The sodium pump in skeletal muscle in relation to energy barriers

Coherent Behavior and the Bound State of Water and K(+) Imply Another Model of Bioenergetics: Negative Entropy Instead of High-energy Bonds

Observations of coherent cellular behavior cannot be integrated into widely accepted membrane (pump) theory (MT) and its steady state energetics because of the thermal noise of assumed ordinary cell water and freely soluble cytoplasmic K(+). However, Ling disproved MT and proposed an alternative based on coherence, showing that rest (R) and action (A) are two different phases of protoplasm with different energy levels. The R-state is a coherent metastable low-entropy state as water and K(+) are bound to unfolded proteins. The A-state is the higher-entropy state because water and K(+) are free. The R-to-A phase transition is regarded as a mechanism to release energy for biological work, replacing the classical concept of high-energy bonds. Subsequent inactivation during the endergonic A-to-R phase transition needs an input of metabolic energy to restore the low entropy R-state. Matveev's native aggregation hypothesis allows to integrate the energetic details of globular proteins into this view.

Regulation of Na,K-ATPase Function in the Lens

The two cell types in the lens, epithelium and fiber, have a very different specific activity of Na,K-ATPase; activity is much higher in the epithelium. However, judged by Western blot, fibers and epithelium express a similar amount of both Na,K-ATPase alpha and beta subunit proteins. Na,K-ATPase protein abundance does not tally with Na,K-ATPase activity. Studies were conducted to examine whether protein synthesis plays a role in maintenance of the high Na,K-ATPase activity in lens epithelium. An increase of cytoplasmic sodium was found to increase Na,K-ATPase protein expression in the epithelium, but not in the fibers. The findings illustrate the ability of lens epithelium to synthesize new Na,K-ATPase protein as a way to boost Na,K-ATPase in response to cell damage or pathological events. Methionine incorporation studies suggested Na,K-ATPase synthesis may also play a role in day to day preservation of high Na,K-ATPase activity. Na,K-ATPase protein in lens epithelial cells appeared to be continually synthesized and degraded. Experiments with cycloheximide suggest that specific activity of Na,K-ATPase in the lens epithelium may depend on the ability of the cells to continuously synthesize fresh Na,K-ATPase proteins. However, other factors such as phosphorylation of Na,K-ATPase alpha subunit may also influence Na,K-ATPase activity. When intact lenses were exposed to the agonist thrombin, Na,K-ATPase activity was diminished, but the response was suppressed by inhibitors of the Src family of non-receptor tyrosine kinases. Thrombin elicited tyrosine phosphorylation of lens epithelium membrane proteins, including a 100 kDa protein band thought to be the Na,K-ATPase alpha 1 subunit. It remains to be determined whether a tyrosine phosphorylation mechanism contributes to the low activity of Na,K-ATPase in lens fibers.

EFFECTS OF EXTERNAL POTASSIUM AND STROPHANTHIDIN ON SODIUM FLUXES IN FROG STRIATED MUSCLE

Unidirectional Na fluxes in isolated fibers from the frog's semitendinosus muscle were measured in the presence of strophanthidin and increased external potassium ion concentrations. Strophanthidin at a concentration of 10^-5M inhibited about 80 per cent of the resting Na efflux without having any detectable effect on the resting Na influx. From this it is concluded that the major portion of the resting Na efflux is caused by active transport processes. External potassium concentrations from 2.5 to 7.5 mM had little effect on resting Na efflux. Above 7.5 mM and up to 15 mM external K, the Na efflux was markedly stimulated; with 15 mM K the Na influx was 250 to 300 per cent greater than normal. On the other hand, Na influx was unchanged with 15 mM K. The stimulated Na efflux with the higher concentrations was not appreciably reduced when choline or Li was substituted for external Na, but was completely inhibited by 10^-5M strophanthidin. From these findings it is concluded that the active transport of Na is stimulated by the higher concentrations of K. It is postulated that this effect on the Na "pump" is produced as a result of the depolarization of the muscle membranes and is related to the increased metabolism and heat production found under conditions of high external K.

Transport of electrolytes in muscle

The muscle fiber stands alongside the red blood cell and the giant axon as one of the three classical cell types that have had major application in investigating ion transport processes in cell membranes. Of these three cell types, the muscle fiber was the first to provide definite evidence for a sodium pump. The ability of the sodium pump to produce an electrical potential difference across the cell membrane was also first demonstrated in muscle fibers. This important property of the sodium pump is now known to have physiological significance in many other types of cells. In this review, electrolyte transport investigations in skeletal muscle are traced from their inception to the current state of the field. Applications of major research techniques are discussed and key results are summarized. An overview of electrolyte transport in muscle, this article emphasizes relationships between the muscle fiber membrane potential and ionic transport processes.

