The effect of external sodium concentration on the sodium fluxes in frog skeletal muscle

Some properties of the external activation site of the sodium pump in crab nerve

1. Methods are described for using the changes in respiration of intact Libinia nerve to follow the rate of energy utilization by the sodium pump in this tissue.2. Short tetani in 10 K(Na)ASW (artificial sea water in which Na is the major cation and the potassium concentration is 10 mM) increased the oxygen uptake which then declined exponentially. From the net influx of Na during the tetanus and the associated oxygen uptake, values between 1.9 and 3.4 were calculated for the Na: approximately P ratio. After longer tetani, the recovery curve was S-shaped.3. The pump was activated by potassium ions in the external medium and this activation was competitively inhibited by external sodium ions. The data are consistent with a Michaelis constant (K(m)) for external potassium of 1 mM and an inhibitor constant (K(i)) for external sodium of 60 mM.4. In activating the pump, K could be replaced by Tl(+), Rb, NH(4) and Cs ions; but, of the monovalent ions tested, sodium seemed to be unique in its inhibitory action.5. In sea waters containing 460 mM-Na, ouabain behaved like a mixed inhibitor of the pump, reducing both the maximum velocity and the apparent affinity for external potassium. At a given ouabain concentration, reducing the sodium content of the medium was without effect on the maximum rate of pumping; but the apparent affinity for potassium increased more steeply than in a ouabain-free solution.6. The rate of energy utilization associated with pumping was unaffected by inclusion of quite high concentrations of sulphydryl-blocking agents in the external medium.

Na Extrusion by the Sartorius of Rana pipiens

Sartorius muscles of R. pipiens may be enriched in sodium and depleted of potassium by prolonged soaking in the cold in K-free or low K Ringer's solution. Such K-depleted muscles take up potassium and extrude sodium when exposed to Ringer's solution containing 10 mM K at ordinary room temperature (ca. 20°C), with no dependence on external Na ion concentration in either phase within fairly wide limits. Extrusion of excess fiber Na is demonstrated on single muscles using Na²⁴-loaded tissues. Rate of exchange of excess fiber Na does not differ significantly from the rate of exchange of Na normally present in muscle fibers.

A hundred years of sodium pumping

This article gives a history of the evidence (a) that animal cell membranes contain pumps that expel sodium ions in exchange for potassium ions; (b) that the pump derives energy from the hydrolysis of ATP; (c) that it is thermodynamically reversible-artificially steep transmembrane ion gradients make it run backward synthesizing ATP from ADP and orthophosphate; (d) that its mechanism is a ping-pong one, in which phosphorylation of the pump by ATP is associated with an efflux of three sodium ions, and hydrolysis of the phosphoenzyme is associated with an influx of two potassium ions; (e) that each half of the working cycle involves both the transfer of a phosphate group and a conformational change-the phosphate transfer being associated with the occlusion of ions bound at one surface and the conformational change releasing the occluded ions at the opposite surface.

The concentration dependence of sodium efflux from muscle

Frog sartorius muscles subjected to overnight loading with Na⁺ in K-free Ringer in the cold were subsequently labeled with Na²⁴ and then immersed in choline Ringer and the efflux of Na²⁴ followed for 4 hours. The initial efflux of Na⁺ appeared to be 17 pmole/cm² sec.; this value was maintained for 20 minutes and was followed by an abrupt decline to about 9 pmole/cm² sec. This latter rate was maintained for the next 20 minutes of efflux. The efflux then declined gradually with time and reached values of the order of 0.1 pmole/cm² sec. The back addition of counts lost from muscles enabled one to calculate the relationship between efflux and [Na]_i for muscle. This roughly approximates an S-shaped curve with a value at half-saturation of about 17 mmole Na per liter of fiber water. The efflux-concentration curve is closely described by assuming that 3 Na⁺ are transported per carrier cycle.

Sodium and potassium ion effluxes from squid axons under voltage clamp conditions

Squid giant axons loaded with Na(24) were subjected to short duration (0.5 msec.) clamped depolarizations of about 100 mv at frequencies of 20/sec. and 60/sec. while in choline sea water. Under such conditions the early outward current was just about maximal at the time of termination of the clamping pulse. An integration of the early current versus time record gave 1.2 mucoulomb/cm(2) pulse, while a measurement of the extra Na(24) efflux resulting from repetitive pulsing gave a charge transfer of 1.4 mucoulomb/cm(2) pulse. In sodium-containing sea water and with pulses 50-75 mv more positive than E(Na) the Na(24) efflux is about 3 times the measured charge transfer. The efflux of K(42) from a previously loaded axon into normal sea water is only 50 per cent of the measured charge transfer when the membrane is held for about 5 msec. at a potential such that there is no early current, and such pulses are at 10-20/sec. The experiments appear to confirm the suggestion that the early current during bioelectric activity is sodium but provide unsatisfactory support for the identification of the delayed but sustained current solely with potassium ions. Resting Na(+) efflux is 0.6 pmole/cm(2) sec. mmole [Na](1), while the apparent K(+) efflux is about 250 pmole/cm(2) sec. and is little affected by hyperpolarization.

