Effect of temperature on membrane potential and ionic fluxes in intact and dialysed barnacle muscle fibres

Comparison of Species and Cell-Type Differences in Fraction Unbound of Liver Tissues, Hepatocytes, and Cell Lines

Fraction unbound (f_u) of liver tissue, hepatocytes, and other cell types is an essential parameter used to estimate unbound liver drug concentration and intracellular free drug concentration. f_u,liver and f_u,cell are frequently measured in multiple species and cell types in drug discovery and development for various applications. A comparison study of 12 matrices for f_u,liver and f_u,cell of hepatocytes in five different species (mouse, rat, dog, monkey, and human), as well as f_u,cell of Huh7 and human embryonic kidney 293 cell lines, was conducted for 22 structurally diverse compounds with the equilibrium dialysis method. Using an average bioequivalence approach, our results show that the average difference in binding to liver tissue, hepatocytes, or different cell types was within 2-fold of that of the rat f_u,liver Therefore, we recommend using rat f_u,liver as a surrogate for liver binding in other species and cell types in drug discovery. This strategy offers the potential to simplify binding studies and reduce cost, thereby enabling a more effective and practical determination of f_u for liver tissues, hepatocytes, and other cell types. In addition, f_u under hepatocyte stability incubation conditions should not be confused with f_u,cell, as one is a diluted f_u and the other is an undiluted f_u Cell density also plays a critical role in the accurate measurement of f_u,cell.

Temperature sensitivity of the pyloric neuromuscular system and its modulation by dopamine

We report here the effects of temperature on the p1 neuromuscular system of the stomatogastric system of the lobster (Panulirus interruptus). Muscle force generation, in response to both the spontaneously rhythmic in vitro pyloric network neural activity and direct, controlled motor nerve stimulation, dramatically decreased as temperature increased, sufficiently that stomach movements would very unlikely be maintained at warm temperatures. However, animals fed in warm tanks showed statistically identical food digestion to those in cold tanks. Applying dopamine, a circulating hormone in crustacea, increased muscle force production at all temperatures and abolished neuromuscular system temperature dependence. Modulation may thus exist not only to increase the diversity of produced behaviors, but also to maintain individual behaviors when environmental conditions (such as temperature) vary.

Effect of membrane properties on skeletal muscle fiber excitability: a sensitivity analysis

In this study, the sensitivity of skeletal muscle fiber excitability to changes in temperature and a range of geometrical, electrical and ionic membrane properties was examined using model simulation. A mathematical model of the propagating muscle fiber action potential (AP) was used to simulate muscle fiber APs while changing individual muscle fiber parameters in isolation to examine how they affect muscle fiber AP amplitude, shape and conduction velocity (CV). The behavior of the model was verified by comparison with previously reported experimental data from both in vivo studies conducted at physiological temperatures and in vitro and in silico studies conducted at lower temperatures. The simulation results presented demonstrate the sensitivity of AP amplitude, shape and CV and, therefore, muscle fiber excitability to small changes in a wide range of different muscle fiber parameters. Furthermore, they demonstrate the potential of computational modeling as a tool for investigating the underlying mechanisms of complex phenomena such as those which govern skeletal muscle excitation.

Mechanisms of cell survival in hypoxia and hypothermia

Most animals experience some degree of hypoxia and hypothermia during the course of their natural life history either as a consequence of ambient ‘exposure’ per se or through metabolic, respiratory and/or circulatory insufficiency. A prevailing experimental approach has been to probe tissues from natural models of hypoxia-tolerant and cold-tolerant vertebrates to look for common mechanisms of defence against O2 lack and hypothermia. The ability to sustain vital cellular functions in severe cases of either condition varies widely amongst the vertebrates. Like humans, the vast majority of mammals are unable to survive prolonged periods of hypothermia or O2 deprivation owing to irreversible membrane damage and loss of cellular ion homeostasis in vital organs such as the brain and heart. However, numerous hibernating endotherms, neonatal and diving mammals as well as many ectotherms can tolerate prolonged periods that would, in clinical terms, be called asphyxia or deep hypothermia. The key to their survival under such conditions lies in an inherent ability to downregulate their cellular metabolic rate to new hypometabolic steady states in a way that balances the ATP demand and ATP supply pathways.