The effect of metabolic inhibition on ion contents and sodium exchange in human lymphocytes

Lymphocytes depleted of ATP by incubation in iodoacetate (IAA) and nitrogen (N2) lost K and gained Na. Isotopic Na exchange showed a fast fraction and a slower exponential fraction, the latter conventionally assumed to reflect surface membrane properties. The gain of cell Na was not accounted for by a decrease in 22Na efflux in either the slow or the fast fraction. After 3-5 hours, Na efflux increased. These results led us to question the concept that normal cell ion levels are maintained by an ATPase pump and could not be explained by exchange diffusion, co-transport, countertransport, or other inherently dissipative mechanisms. The data are, on the other hand, consistent with the concept that cell ion contents are determined by their relative exclusion from cell water coupled with selective adsorption onto fixed macromolecular anionic sites within the cell. In this view, the IAA,N2-induced rise in cell Na is due to the occupancy of adsorption sites losing K, while the increased isotopic exchange is due to a decreased activation energy for ion-site interaction.

Potassium induced potential changes in rat diaphragm muscle

The effect of different potassium concentrations on the membrane potential and membrane resistance of rat diaphragm muscle fibres was measured by means of a double sucrose gap method and a microelectrode technique. Concentration measurements showed that the muscle fibres gained sodium and lost potassium in the equilibration period. In the absence of external chloride changing the external potassium concentration from 2.8 mM to potassium-free caused a depolarization of the membrane of about 30 mV and a small increase in membrane resistance. This K-dependent potential change (K-response) was induced by ouabain, K-strophanthin, 2,4-dinitrophenol and cyanide, indicating that an energy requiring process is involved. The temperature dependence of the K-response found is consistent with this assumption. Variation in potassium permeability in the absence and presence of external potassium could account for only 13% of the K-response. The K-response amplitude appeared to depend on the external potassium and the internal sodium concentration. Hyperpolarization of the membrane could not only be produced after readmission of potassium but also after addition of thallium, the latter being more potent. Raising the external chloride concentration resulted in a decrease of the K-response and membrane resistance. The current, generating the K-response was shown to be hardly influenced by conditional polarization of the membrane. It is concluded from these results that the K-response is mainly due to the operation of an electrogenic sodium pump.

The membrane of giant molluscan neurons: electrophysiologic properties and the origin of the resting potential

The molluscan neuron, because of its large size and accessibility, has been an important model for studying the electrophysiology of nerve cells. This review catalogs data about specific molluscan neurons, but the greater importance of this material is in the broad picture of how a neuronal membrane maintains internal potential and is responsive to changes in the environment. Electrical properties of the membrane. The mechanisms which contribute to the resting potential in molluscan neurons can be separated into ionic and metabolic components. When the electrogenic sodium pump is eliminated experimentally, the ionic component of the potential follows the constant field equation quite closely. Many of the "constants" and "parameters" which characterize the membrane of molluscan neurons are actually variables which depend upon temperature, ionic environment, and membrane potential. The evaluation of the electrical parameters is complicated by extensive infoldings of the somatic membrane, and by large axons which drain current from the soma. Most molluscan neurons have a very high specific membrane resistance and a correspondingly low potassium permeability. Membrane capacitance is close to the 1 microF/cm2 value which characterizes biological membranes. The current-voltage relation of molluscan neurons may be complicated by inward-going rectification, but if that is inhibited the I-V curve follows the prediction of either the constant field equation or a simple electrical model. Factors which modify membrane behavior. The resting potential of molluscan neurons is very sensitive to changes in temperature and Ko, through a combination of effects upon the electrogenic sodium pump, inward-going rectification, and the membrane "parameters". Inward-going rectification depends upon a rectifying K conductance, and can be eliminated by cold or the removal of Ko. Strong or prolonged currents have time-dependent effects upon the membrane, and excessive polarization leads to a "high conductance state". The underlying (non-rectifying) K permeability of the membrane is relatively insensitive to temperature and ionic changes, whereas the Na permeability increases with warming. Membrane resistance varies with both temperature and ions (because the I-V curve is sensitive to these conditions) but membrane capacitance is relatively insensitive to external factors. Electrogenic sodium transport. Sodium transport is electrogenic in molluscan neurons. It can be stimulated by warm temperatures and an excess of substrate (e.g. high Nai); it can be inhibited by cold, by an absence of substrate (e.g. low Ko), or by pharmacologic agents such as cyanide or ouabain.(ABSTRACT TRUNCATED AT 400 WORDS)