Calcium flux and contractility in guinea pig atria

The calcium in guinea pig atria can be divided into three components by kinetic studies with Ca⁴⁵: (a) a rapidly exchangeable fraction with a half-time of 4.5 minutes; (b) a slowly exchangeable fraction with a half-time of 86 (or 168) minutes; and (c) an inexchangeable fraction. In Krebs-Henseleit solution containing 2.5 mM calcium, the calcium content of the tissue at rest remains constant, the flux being about 0.02 µµmol/cm²-second. An increase or a decrease in extracellular calcium concentration by 1.25 mM causes a proportionate change in influx. A large increase in Ca⁴⁵ entry, equivalent to as much as 0.55 µµ/mol/cm² accompanies a contraction. When the strength of contraction is varied by stimulating at different frequencies or in solutions containing calcium at different concentrations, the increment of Ca⁴⁵ uptake per beat changes proportionally with the strength of the beat. Total atrial calcium is not increased by stimulation; however, the increase in outflux of Ca⁴⁵ during contraction that this constant tissue calcium implies could not be demonstrated under the experimental conditions employed. The observations are discussed in the light of the possible role of calcium transfer in excitation-contraction coupling.

Adrenaline, anti-adrenaline drugs and potassium movements in rabbits auricles

Adrenaline (2x10(-6) M) or isoprenaline (7.5x10(-8) M) increased the rate of (42)K uptake and the potassium content of right (spontaneously beating) auricles, but had no effect on potassium movements in quiescent left auricles. Although faster beating induced by electrical stimulation increased the rate of (42)K uptake, the actions of adrenaline were also apparent in auricles which were electrically stimulated so that they beat at a constant rate. The increase in (42)K uptake produced by adrenaline accounted entirely for the increase in potassium content of the tissue. Adrenaline, in concentrations ranging from 2x10(-6) M to 2x10(-4) M, had no effect on (42)K loss from electrically stimulated auricles. The action of adrenaline on (42)K uptake was blocked by dichloroisoprenaline (4x10(-6) M) but not by phenoxybenzamine (1.6x10(-6) M).

THE INFLUENCE OF SODIUM-FREE SOLUTIONS ON THE MEMBRANE POTENTIAL OF FROG MUSCLE FIBERS

The membrane potential of frog sartorius muscle fibers in a Cl- and Na-free Ringer's solution when sucrose replaces NaCl is about the same as that in normal Ringer's solution. The K⁺ efflux is also about the same in the two solutions but muscles lose K and PO₄ in sucrose Ringer's solutions. The membrane potential in sucrose Ringer's solution is equal to that given by the Nernst equation for a K⁺ electrode, when corrections are made for the activity coefficients for K⁺ inside and outside the fiber. For a muscle in normal Ringer's solution, the measured membrane potential is within a few millivolts of E_K. This finding is incompatible with a 1:1 coupled Na-K pump. It is consistent with either no coupling of Na efflux to K influx, or a coupling ratio of 3 or greater.

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.

EFFECTS OF SODIUM AZIDE ON SODIUM FLUXES IN FROG STRIATED MUSCLE

Unidirectional Na fluxes from frog's striated muscle were measured in the presence of 0 to 5 mM sodium azide. With azide concentrations of 2 and 5 mM the Na efflux was markedly stimulated; the Na efflux with 5 mM azide was about 300 per cent greater than normal. A similar increase was present when all but the 5.0 mM sodium added with azide was replaced by choline. 10^-5M strophanthidin abolished the azide effect on Na²⁴ efflux. Concentrations of azide of 1.0 mM or less had no effect on Na efflux. The Na influx, on the other hand, was only increased by 41 per cent in the presence of 5 mM NaN₃. From these findings it is concluded that the active transport of Na is stimulated by the higher concentrations of azide. The hypothesis is advanced that the active transport of Na is controlled by the transmembrane potential and that the stimulation of Na efflux is produced as a consequence of the membrane depolarization caused by the azide.

