Effects on sodium efflux of treating frog sartorius muscles with hypertonic glycerol solutions

Resting and action potentials under hypotonic conditions, unlike Na+ pump activity, depend only on the alteration of intracellular [Na+] and [K+] in frog skeletal muscle

It is well established that hypotonicity generates a marked and unexpected increase in active Na(+) efflux in frog muscle fibers as well as in other cells like cardiac myocytes, astrocytes, brain synaptosomes and renal cells. The effect of hypotonicity on the electrical activity of skeletal muscle related to Na(+) and K(+) voltage-gated channels, however, has not been specifically addressed. The results of the present investigation show that the changes in resting and action potentials produced by hypotonicity can be fully explained by the reduction of intracellular [Na(+)] and [K(+)] due to the increase in cellular water content.

Na+-K+ Pump Regulation and Skeletal Muscle Contractility

Clausen, Torben. Na+-K+ Pump Regulation and Skeletal Muscle Contractility. Physiol Rev 83: 1269-1324, 2003; 10.1152/physrev.00011.2003.—In skeletal muscle, excitation may cause loss of K+, increased extracellular K+ ([K+]o), intracellular Na+ ([Na+]i), and depolarization. Since these events interfere with excitability, the processes of excitation can be self-limiting. During work, therefore, the impending loss of excitability has to be counterbalanced by prompt restoration of Na+-K+ gradients. Since this is the major function of the Na+-K+ pumps, it is crucial that their activity and capacity are adequate. This is achieved in two ways: 1) by acute activation of the Na+-K+ pumps and 2) by long-term regulation of Na+-K+ pump content or capacity. 1) Depending on frequency of stimulation, excitation may activate up to all of the Na+-K+ pumps available within 10 s, causing up to 22-fold increase in Na+ efflux. Activation of the Na+-K+ pumps by hormones is slower and less pronounced. When muscles are inhibited by high [K+]o or low [Na+]o, acute hormone- or excitation-induced activation of the Na+-K+ pumps can restore excitability and contractile force in 10-20 min. Conversely, inhibition of the Na+-K+ pumps by ouabain leads to progressive loss of contractility and endurance. 2) Na+-K+ pump content is upregulated by training, thyroid hormones, insulin, glucocorticoids, and K+ overload. Downregulation is seen during immobilization, K+ deficiency, hypoxia, heart failure, hypothyroidism, starvation, diabetes, alcoholism, myotonic dystrophy, and McArdle disease. Reduced Na+-K+ pump content leads to loss of contractility and endurance, possibly contributing to the fatigue associated with several of these conditions. Increasing excitation-induced Na+ influx by augmenting the open-time or the content of Na+ channels reduces contractile endurance. Excitability and contractility depend on the ratio between passive Na+-K+ leaks and Na+-K+ pump activity, the passive leaks often playing a dominant role. The Na+-K+ pump is a central target for regulation of Na+-K+ distribution and excitability, essential for second-to-second ongoing maintenance of excitability during work.

Ionic movements mediated by monensin in frog skeletal muscle

Monensin-mediated ionic movements were studied in frog skeletal muscle. The ionophore, which forms electrically neutral complexes with monovalent cations, induced dose dependent fluxes of Na+, K+ and H+ in and out of the fibers. Monensin concentrations ([MON]) ranged from 2 to 40 microM. In the presence of normal Ringer's solution the following maximum ionic exchanges were generated by monensin (in pmol cm-2 s-1): (1) Nai+/Nao+ 112, (2) Nai+/Ho+ 30.7, (3) Ki+/Nao+ 14.2 (4) Hi+/Nao+ 49. The maximum net fluxes produced by these exchanges (i.e. for [MON] = infinity) are (in pmol cm-2 s-1): Na+ (inward) 32.5, K+ (outward) 14.2, H+ (outward) 18.3. The last one appears to be largely offset by a passive (monensin-independent) H+ influx down an inwardly directed electrochemical gradient promoted by pH reduction of the T-tubular lumen content as a consequence of the monensin-mediated net H+ efflux. Maximum unidirectional cationic fluxes mediated by monensin amounted to 206 pmol cm-2 s-1 and had the following composition: influx: 85% Na+ and 15% H+; efflux: 69% Na+, 7% K+, 24% H+.