Rubidium influx into rat skeletal muscles in relation to electrical activity

1. Rates of (86)Rb influx were compared in vivo over 2, 4 and 6 hr periods in various tonic and phasic muscles of rat following its I.P. injection. During the 2 hr period its influx rate into soleus was about 4 times that of the vastus with the EDL muscles at an intermediate rate. Uptake by diaphragm was fastest reaching equilibrium within 2 hr.2. Unilateral section of the sciatic nerve 48 hr before (86)Rb injection reduced isotope uptake into soleus to about 50% of its contralateral control muscle over a 4 hr period. In EDL muscles on the other hand nerve section increased influx by about 75% of control in conscious rats and more than doubled influx in anaesthetized rats.3. Tenotomy of soleus reduced (86)Rb influx to 40% of control, but tenotomy in EDL was without effect in influx.4. Uptake of urea into muscles within 5 min of its I.V. injection was used to determine the possibility of muscle blood flow determining (86)Rb influx. Accumulation of urea was not significantly different in control and denervated EDL muscles nor between soleus and vastus muscles in anaesthetized rats, so it seems unlikely that blood flow is important here.5. Membrane depolarization in response to addition of 30 mM rubidium to external bathing fluid was greater in the case of denervated than in control EDL muscles which was in keeping with the greater (86)Rb influx seen in the former muscles. The ouabain sensitivity of rubidium-induced depolarization in the denervated EDL muscles would suggest, however, that rubidium enters the fibres actively.

Some further observations on the electrogenic sodium pump in non-myelinated nerve fibres

1. A study has been made of the hyperpolarization that follows a period of electrical activity (post-tetanic hyperpolarization) and of the hyperpolarization which develops when potassium is readmitted after bathing the desheathed vagus nerve of the rabbit in potassium-free Locke solution during 15 min (potassium-activated response).2. Reduction of the external chloride concentration increases the membrane resistance and the potassium-activated response without changing the time constant of the response. A linear relation between the amplitude of the potassium-activated response and the membrane resistance was found. When chloride was replaced completely by isethionate (sodium salt) and by sulphate (other salts) the potassium-activated response increased by a factor of 5.3. The membrane resistance is decreased during the post-tetanic hyper-polarization elicited in isethionate Locke solution: the decrease is more pronounced after a longer period of electrical stimulation of the nerve.4. A small increase of the membrane resistance was found during the potassium-activated response. The changed membrane potential during the response can account for the alteration of the membrane resistance observed.5. The amplitude of the potassium-activated response is increased during hyperpolarization and reduced during external depolarization of the nerve, whereas the time constant is not affected. The potassium-activated response appears to be independent of polarization of the membrane after correction for the changed membrane resistance.6. The maximum amplitude of the activated response and the external potassium concentration are related following Michaelis-Menten kinetics; the time constant of the response is inversely related to the external potassium concentration.7. The area of the electrogenic response activated by high potassium concentrations (5.6-20 mM) is almost constant, but is reduced at lower potassium concentrations. The amplitude and area of the thallium-activated response are increased (about 1.5 times) compared with the potassium-activated response.8. It was concluded that the electrogenic response, reflected by post-tetanic hyperpolarization, is not directly related to activity of the electrogenic pump, which is probably due to accumulation of potassium in the periaxonal space; that the potassium activated response is produced entirely by activity of the electrogenic sodium pump; and that the current produced by activity of the electrogenic sodium pump is independent of the electrochemical gradient and membrane resistance.

The independence of electrogenic sodium transport and membrane potential in a molluscan neurone

1. The current-voltage relations of the Anisodoris giant neurone (G cell) were studied in the presence and absence of Na pump activity.2. Inhibition of the electrogenic Na pump with ouabain had no effect on either the presence at warm temperatures (10-15 degrees C), or absence at cold temperatures (0-5 degrees C), of inward-going rectification.3. Abolition of inward-going rectification in the warm, by replacement of external K with Rb, did not affect the electrogenic Na pump.4. The current generated by the electrogenic pump was essentially constant between the membrane potentials of - 30 and - 100 mV.5. The potential produced by the electrogenic pump can be predicted by a modification of the constant field equation.6. It is estimated that the energy required to extrude Na was between 3160 and 3700 cal/g-atom, and that uncoupled Na efflux during pump activity was typically between 0.2 and 4.0 p-mole/cm(2).sec.