MUSCLE: VOLUME CHANGES IN ISOLATED SINGLE FIBERS

Volumetric experiments on single fibers isolated from semiten-dinosus muscles of frogs, some performed in correlation with measurements of membrane potential, confirm the data obtained on whole muscles, but only for the specific range of conditions in which most of the latter experiments have been done. These conditions are restricted to media in which the anion ( Cl usually) is permanent and the K is 10 to 12.5 meqlliter, or four to five times above the normal level in Ringer's solution. When other ionic conditions are employed, phenomena are disclosed which have not previously been described. The findings throw doubt upon the validity of some generally accepted views regarding the permeability properties of the membrane of frog muscle fibers and regarding the nature of the mechanisms which regulate their volume.

THE EXTRUSION OF SODIUM FROM CAT SPINAL MOTONEURONS

Sodium ions were injected into cat spinal motoneurons electrophoretically through an intracellular NaCl-filled microelectrode. Following an injection there were characteristic changes in the resting and spike potentials, the after-potential and the inhibitory postsynaptic potential, all of which recovered within about 7 min. The maximum rising slope of the spike recovered exponentially, suggesting the exponential decrease of the intracellular sodium concentration by the operation of the sodium pump in actively extruding excess sodium. The time course of the recovery of the maximum falling slope of the spike paralleled that of the rising slope, indicating a reciprocal change in the intracellular sodium and potassium concentrations. There was a good parallelism in the time courses of the recovery of the amplitude of the after-potential and the maximum falling slope of the spike, as would be expected from their postulated dependence on the same internal potassium concentration. The inhibitory postsynaptic potential recovered from its displacement in the depolarizing direction with the same time course as did the other potentials, which indicates parallel decreases of the intracellular sodium and chloride concentrations. From the exponential recovery curves obtained for these potentials, the rate constant of active sodium extrusion was estimated as 40 h -1 . The fast rate of sodium extrusion in cat motoneurons is related to the dynamic ionic balance in neurons of the central nervous system, and is explained by the geometry and by the membrane properties of motoneurons.

SODIUM PUMP: ITS ELECTRICAL EFFECTS IN SKELETAL MUSCLE

The variations in the membrane potentials of skeletal muscle fibers which follow high rates of sodium extrusion are not due to changes in the ionic concentrations of the fiber; experiments suggest that sodium is extruded by an electrogenic mechanism.

THE EFFLUX OF SUBSTANCES FROM FROG VENTRICLES TO SUCROSE AND TO RINGER'S SOLUTIONS

The frog ventricle in sucrose solution contracts for several hours at 25°C, and for as long as 24 hours at 5°G. The possibility that a fraction of the extracellular fluid remains outside of the excitable membrane was examined by measuring the efflux of tracers. The half-time for the efflux to sucrose solution at 25°C of C¹⁴ sucrose is about 1 minute, for Na²⁴ is 6.5 minutes, and for Cl⁸⁶ is 4 minutes. There is no evidence for the retention of an extracellular Na fraction. The Q₁₀ for Na and Cl efflux is about 1.3. The half-time for K⁴² efflux is about 180 minutes; the Q₁₀ is 1.7. The efflux rates of Na²⁴, Cl³⁶ and K⁴² to sucrose and to Ringer's solutions are quite similar. Ca⁴⁵ efflux is only one-fifth as fast to sucrose solution as to Ringer's; the retention of Ca⁺⁺ may be important for maintaining excitability in sucrose solution. P³² efflux is five times faster to sucrose solution than to Ringer's solution, and there is a similar increase in the rate of inosine loss to sucrose solution. The Q₁₀ for efflux to sucrose solution is 2.2 for P³²O₄ and 2.4 for inosine. We suggest that energy metabolism is abnormal in ventricles in sucrose solution and that low temperature prolongs excitability by slowing the metabolic change.

THE CONTROL OF THE MEMBRANE POTENTIAL OF MUSCLE FIBERS BY THE SODIUM PUMP

Frog sartorius muscles were made Na-rich by immersion in K-free sulfate Ringer's solution in the cold. The muscles were then loaded with Na²⁴ and the extracellular space cleared of radioactivity. When such Na-rich muscles were transferred to lithium sulfate Ringer's solution at 20°C, Na efflux was observed to increase with time, to reach a maximum about 15 minutes after the transfer of the muscles to Li₂SO₄, and then to decline. The decline in efflux from these muscles was proportional to ([Na]_i)⁸ over a considerable range of [Na]_i. The membrane potential of Na-rich muscles was about -48 mv in K-free sulfate Ringer's at 4°C but changed to -76 mv in the same solution at 20°C and to -98 mv in Li₂SO₄ Ringer's at 20°C. By contrast, muscles with a normal [Na]_i showed a fall in membrane potential when transferred from K-free sulfate Ringer's to Li₂SO₄ Ringer's solution. The general conclusions from this study are (a) that Na extrusion is capable of generating an electrical potential, and (b) that increases in [Na]_i lead to reversible increases in P_Na of muscle fibers.

