observed upon transferring labeled sodium-enriched muscles from Ringer's solution to the sodium-free medium. It is concluded that lithium ions have a direct stimulating action on the sodium pump in skeletal muscle cells and that a significantly large external sodium-dependent component of sodium efflux is present in muscles with an elevated sodium content. In the sodium-rich muscles, about 23% of the total sodium efflux was due to strophanthidin-insensitive Na-for-Na interchange, about 67% being due to strophanthidin-sensitive sodium pumping.

Abstract: The relation between oxygen consumption and motility of Ciona spermatozoa has been measured by using pH stats to measure the acid production of spermatozoa swimming in dilute suspensions where their motility can be analyzed accurately, and calibrating the acid production by measuring it simultaneously with measurements of oxygen consumption, using more concentrated sperm suspensions. When the motility of the spermatozoa is inhibited by thiourea or by increased viscosity, their oxygen consumption decreases in proportion to the decrease in beat frequency. 80–85 % of their oxygen consumption appears to be tightly coupled to motility. The amount of movement-coupled oxidative metabolism per beat remains nearly constant, even when there are significant changes in the energy required per beat for movement against the viscous resistance of the medium. This implies that under these conditions, where the radius of curvature of flagellar bending remains constant, the amount of ATP used is determined by a stoichiometric relation to bending rather than by the energy requirement. The movement-coupled oxidative metabolism appears to be sufficient to generate approximately two molecules of ATP per beat for each molecule of the flagellar ATPase, dynein.