Effect of hypothermia on the volume of rat glial cells

1. The cell volume of suspended C6 glioma cells and primary cultured rat astrocytes was measured at normothermia (37 degrees C), and at mild (32 degrees C) and moderate (27 degrees C) hypothermia by flow cytometry with electrical cell sizing. 2. Under control conditions (37 degrees C), C6 glioma cells had a volume of 809 +/- 29 microm3. Moderate hypothermia (27 degrees C) led to rapid cell swelling, with a maximum volume of 113.1 +/- 1.3 % of control being achieved after 50 min. After rewarming to 37 degrees C, cell volume recovered very slowly and incompletely (to 107.2 +/- 0.4 % of control). Less severe hypothermia (32 degrees C) led to a smaller increase in cell volume (108.7 +/- 0.5 % of control). 3. The maximal cell swelling response and the kinetics of swelling were similar in C6 glioma cells and primary cultured astrocytes. 4. Hypothermia-induced cell swelling was dependent on the presence of extracellular Na+ and was reduced by the Na+-H+ antiporter inhibitor EIPA. 5. The underlying mechanisms of hypothermia-induced cell swelling are an intracellular accumulation of Na+ by (1) differential effects of hypothermia on the membrane permeabilities of Na+ and K+ and (2) activation of the Na+-H+ antiporter by a shift of its activation curve to a more alkaline value.

Changes in membrane potential associated with cell swelling and regulatory volume decrease in barnacle muscle cells

Our aim was to test the effect of hypotonicity and extracellular Ca2+ (Cao) on cell volume and membrane potential (VM) in barnacle muscle cells. Under isotonic conditions the resting VM of isolated cells mounted in the experimental chamber exposed to either Ca(2+)-free or Ca(2+)-containing (11 mM) solutions was -46.3 +/- 1.0 mV (n = 24) and -56.2 +/- 0.9 mV (n = 38), respectively. In the absence of Cao, the cells depolarized at a rate of 2.3 +/- 0.47 mV/hr; the presence of Cao reduced this rate of depolarization by 2.9-fold. Both in the absence or presence of Cao, the cells swelled in response to hypotonicity but underwent regulatory volume decrease (RVD) when Cao was present. Addition of the Ca2+ channel blocker, verapamil (0.1 mM), inhibited the Cao-dependent RVD. The percentage of cells responding with RVD increased with larger hypotonic challenges. There was a Cao-independent direct relationship between cell swelling and membrane depolarization which can be explained by dilution of the concentration of intracellular K+ ([K+]i). RVD was accompanied by a small hyperpolarization (3.0 +/- 0.38 mV/2 hr) which may represent increases in [K+]i during cell shrinking and activation of a conductive pathway. The results indicate the following: (1) the presence of Cao stabilizes VM; (2) cell swelling produces a depolarization which can be explained by dilution of [K+]i; (3) cell swelling activates a verapamil-sensitive Ca2+ influx responsible for promoting RVD; and (4) RVD is accompanied by a hyperpolarization which may result from activation of a conductive pathway.

Na/Ca exchange in barnacle muscle cells has a stoichiometry of 3 Na+/1 Ca2+

The portions of the 45Ca influx and 22Na efflux that were activated by physiological concentrations of intracellular free Ca2+, [Ca2+]i, were studied in internally perfused single giant barnacle muscle cells. Since both fluxes were activated by intracellular Ca2+ (Cai) and the Ca influx was dependent on internal Na+ (Nai), the fluxes appear to be coupled (Na/Ca exchange). Tracer Ca/Ca and Na/Na exchanges were eliminated by employing tris(hydroxymethyl)aminomethane (Tris) as the predominant external cation. Under these circumstances, the ratio of the external Ca2+ (Cao)-dependent, Cai-activated Na+ efflux to the Nai-dependent, Cai-activated Ca influx was 3.1-3.2 Na+/1 Ca2+, when the intracellular Na+ concentration, [Na+]i was either 30 or 46 mM. This is the first direct measurement of the Na/Ca exchange stoichiometry. In many types of cells, the Na/Ca exchange system appears to operate in parallel with a plasma membrane ATP-driven Ca pump that has a lower capacity (maximum velocity), but higher affinity for Ca2+ than the Na/Ca exchanger. The data on the stoichiometry and activation by internal Ca2+ imply that the turnover of the Na/Ca exchanger is modulated during periods of cell activity. When the cells are depolarized, the Na/Ca exchange system is activated by the rising [Ca2+]i, and Ca2+ entry via the exchanger is promoted. Then, at repolarization, Ca2+ exits rapidly, primarily via the exchanger. However, in resting cells, with a low [Ca2+]i, much (but not all) of the Ca2+ efflux is probably mediated by the ATP-driven Ca pump.