Effects of external lithium on the physiology of Limulus ventral photoreceptors

We have examined some of the physiological effects associated with the replacement of extracellular Na+ with Li+ in nominally Ca(2+)-free saline in the ventral photoreceptors of the horseshoe crab Limulus polyphemus. We observed that replacement of Na+ saline with Li+ saline induced larger voltage-activated inward currents with similar voltage dependence. These currents were absent in Tris+ saline. Anode-break excitation was maintained in Li+ saline but blocked in Tris+ saline. Regenerative events associated with quantum bumps in dark-adapted cells illuminated with dim lights were maintained in Li+ saline. Regenerative events associated with responses to moderately bright illumination were also maintained in Li+ saline. The post-illumination hyperpolarization associated with the Na+/K(+)-exchange pump (Brown & Lisman, 1972) was present after brief exposure to Li+ saline but disappeared after longer exposure. Following return to Na+ saline, the post-illumination hyperpolarization reappeared. We conclude that (1) Li+ permeates the voltage-dependent Na+ channel, GNa(V), in the photoreceptor plasma membrane; (2) Li+ supports voltage-activated physiological events normally mediated by Na+; and (3) Li+ substitution briefly supports and later inhibits the electrogenic effects of the Na+/K(+)-exchange pump. The effects of external Li+ on cellular physiology have implications for the interpretation of other studies employing Li+ extracellularly.

Electrogenic pump (Na+/K(+)-ATPase) activity in rat optic nerve

Rat optic nerves were studied in a sucrose gap chamber in order to study the origin of a late afterhyperpolarization that follows repetitive activity. The results provide evidence for electrogenic pump (Na+/K(+)-ATPase) activity in central nervous system myelinated axons and demonstrate an effect on axonal excitability. Repetitive stimulation (25-200 Hz; 200-5000 ms) led to a prolonged, temperature-dependent post-train afterhyperpolarization with duration up to about 40 s. The post-train afterhyperpolarization was blocked by the Na+/K(+)-ATPase blockers strophanthidin and ouabain, and the substitution of Li+ for Na+ in the test solution, which also blocks Na+/K(+)-ATPase. The peak amplitude of the post-train afterhyperpolarization was minimally changed by the potassium-channel blocker tetraethylammonium (10 mM), and the Ca2(+)-channel blocker CoCl2 (4 mM). Hyperpolarizing constant current did not reverse the afterhyperpolarization. The amplitude of the hyperpolarization was increased in the presence of the potassium-channel blocker 4-aminopyridine (1 mM). In the presence of 4-amino-pyridine, the post-train hyperpolarization was much reduced by strophanthidin, except for a residual early component lasting several hundred milliseconds which was blocked by the potassium-channel blocker tetraethylammonium. This finding indicates that after exposure to 4-aminopyridine, repetitive stimulation leads to activation of a tetraethylammonium-sensitive K(+)-channel that contributes during the first several hundred milliseconds to the post-train afterhyperpolarization. The amplitude of the compound action potential elicited by a single submaximal stimulus during the post-train hyperpolarization was smaller than that of the control response. The decrement in amplitude was not present under identical stimulation conditions when the post-train hyperpolarization was blocked by strophanthidin, indicating that the hyperpolarization associated with repetitive stimulation reduced excitability.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the sodium pump in pacemaker generation in dog colonic smooth muscle

1. The role of the Na+ pump in the generation of slow wave activity in circular muscle of the dog colon was investigated using a partitioned 'Abe-Tomita' type chamber for voltage control. 2. Blockade of the Na+ pump by omission of extracellular K+, by ouabain, or the combination of 0 mM-Na+ and ouabain, depolarized the membrane up to approximately -40 mV and abolished the slow wave activity. Repolarization back to the control membrane potential by hyperpolarizing current restored the slow wave activity. 3. Slow waves continued to be present in 0 Na+, Li+ HEPES solution. 4. The depolarization induced by the procedures to block Na+ pump activity was associated with an increase in input membrane resistance. 5. Voltage-current relationships show the presence of an inward rectification. 6. Reduction of temperature depolarized the membrane, and decreased the slow wave frequency and amplitude. The slow wave amplitude was restored by repolarization of the membrane. 7. Brief depolarizing pulses evoked premature slow waves. Brief hyperpolarizing pulses terminated the slow waves. 8. We conclude that abolition of slow wave activity by Na+ pump blockade is a direct effect of membrane depolarization and that the Na+ pump is not responsible for the generation of the slow wave. 9. Our results are consistent with the hypothesis that pacemaker activity in smooth muscle is a consequence of membrane conductance changes which are metabolically dependent.