The hyperpolarization of frog skeletal muscle fibres induced by removing potassium from the bathing medium

1. The time course of changes in resting potential after removing K(0) was studied in twenty-four single fibres and in 136 fibres from small bundle (two to four fibres) preparations of frog semitendinosus muscles.2. The initial resting potentials in the control saline ranged between -88 and -98 mV. The potentials returned to nearly the initial values when control conditions were reinstated after 3-8 hr of experimentation. All the fibres twitched at the end of the experiment.3. Only about one third of the fibres hyperpolarized for any length of time on exposure to a K-free saline at room temperature (20-28 degrees C). The hyperpolarization was reversed to depolarization after a variable delay. The resting potential could fall to -50 or -40 mV.4. The remainder of the fibres depolarized with little or no prior hyperpolarization.5. Both patterns of response could be replicated in the different fibres.6. Hyperpolarization induced by K-free solution was reduced or abolished on cooling to ca 10 degrees C; on substitution of Tris or Li for Na; and upon inhibition of the Na pump with DNP (0.025-0.2 mM) or ouabain (0.05 mM). The latter agent was not as effective as the other conditions.7. Only small, slowly developing depolarization occurred when Na was replaced with Tris or Li.8. The various effects in K-free solutions were reversed on returning to the control conditions.9. It is suggested that removal of K(0) itself has little or no direct effect on the resting potential and that the initial hyperpolarization is due to the pumped efflux of Na without a compensatory influx of K. Block of the pump electrogenesis is manifested by depolarization of the fibres as K(1) is depleted and Na(1) increased.10. The Na pump appears to be dependent upon the nutritional status of the frogs and variations of the latter probably cause the different responses of fibres to removal of K(0).

The kinetics of sodium extrusion in striated muscle as functions of the external sodium and potassium ion concentrations

After a 20 min initial washout, the rate of loss of radioactively labeled sodium ions from sodium-enriched muscle cells is sensitive to the external sodium and potassium ion concentrations. In the absence of external potassium ions, the presence of external sodium ions increases the sodium efflux. In the presence of external potassium ions, the presence of external sodium ions decreases the sodium efflux. In the absence of external potassium ions about one-third of the Na(+) efflux that depends upon the external sodium ion concentration can be abolished by 10(-5)M glycoside. The glycoside-insensitive but external sodium-dependent Na(+) efflux is uninfluenced by external potassium ions. In the absence of both external sodium and potassium ions the sodium efflux is relatively insensitive to the presence of 10(-5)M glycoside. The maximal external sodium-dependent sodium efflux in the absence of external potassium ions is about 20% of the magnitude of the maximal potassium-dependent sodium efflux. The magnitude of the glycoside-sensitive sodium efflux in K-free Ringer solution is less than 10% of that observed when sodium efflux is maximally activated by potassium ions. The inhibition of the potassium-activated sodium efflux by external sodium ions is of the competitive type. Reducing the external sodium ion concentration displaces the plots of sodium extrusion rate vs. [K](o) to the left and upwards.

Accumulation of caesium and rubidium in vivo by red and white muscles of the rat

1. Rats were given drinking water containing either 20 mM-CsCl or 20 mM-RbCl for a period of 2 weeks. Samples of blood were then taken from the rats under anaesthetic. They were immediately centrifuged and the plasma taken for analysis. Soleus muscles, diaphragm, extensor digitorum longus, white gastrocnemius and vastus lateralis muscles were then taken from the dead animals and these and the plasma were analysed for potassium, and for caesium or rubidium by means of the flame photometer.2. The concentrations of potassium and rubidium or caesium in the fibre water of these various muscles and in the samples of plasma water were then calculated.3. It was found that the red muscles including soleus and diaphragm generally tended to accumulate caesium and rubidium to a greater extent than did the white muscles such as the white gastrocnemius and vastus lateralis.4. When the concentration ratio [K](i)/[K](o) was divided into the ratio [Rb](i)/[Rb](o) for the different muscles, values of about 1.3 were obtained for the red muscles compared with values about 1.14 for white muscles.5. When in the case of the caesium-treated rats the ratio [K](i)/[K](o) was divided into the ratio [Cs](i)/[Cs](o) values ranged from 1.94 +/- 0.12 for the red soleus to 1.08 +/- 0.09 for the white gastrocnemius.6. When these values in the caesium-treated animals were plotted against the percentage of red fibres in the five muscle types (as obtained from the data of Sreter & Woo, 1963) the graph indicated that the white fibres had similar ionic gradients for Cs(+) and K(+) and that affinity for Cs(+) was confined to the red fibres.7. The membrane potential measured in soleus and extensor muscles immersed in plasma from the same animal was not significantly different from E(K) but was much less than E(Cs).8. These results are interpreted in terms of permeability differences between the slow red fibres and white twitch fibres.