Effects of calcium on membrane potential and sodium influx in barnacle muscle fibers

The influence of internal and external Ca2+ on membrane potential and 22Na influx were tested in internally perfused giant barnacle muscle fibers. The fibers depolarized by about 2-3 mV, and Na+ influx increased when external Ca2+ was removed. These effects were inhibited and reversed by adding 2 mM La3+ externally but not by tetrodotoxin (TTX). Ca2+ channel blockers did not prevent the depolarization. Increasing internal free Ca2+ ([Ca2+]i) from 10(-7) to 10(-5) M also stimulated Na+ influx and depolarized the fibers by a few millivolts. Neither external La3+ nor TTX prevented the effects of raising [Ca2+]i; however, internal tetrabutylammonium ions depolarized the fibers and attenuated the internal Ca2+-dependent effects. These data are consistent with the idea that removal of external Ca2+ activates a La3+-sensitive channel that is permeable to Na+; raising [Ca2+]i activates a La2+-insensitive, Na+-permeable channel that may be similar to the internal Ca2+-activated nonselective cation channels observed in cardiac muscle. The results demonstrate that all Na+ (and Ca2+) fluxes that do not contribute to Na-Ca exchange must be carefully identified before the exchange stoichiometry can be determined from Na+ and Ca2+ flux measurements.

Properties of chloride transport in barnacle muscle fibers

Unidirectional chloride-36 fluxes were measured in internally dialyzed barnacle giant muscle fibers. About 50--60% of the Cl efflux was irreversibly blocked by the amino-group reactive agent, 4-acetamido-4'-isothiocyano-stilbene-2,2'-disulfonic acid (SITS), when it was applied either intra- or extracellularly. Similarly, Cl influx was also blocked by SITS. No significant effect on [Cl]i of SITS was noted in intact muscle fibers. However, the rate of net Cl efflux from muscle fibers which were Cl-loaded by overnight storage at 6 degrees C could be slowed by SITS treatment. Two classes of anions were defined based upon their effects on Cl efflux. Methanesulfonate and nitrate inhibited Cl efflux either when they replaced external chloride or when they were added to a constant external chloride concentration. The other group of anions (propionate, formate) stimulated both Cl efflux and influx and such stimulation could be blocked by SITS. Propionate influx was not nearly as large as the stimulated Cl efflux and was unaffected by SITS. Neither the effects of SITS nor those of the anion substitutes could be simply accounted for by changes in the membrane resting potential or conductance. These results suggest a mediated transport system for chloride across the barnacle sarcolemma.

Contribution of sodium pump to resting potential of squid giant axon

The effect of the cardiotonic aglycone, strophanthidin, on sodium and potassium efflux, membrane potential, membrane conductance, potassium permeability, and the shape of the action potential of the giant axon of the squid, Loligo pealei, was examined. Strophanthidin depolarized the membrane to an extent determined by the intracellular sodium concentration, except in axons pretreated with cyanide, in which the effect is abolished. Cyanide itself hyperpolarized the axon membrane. Axons treated with strophanthidin appear to be better potassium electrodes, but this observation is fully accounted for by the stimulating effect of [K]o on an electrogenic sodium pump. The increase in potassium efflux produced by strophanthidin is also well accounted for by the observed membrane depolarization and the known dependence of potassium permeability on membrane potential (e-fold increase in efflux per 6.4 mV depolarization). Strophanthidin has no demonstrable effect on membrane conductance apart from that due to the observed depolarization. These findings support the view that cardiotonic steroids, at least in nerve, are specific inhibitors of the sodium pump, devoid of effects on permeability that might interfere with the study of electrogenic pumping. The alteration in the shape of the action potential after exposure to strophanthidin (deepening of the "underswing") suggests that the strophanthidin-induced membrane depolarization results from the inhibition of a true electrogenic pump, and not from ion redistributions in the vicinity of the membrane.