Hypo-osmotic stimulation of active Na+ transport in frog muscle: apparent upregulation of Na+ pumps

The purpose of this work was to determine if hypotonicity, in addition to the stimulation of active Na+ transport (Venosa, R.A., 1978, Biochim. Biophys. Acta 510:378-383), promoted changes in (i) active K+ influx, (ii) passive Na+ and K+ fluxes, and (iii) the number of 3H-ouabain binding sites. The results indicate that a reduction of external osmotic pressure (pi) to one-half of its normal value (pi = 0.5) produced the following effects: (i) an increase in active K+ influx on the order of 160%, (ii) a 20% reduction in Na+ influx and K+ permeability (PK), and (iii) a 40% increase in the apparent density of ouabain binding sites. These data suggest that the hypotonic stimulation of the Na+ pump is not caused by an increased leak of either Na+ (inward) or K+ (outward). It is unlikely that the stimulation of active Na+ extrusion and the rise in the apparent number of pump sites produced by hypotonicity were due to a reduction of the intracellular ionic strength. It appears that, at least in part, the stimulation of active Na+ transport takes place whenever muscles are transferred from one medium to another of lower tonicity even if neither one was hypotonic (for instance pi = 2 to pi = 1 transfer). Comparison of the present results with those previously reported indicate that in addition to the number of pump sites, the cycling rate of the pump is increased by hypotonicity. Active Na+ and K+ fluxes were not significantly altered by hypertonicity (pi = 2).

Role of sodium and potassium permeabilities in the depolarization of denervated rat muscle fibres

1. Na+ and K+ flux measurements and membrane potential (Vm) determinations were performed on normal and denervated rat extensor digitorum longus (e.d.l.) muscles. 2. The mean Vm in normal muscle fibres was -74.6 mV. During the first week after denervation Vm fell about 20 mV following an S-shaped time course. 3. In that period the Na+ permeability (PNa) increased and the K+ permeability (PK) decreased, so that by the sixth day post-denervation, the PNa/PK ratio was increased by a factor of 2.7. 4. The decrease in PK preceded the increase in PNa. 5. No major contribution to the fall of Vm by a reduced activity of an electrogenic Na+ pump could be detected. 6. A good agreement was found between the experimental values of the depolarization and those calculated using the constant-field equation assuming Cl- is at equilibrium and no significant change of the intracellular K+ concentration ([K+]i) during the first week after denervation. 7. It is concluded that the depolarization promoted by denervation in e.d.l. rat muscle fibres can be fully explained in terms of changes in PNa and PK.

Effects of extracellular sodium concentration on null potential, conductance and open time of endplate channels

(i) Effects of extracellular sodium concentration, [Na]o, on endplate channel characteristics were investigated in voltage-clamped, glycerol-treated toad sartorius fibres. (ii) The relation between [Na]o (and [K]o) and acetylcholine null potential could be reasonably well fitted by the Goldman-Hodgkin-Katz type of equation, except when [Na]o was higher than normal. Anions had no significant effect on the null potential. (iii) Endplate channel open time (phi), whether measured from miniature endplate currents or from current fluctuations induced by iontophoresis of acetylcholine, varied inversely with [Na]o. The relation between phi-1(= alpha) and [Na]0 could be fitted by alpha = alpha max [Na]o/(Km + [Na]o) with a Km of 92 mM. (iv) Endplate conductance, measured at the peak of endplate currents or at the peak of spontaneous miniature endplate currents, increased nonlinearly with [Na]o. (v) Single channel conductance, gamma, also increased nonlinearly with [Na]o. Experimental observations at -90 mV could be fitted by the relation gamma = gamma max [Na]o/(Km + [Na]o), giving values for gamma max and Km of 47 pS and 146 mM respectively. Correcting channel conductance for the contribution from potassium ions gave values of gamma max and Km of 78 pS and 423 mM respectively. (vi) The results are consistent with the hypothesis that binding sites for Na ions can modulate both channel lifetime and conductance and that these sites become saturated at higher sodium concentrations.

The interaction of potassium with the activation of anomalous rectification in frog muscle membrane

1. Inward rectification of frog muscle membrane was analysed with the Vaseline gap method. 2. Hyperpolarization, under voltage clamp, produced inward potassium currents, which had a component that activated with a time constant, tau K. 3. The activation time constant tau K of the inward potassium currents was voltage dependent. For a given external potassium concentration, the time constant was maximal for potentials near the potassium equilibrium potential, EK. 4. The potassium chord conductance gK, had a sigmoidal voltage dependency, increasing initially e-fold per 11.6 mV of hyperpolarization. 5. When the internal potassium concentration was fixed, raising external potassium induced a shift of the tau K-V and the gK-V relations in the positive direction along the voltage axis. That shift was comparable to the change in EK. 6. No shift of the tau K-V and the gK-V relations was observed when the internal potassium was reduced from 150 to 50 mM. 7. Changes of internal sodium concentration between 5 and 100 mM did not significantly effect the magnitude of inward rectification.