On the electrogenic sodium pump in mammalian non-myelinated nerve fibres and its activation by various external cations

1. A study has been made of the hyperpolarization that follows a period of electrical activity (the post-tetanic hyperpolarization) in mammalian non-myelinated nerve fibres.2. Evidence is presented that under certain circumstances this postetanic hyperpolarization is a result of activity of an electrogenic sodium pump that normally is absolutely dependent on the external presence of potassium.3. When the external chloride is replaced by sulphate or by isethionate the post-tetanic hyperpolarization, which in normal Locke solution is only a few millivolts in amplitude, is increased usually to about 20 mV, and on occasion to 35 mV.4. This effect of removing the chloride takes several minutes to develop and is consistent with the idea that the increase in the post-tetanic response is the result of removing the short-circuiting effect of internal chloride ions (by their being washed out into the chloride-free bathing medium).5. Small anions, such as chloride, nitrate, iodide, bromide, and thiocyanate can short-circuit the electrogenic pump, whereas larger anions such as sulphate and isethionate cannot. The bicarbonate ion, which is larger than chloride, short-circuits the pump but less effectively.6. In Locke solution containing 5 mM potassium the post-tetanic hyperpolarization declines exponentially, with a time constant of about 1-3 min. The time constant is inversely related to the external potassium concentration.7. However, when the external potassium concentration is zero the hyperpolarization declines rapidly to a very small value. Subsequent addition of potassium to the bathing medium causes a marked redevelopment of the hyperpolarization.8. This potassium-activated response declines exponentially with a time constant that is inversely related to the potassium concentration. When the added potassium concentration is 5 mM, the time constant is 1.9 min.9. The amplitude of the potassium-activated response increases with increasing concentrations of potassium.10. Other cations can produce this activated response. Thus, thallium is more effective than, rubidium as effective as, caesium and ammonium about 1/10 as effective as, and lithium ions about 1/30 as effective as potassium in producing the activated response. Choline is quite ineffective.11. The size of the post-tetanic response is little affected by changes in the duration of the period of stimulation. However, increasing the duration definitely increases the time constant of recovery.12. Reducing the external sodium concentration increases the size of the post-tetanic hyperpolarization (by about 25%), but the effect is complex and requires further study.13. Reducing the calcium of the Locke-solution from 2.2 to 0.2 mM has no appreciable effect on the post-tetanic response, nor has increasing the pH of the Locke from 7.2 to 9.2.14. When the membrane potential is increased or decreased, by externally applied currents, there is relatively little change in the post-tetanic response.15. A mathematical model of the electrogenic pump, devised to mimic the experimental results, was analysed with an analogue computer. A satisfactory agreement between model and experiment was achieved by a model in which: (1) the rate of extrusion of sodium ions depends on the degree to which a pool of carrier molecules on the inside surface of the membrane is combined with sodium; (2) each carrier molecule transfers three sodium ions at a time; (3) the rate constant for extrusion of sodium ions also depends on the presence externally of potassium ions, which combine with some sites on the external surface of the membrane that are half-saturated when the external concentration of potassium is 2.8 mM.

Partition of sodium fluxes in isolated toad oocytes

1. The rate constant for Na efflux from the oocyte calculated from (d/dt) (ln [Na(*)](i)]) is only approximately 52% of that calculated from (d/dt)[(ln(d[Na(*)](i))dt)]. The difference may be interpreted by supposing that 48% of the internal Na of the oocyte is either bound to proteins or sequestered in cell organelles.2. The mean rate constant for Na efflux was 6.4 x 10(-3) min(-1) corresponding to an apparent Na efflux rate of 13.3 p-mole/cm(2).sec. When this is corrected for the increase in surface area produced by microvilli the true efflux rate is 1.1-1.3 p-mole/cm(2).sec.3. The action of ouabain (1-5 muM) appears to involve two different effects: (a) there is 48-65% inhibition of the membrane Na pump, and (b) there is a release of some of the sequestered Na in the cell.4. Removal of external K causes a 40% reduction in Na efflux although this value may be an underestimation owing to the presence of K which has leaked from the cell and may be retained near the cell surface.5. Raising the external K concentration to 15 mM reduces the inhibitory effect of ouabain by approximately a half.6. It was concluded that the Na pump in the toad oocyte may have a slightly lower level of activity than that in frog muscle, but that its general properties are similar to those in frog muscle and some other animal cells.