Contractile activation phenomena in voltage-clamped barnacle muscle fiber

Tension development in voltage-clamped barnacle muscle fibers occurs with depolarizing pulses so small as not to activate the potassium and calcium conductance systems. Peak tension and the tension time integral appear to be graded by both amplitude and duration of the depolarizing pulses. Subthreshold depolarizing conditioning pulses shorter than 500 ms potentiate the response to a given test pulse. This effect diminishes and reverts when the duration of the conditioning pulse is increasingly prolonged. The relationship between fiber membrane potential and tension developed in response to depolarizing pulses is described by an S-shaped curve. The tension saturates at a membrane potential of about +10 mV (inside positive). For a given pulse duration the saturation value remains constant even when the fiber interior reaches a value of +230 mV, which is well above what may be estimated to be the equilibrium potential of calcium ions (Eca = +120). In the presence of 5 mM external procaine, the shape of the tension-potential curve changes; the maximum value tension besides being diminished is not sustained by falls when the potential approaches the estimated value for Eca. These results suggest that under physiological conditions the contractile activator is probably released from an internal store, and that the calcium entering the fiber as inward current does not play a direct major role in contractile activation.

The influence of cardioactive steroids, metabolic inhibitors, temperature and sodium on membrane conductance and potential of crayfish giant axons

1. The resting membrane potential and the current-voltage relation were measured in crayfish giant axons before and after treatment with cardioactive steroids, metabolic inhibitors, extracellular sodium depletion and low temperature. 2. The membrane resistance of axons treated with cardioactive steroids, metabolic inhibitors, and low extracellular sodium was reduced by 30-53% depending on the treatment. Low temperature also caused a decrease in the membrane resistance of the axon but the decrease was limited to potentials around the resting membrane potential. The temperature response of sodium depleted or ouabain treated axons was an increase in resistance at all points along the current-voltage relation. 3. All inhibitors and low temperature caused a depolarization of the membrane potential. Ouabain and strophanthidin were the most effective, reducing the membrane potential by an average of 9.6 mV in 10-20 min. Low sodium did not cause a depolarization but consistently reduced the membrane resistance by an average of 30%. 4. The data suggest that there is an interaction between the activity of the ouabain-sensitive transport system and resting membrane resistance in the crayfish axon.

The influence of chloride on the ouabain-sensitive membrane potential and conductance of crayfish giant axons

1. Resting potential and current-voltage relation were measured in crayfish giant axons bathed in chloride-free and sodium-free solutions with and without ouabain. 2. Chloride-free solution caused a transient depolarization but did not alter the steady-state membrane potential. Utilizing isethionate as an anion substitute, the membrane resistance increased 12.5%. 3. In the absence of extracellular chloride, ouabain (0.5-1 mM) depolarized the axon 6-7 mV. The shape of the current-voltage relation did not change but the curve was shifted along the current axis. 4. These results indicate that ouabain inhibits a steady-state hyperpolarizing electrogenic pump current of approximately 3 muA/cm2. 5. Extracellular sodium removal from axons equilibrated in chloride-free solutions transiently hyperpolarized the membrane 6-7 mV without a change in membrane resistance. The transient hyperpolarization was ouabain and temperature sensitive. The steady-state potential reached in sodium-free and chloride-free solution was not ouabain sensitive. Temperature sensitivity of the steady-state membrane potential was greatly reduced. 6. The transient hyperpolarization produced by extracellular sodium removal was metabolically driven and may present the expression of a sodium efflux transport current of 7.0-7.5 muA/cm2. 7. Using electrophysiologically measured parameters, sodium and potassium conductance, influx and efflux currents and the coupling ratio for sodium/potassium transport are calculated from a modification of the conductance equation. 8. The sodium/potassium transport coupling ratio for steady-state conditions was estimated at 5:3 (1.67:1).