The subcellular location of potassium flux pathways in frog skeletal muscle

The influence of ouabain and physostigmine on 42K and 86Rb uptake in isolated from sartorii with normal [Na]i(12-14 mmol.kg -1 wet weight) and low [Na]i (6 mmol.kg-1 wet weight) was compared. Both in normal sodium and in low sodium muscles application of 10-3 M physostigmine reduces potassium influx by about 70%. About one forth of potassium-uptake in normal-sodium muscles is inhibited by ouabain (10-4 M) and only a very slight fraction of potassium uptake is ouabain-sensitive in low-sodium muscle. The effects of ouabain and physostigmine on 42K influx are additive. The greater parts of the Rb-fluxes are through the ouabain-sensitive pathway. Glycerol treatment has no effect on ouabain-sensitive channels although it inhibits markedly the K-flux through the physostigmine-sensitive pathway. The results suggest that the Na-K-ATPase is located in the surface membrane while most of the physostigmine-sensitive K-exchange is within the tubules.

Density and apparent location of the sodium pump in frog sartorius muscle

The binding of the cardiosteroid 3H-ouabain to frog skeletal muscle was determined by studying the kinetics of its uptake and release. The amount of ouabain bound as a function of drug concentration in the external medium follows a hyperbolic relationship with a maximum binding (Bmax) of the order of 2500 molecules per square micrometer of surface membrane and an affinity constant (K) of 2.2 X 10(-7)M. The data do not suggest a drug-receptor (Na pump site) relation other than one-to-one. Ouabain molecules are released from whole muscle into ouabain-free media very slowly. The release is a single exponential function of time (tau approximately equal to 25 hr). When re-binding is prevented by the presence of unlabeled ouabain in the external medium, the loss of labeled ouabain is increased (tau approximately equal to 15 hr). Increasing [K+]O from 2.5 to 10 mM slows the time course of binding without any significant change in binding capacity of the muscle fibers. Experiments on detubulated muscles indicate that the density of pump sites is considerably higher in the surface than in the T-tubular membrane. These findings agree with the report by Narahara et al. [Narahara, H.T., Vogrin, V.G., Green, J.D., Kent, R.A., Gould, M.K. (1979) Biochim. Biophys. Acta 552:247] on the distribution of (Na+ + K+)- ATPase among different cell membrane fractions from frog skeletal muscle.

Distribution of potassium and chloride permeability over the surface and T-tubule membranes of mammalian skeletal muscle

The distribution of K and Cl permeability, PK and PCl, over the surface and T-tubule membranes of red rat sternomastoid fibers has been determined. Membrane potential, Vm, was recorded with 3-M KCl-filled glass microelectrodes. Changes in Vm with changes in [K]o or [Cl]o were used to estimate PCl/PK in normal and detubulated preparations. The results show that the T-tubule membrane has a high PCl and is therefore different from the T-tubule membrane of amphibian fibers. Analysis of the time course of depolarization when [K]o was raised (in SO4 solutions) showed that PK was distributed over the surface and T-tubule membranes. Two observations suggested that T-tubule PCl was higher than the surface PCl. Firstly, in normal fibers, the depolarization caused by an increase in [K]o was 3.5 times greater in SO4 solutions than in Cl solutions. In marked contrast, the depolarization in glycerol-treated fibers was independent of [Cl]o. Secondly, the rapid change in Vm when [Cl]o was changed was reduced by 80% after glycerol treatment. Both observations suggest that PCl was low in glycerol-treated fibers. PCl/PK was calculated from the Vm data using Goldman, Hodgkin and Katz equations for Na and K or for Na, K, and Cl. In normal fibers PCl/PK = 4.5 and in glycerol-treated fibers PCl/PK = 0.28. Since it is unlikely that glycerol treatment would increase PK, the reduction in the ratio must follow the loss of Cl permeability "channels" in the T-tubule membrane.

Stimulation of the Na+ pump by hypotonic solutions in skeletal muscle

The fractional loss of 22 Na+ from frog sartorius muscle is increased when the tonicity of the external solution is reduced. The effect, which is larger the lower the osmolarity, exhibits the following characteristics: (1) quick onset and reversibility, (2) is not reduced in the absence of external Na+, (3) is completely abolished by strophanthidin (3. 10-5 M), (4) is neither the result of membrane depolarization nor K+ accumulation in the extracellular space.

Energy coupling to sodium transport in frog skeletal muscle

In addition to a strophanthidin-sensitive (SS) sodium efflux, a large component of the sodium efflux in freshly isolated frog skeletal muscle is sodium-activated and strophanthidin-insensitive (SASI). The amount of metabolic energy associated with sodium movement by each of these components was measured and the coupling between sodium movement and adenosine 5'-triphosphate (ATP) hydrolysis in muscle was calculated. Energy production was blocked by iodoacetate and cyanide. Energy turnover was estimated from the change in creatine phosphate (CrP) and ATP contents and expressed as potential energy (PE = CrP + 2ATP). After metabolic poisoning a linear fall of PE occurred (6.3 mumol/g.h). Metabolic poisoning had no effect on the magnitude of the SS or SASI components of sodium efflux. In 2 h the sodium moved, and PE change due to the SS component was 4.35 and 1.66 mumol/g.h, respectively, which gave a coupling factor of 2.6. The amount of sodium moved by the SASI component was similar to that moved by the SS component in 2 h whereas no energy change was observed. It was, therefore, concluded that sodium movement by the SASI component requires no energy input.