Regulation of intracellular sodium concentrations in rat diaphragm muscle

The concept is suggested that the sodium pump mechanism is influenced by the transmembrane electrical potential, and that the pump acts to maintain a constant electrochemical gradient for sodium. Evidence leading to this suggestion was obtained in rat diaphragm muscle by altering systematically the transmembrane chemical gradient for sodium ions and the transmembrane voltage. The voltage changes were produced by varying the extracellular and intracellular potassium ion concentrations. In each case the intracellular sodium concentration changed, presumably by activity of the sodium pump, so that the total electrochemical gradient for sodium was restored.

Effects of sodium extrusion and local anaesthetics on muscle membrane resistance and potential

1. Membrane potentials and resistances of K-depleted muscles were measured in the cold and again after warming in K-containing media so that active ion movements occurred.2. On warming there was a fall of resistance and a gradual rise of potential which passed through a maximum. Later measurements of resistance in a chloride medium showed that values were, if anything, higher than initially in the warm.3. The excess potentials measured approximated to those required to induce passive inward movement of the K ions through the measured K resistance.4. Permeabilities for K(+) and Cl(-) were deduced. When cocaine, procaine, amytal or mepyramine were added or when K(+) was replaced by Rb(+) in the Cl(-)-free solution the K(+) permeability was eventually reduced. The same agents led to an enhanced initial response of potential to warming, but later the potentials in Cl(-)-free media fell to less than the K(+) equilibrium values.5. A method for obtaining the resistivity of the membrane from measurements made in conditions of non-linear voltage-current dependence was applied.

Energy barriers to sodium extrusion from sodium-rich kidney cortex slices

1. Rat and guinea-pig kidney cortex slices were made Na-rich by leaching in cold NaCl and then allowed to recover in Ringer solution at 30 or 25 degrees C. Net Na extrusion was systematically decreased by the use of re-immersion media with constant Na and diminishing K, or with constant K and increasing Na, and the energy requirements for Na extrusion under these conditions were calculated.2. The critical energy barrier at which no net Na loss can be achieved was somewhat lower in these tissues than found by other authors for frog sartorius muscle.3. Inclusion of pyruvate or insulin and lactate in the recovery fluid had no effect.4. Methylsulphate apparently enters renal cells during incubation and is therefore unsuitable for use with this tissue as a source of impermeant anion.

Active transport of sodium and potassium in mammalian skeletal muscle and its modification by nerve and by cholinergic and adrenergic agents

1. Active transport of Na(+) and K(+) by Na-rich extensor digitorum and soleus muscles of rat was found to be increased considerably when muscles were innervated during enrichment with Na(+) in K-free modified Krebs solution containing 160 mM-Na at 2 degrees C and recovery in a similar fluid with 10 mM-K and 137 mM-Na at 37 degrees C, bubbled with oxygen.2. Addition of acetylcholine (2.0 mug/ml.) to recovery fluid containing denervated extensors increased active transport, whereas addition of eserine (50 mug/ml.), decamethonium (0.1 mug/ml.) and to a lesser extent tubocurarine (0.26 mug/ml.) inhibited active transport. Blocking of nerve conduction in innervated extensor inhibited K(+) uptake more than Na(+) excretion.3. The membrane potential of Na-rich extensor muscles measured soon after re-immersion in recovery fluid was higher in denervated than in innervated muscles. In the latter it was close to the K-equilibrium potential (E(K)). It is suggested that denervation here makes the Na-pump electrogenic by decreasing K(+) uptake either by decreased permeability or by inactivating a K-pump. Evidence is presented that the latter is more likely.4. Addition of isoprenaline to Na-rich soleus muscles in recovery fluid increased active transport and reduced the membrane potential measured soon after re-immersion in recovery fluid. The Na-pump still remained electrogenic in the presence of isoprenaline. It was suggested that isoprenaline might also stimulate the Na-pump, perhaps through activation of lactic dehydrogenase.