Calcium fluxes in internally dialyzed giant barnacle muscle fibers

Calcium-45 fluxes have been examined in isolated giant barnacle muscle fibers subjected to internal solute control by means of "internal dialysis". The 45Ca efflux was dependent upon the concentrations of both total and ionized internal Ca (Ca2+ buffered with EGTA). With a total Ca concentration of 2.0 mM and a 1:2 Ca/EGTA ratio (nominal [Ca2+]i =0.13 muM), the Ca efflux averaged 1.2 pmoles/cm2 sec. Under identical conditions, the mean Ca influx was only 0.36 pmoles/cm2 sec. The Ca efflux may not be attributed to leak of the CaEGTA complex, since a 2.5-fold increase in the EGTA concentration (nominal [Ca2+]i=0.032 muM) reduced the 45Ca efflux by one-third. Furthermore, when EDTA was used to buffer the internal Ca concentration (in the absence of internal Mg), the steady efflux of 14C-EDTA was only about 10% of the 45Ca efflux (in parallel experiments). The timecourse of the 45Ca fluxes also appeared anomalous in the 45Ca influx reached a steady level much more rapidly than 45Ca efflux in fibers of comparable diameters. If the muscle fibers are treated as right circular cylinders, these data imply that the apparent diffusion coefficient for inwardly-moving Ca is much larger than for outwardly-moving Ca. In contrast to Ca efflux, the outward diffusion of 22Na, 14C-EDTA and 3H2O appears to be limited primarily by the permeability of the dialysis tube wall. Some, but not all, of the anomalous behavior of the Ca fluxes can be reconciled if the deep, branched infoldings of the barnacle muscle surface membrane are taken into account.

Studies on bursting pacemaker potential activity in molluscan neurons. II. Regulations by divalanet cations

Identified cells in Aplysia (R15, R2, LPG) and cell 11 in Otala have been used to investigate the effects of divalent cations, temperature, pH and ouabain on neuronal activity. Divalent cations act primarily to regulate the appearance of bursting pacemaker potential (BPP) activity in these cells. These ions are necessary for B generation and will inhibit its appearance at high concentrations (Ca greater than Mg greater that greater than Sr). In addition, Ca is involved in the seasonal modulation of BPP activity in a neurosecretory cell in Otala. Monovalent cations play secondary roles as regulators of BPP activity by competing with divalent cations for the sites involved in the regulation of the (probable) monovalent conductances underlying BPPs. The effects of pH, temperature and ouabain on membrane properties and BPP activity are partly related to their interaction with divalent cations. The results described indicate important roles for divalent cations in the regulation of the expression of BPP activity and its underlying membrane properties both in different nerve cells and in the same cell during dormancy and activity of the snails.

Effects of anions and cations on the resting membrane potential of internally perfused barnacle muscle fibres