Density and distribution of tetrodotoxin receptors in normal and detubulated frog sartorius muscle

Tetrodotoxin (TTX) binding was measured in muscles which were either in normal condition or which had been "detubulated" by glycerol-induced osmotic shock. In both cases the binding of TTX was found to saturate at high TTX concentrations. Maximum binding in normal fibers was 35 pmol/g wet weight, and that figure was reduced to 16 pmol/g after glycerol treatment. The dissociation constant for binding to the surface membrane was 3 nM, which is the range of values obtained by electrophysiological measurements of the effect of TTX on the maximum rate of rise of the action potential. The results suggest that the dissociation constant in the transverse tubules may be higher than that in the surface. Control experiments indicate that the effects of glycerol treatment are limited to the accessibility of the receptors to the toxin and result in no alteration of the affinity of the binding site. TTX receptors in the transverse tubules may be recovered after glycerol treatment by homogenization of the fibers. The measurements suggest that the density of sodium channels in surface membrane is about 175/muM2 and that in the transverse tubular membrane is 41-52/mum2.

Sodium fluxes in single amphibian oocytes: further studies and a new model

1. The kinetics of Na efflux were studied in oocytes of Bufo bufo, Rana temporaria and R. pipiens. 2. Rate constants for Na efflux into Ringer solution varied from 0-002 min-minus 1 to 0-017 min-minus 1 and did not vary significantly from one species to another. 3. Na efflux is rapidly reduced by 30-50% on removing external K or applying ouabain but is reduced by 90% on cooling to 0 degrees C. The effects of K and cooling are also rapidly reversible. 4. Substitution of external Na by Li produces a slow decline of Na efflux. Reversal on restoring external Na is, however, rapid even in the presence of ouabain. 5. When external Na is replaced by Li in the presence of ouabain, the normal decline in Na efflux does not occur. 6. When external Na has been replaced by Li, application of ouabain causes little or no further decline in Na efflux. 7. These results are interpreted quantitatively by means of a model which proposes that intracellular membrane-bounded channels (IMBC) contain 10-30% of the intracellular Na and provide a channel for its expulsion from the cell via connexions with the cell surface. It is supposed that Na is expelled actively from the cytoplasm into the IMBC as well as at the cell surface. Na expulsion via the IMBC is supposed to be insensitive to external K or ouabain. This model accounts for the results using parameters consistent with other investigations by autoradiography and Na-sensitive micro-electrodes. 8. Preliminary electron micrographic evidence shows channels which appear to lead from the cell surface into the cytoplasm and which may correspond with the proposed IMBC of the model.

Inward movement of sodium ions in resting and stimulated frog's sartorius muscle

1. Paired frog sartorius muscles were exposed to Ringer solutions labelled with (22)Na(+) for about 20 min. At the end of this exposure one of them was stimulated supramaximally one hundred to two hundred times. Immediately after the stimulation both members of the pair were washed in a series of tubes filled with a Na(+)-free medium containing 3 x 10(-5)M strophanthidin.2. Under the above conditions the intracellular component of the efflux was exponential with an average time constant (tau) of 388 min, that is, approximately four times longer than in the presence of normal Ringer. On the other hand the mean tau for the washout of the interfibre space was 3.2 min.3. From the extrapolation to time zero of the intracellular component of the washout curve the initial intracellular radioactivity of both muscles was obtained and the resting and extra Na(+) influx were calculated.4. The mean surface membrane area/muscle weight ratio was found to be 552 cm(2).g(-1) and the mean fibre diameter 53.4 mum for muscles weighing on the average 60 mg.5. The average resting Na(+) influx in the presence of normal Ringer was 4.7 p-mole.cm(-2).sec(-1). As the external Na(+) concentration ([Na(+)](0)) was reduced the Na(+) influx diminished in a non-linear fashion. This non-linearity could be accounted for by the presence in the influx of a Na(+) for Na(+) exchange fraction which saturates at low [Na(+)](0).6. The mean extra Na(+) influx in the presence of normal Ringer was 27.4 p-mole.cm(-2).impulse(-1) and was not significantly affected either by halving [Na(+)](0) or by varying the frequency of stimulation. When [Na(+)](0) was reduced to 45 mM by partial replacement of Na(+) by Tris(+) the extra influx was significantly higher than when choline(+) instead of Tris(+) was used to substitute for Na(+).