1. Single barnacle muscle fibres from Megabalanus psittacus (Darwin) were internally perfused with a number of K salt solutions (200 mM) which were made isotonic to the barnacle saline with sucrose.2. 200 mM-K acetate solution, in general, was found to be more effective than other solutions of K salts in generating and maintaining stable resting membrane potential of -56.0 +/- 0.7 mV (all potentials are referred to the external solutions as ground). The various K salts, on the basis of the magnitude of the resting potential they generated in the muscle fibres, followed the sequence, acetate > isethionate > aspartate > glutamate > fluoride > monohydrogen phosphate > succinate > citrate > sulphate > oxalate > iodobenzoate > ferrocyanide > chlorate > nitrate > chloride > thiocyanate > iodide > bromide > cyanide.3. The resting potential in muscle fibres perfused with solutions of acetate, aspartate and glutamate increased linearly with the logarithm of the K concentration (slope = 30.4 mV for K acetate and 27.4 for K aspartate and glutamate) when the ionic strength of the solutions was progressively increased from 50 to 650 mM. On the other hand, similar increase of ionic strength beyond 200 mM of solutions of K isethionate, fluoride, monohydrogen phosphate, succinate and citrate depolarized the muscle fibres.4. Perfusion of acetate solutions of other alkali metal ions gave low values for the resting potential and followed the sequence K > Na > Rb > Li > Cs. Also NH(4) and Tris ions gave low values for the resting potential which underwent oscillations associated with the twitching of the fibre and occasionally became positive in value (action potential).5. Addition of tetraethyl ammonium chloride (TEA-Cl), 20-100 mM, to K acetate solutions (200 mM) depolarized the fibre membrane and the consequent reduction of resting potential varied linearly with the logarithm of TEA concentration.6. Replacement of chloride ion by acetate or isethionate in the external solution did not change significantly the resting potential although the values were consistently lower by about 2 mV.7. Complete elimination of K in the external solution and reduction of its ionic strength using sucrose depolarized the muscle fibres by about 27 mV when Na was changed from 475 to 1 mM. Under these conditions, external solutions completely in acetate form gave resting potentials which were more positive than those observed in completely chloride solutions by 6-8 mV.8. Replacement of Na by Li, Tris, choline, tetramethyl or tetraethyl ammonium ion in the external solution made the values of the resting potential more positive (depolarization). Similarly increasing the concentration of K (or Cs or Rb in place of K) by correspondingly decreasing the concentration of Na in the outside solution depolarized the fibres and the resting potential became zero at a concentration of 280 mM (or 308 or 1500 mM for Rb or Cs, respectively) on extrapolation.

Calcium and potassium systems of a giant barnacle muscle fibre under membrane potential control

1. Single barnacle muscle fibres from Megabalanus psittacus (Darwin) were internally perfused and the effects of various internal and external solutions on voltage clamp currents were examined.2. The usual internal solution was 180 mM-K(+) aspartate (osmotic pressure adjusted to 1000 m-osmole by adding sucrose). Fibres perfused with this solution gave an average resting potential of -55 +/- 5 mV (all potentials are referred to the external solutions as ground). Further increase in internal K concentration depolarized the fibres.3. With membrane current control the total capacitance, referred to apparent membrane surface area, was 21.2 +/- 2.4 muF/cm(2).4. Under normal conditions, with demonstrably good longitudinal space clamp control, voltage clamp currents associated with certain depolarizing pulses showed oscillations. These oscillations were reduced in frequency and magnitude by lowering the temperature from 20 to 10 degrees C, by eliminating the inward currents with external Ca-free saline or by reducing the outward currents with internal tetraethylammonium (TEA) or replacement of internal K by Cs.5. With a Na- and Ca-free, 60 mM-MgCl(2) solution outside depolarizing voltage clamp pulses produced only outward currents. On repolarization the current tail reversed direction at about -70 mV for pulses of less than 50 msec duration. For longer pulses this reversal potential was less negative, suggesting an accumulation of external, or depletion of internal K.6. Both the size of the outward currents and the rate at which they reached their maximum value increased with temperature. The activation energy for the rate constant was about 63 kJ/mole.7. Fibres bathed in Na- and Mg-free, 60 mM-CaCl(2) saline were excitable. After replacement of the internal K(+) with Cs(+) or adding 60 mM-TEA to the internal solution only sustained inward currents were recorded with depolarization.8. Sustained inward currents could be reduced by external application of 5 mM-LaCl(3). Tetrodotoxin was not effective even at a concentration of 1000 nM.9. The rate at which these inward currents reached a maximum value increased with increase in temperature of the bathing solution with an activation energy of the order of 42 kJ/mole.10. The reversal potential of the inward currents changed with the level of internal Ca ions. For a fibre perfused without ethyleneglycol-bis (beta-aminoethyl ether) N,N'-tetraacetic acid (EGTA) this reversal potential was 175 mV (internal free Ca 5 x 10(-7)M), and was 196 mV for a fibre perfused with 20 mM-Tris EGTA (internal free Ca 0.26 x 10(-8)M).11. We propose an electrical equivalent circuit to account for most of the observed electrical properties of barnacle muscle fibres. In this model the Ca and the K system are located at different anatomical places and they interact through a series resistance.