Effects of internal Na and external K concentrations on Na/K coupling of Na,K-pump in frog skeletal muscle

To clarify the dependency of the Na/K coupling of the Na,K-pump on internal Na and external K concentrations in skeletal muscle, the ouabain-induced change in membrane potential, the ouabain-induced change in Na efflux and the membrane resistance were measured at various internal Na and external K concentrations in bullfrog sartorius muscle. Upon raising the internal Na concentration from 6 mmol/kg muscle water to 20 mmol/kg muscle water, the magnitude of the ouabain-induced change in membrane potential increased about eightfold and the magnitude of the ouabain-induced change in Na efflux increased about fivefold while the membrane resistance was not significantly changed. As the external K concentration increased from 1 to 10 mM, the magnitude of the ouabain-induced change in membrane potential decreased (1/5.5 fold), while the magnitude of the ouabain-induced change in Na efflux increased (about 1.5-fold). The membrane resistance decreased upon raising the external K concentration from 1 to 10 mM (1/2-fold). These observations imply that the values of the Na/K coupling of the Na,K-pump increases upon raising the internal Na concentration and decreases upon raising the external K concentration.

Relationship between ionic surroundings and insulin actions on glucose transport and Na,K-pump in muscles

1. It is well known that insulin has various effects on glucose transport and the Na,K-pump in muscles. It is also known to have some effects on the membrane potential--in general, insulin induces a hyperpolarization of the membrane in muscles. Furthermore, it is suggested that the actions of insulin are modified by changes in ionic surroundings. 2. In this review article, the actions of ionic surroundings and insulin on glucose transport in muscles are discussed; in particular, the effects of changes in extracellular and/or intracellular concentrations of Na, K, Ca and H ions will be mentioned. 3. The actions of ionic surroundings and insulin on the Na,K-pump in muscles are discussed; in particular, the effects of changes in extracellular an/or intracellular concentrations of Na, K, Ca and H ions will be examined. 4. The relationship between the actions of ionic surroundings and insulin are discussed. 5. In particular, the effects of changes in ionic surroundings on the insulin-induced hyperpolarization of the membrane are discussed by relating it to the Na,K-pump function. The relationship between the insulin-induced change in membrane potential and glucose transport will be also mentioned.

Effects of external K concentration on the electrogenicity of the insulin-stimulated Na,K-pump in frog skeletal muscle

Insulin hyperpolarized the membrane of frog skeletal muscle by stimulating the electrogenic Na,K-pump. At external K concentrations of 1,2,5 and 10 mM, both the insulin-induced hyperpolarization and the insulin-stimulated ouabain-sensitive Na efflux (an index of Na,K-pump activity) were observed. By increasing the external K concentration, the insulin-stimulated Na efflux increased, but the magnitude of the insulin-induced hyperpolarization decreased; i.e., although the activity of the insulin-stimulated Na,K-pump increased, on the contrary, the magnitude of the hyperpolarization decreased. To clarify the causes of this phenomenon, the specific membrane resistance was measured and found to decrease upon increasing the external K concentration. One of the reasons for the decrease in magnitude of the hyperpolarization is the decrease in the specific membrane resistance. However, the decrease in magnitude of the hyperpolarization with a rise of the external K concentration, which increased the insulin-stimulated Na,K-pump activity, cannot be explained only by the decrease in the specific membrane resistance. It is suggested that the decrease in magnitude of the hyperpolarization is mainly caused by a decrease in the electrogenicity of the insulin-stimulated Na,K-pump upon an increase in the external K concentration. The conclusion of the present study is that the electrogenicity of the insulin-stimulated Na,K-pump in muscles is variable and decreases with increasing the external K concentration.

Saturation of the internal sodium site of the sodium pump can distort estimates of potassium affinity

The Na+/K+ exchange pump in cardiac Purkinje strands has been well studied with the voltage clamp and Na+-selective microelectrodes. Models describing the observed results suggest that the pump rate can be considered proportional to [Na+]i over the range examined and depends on external [K+] in accordance with Michaelis-Menten kinetics. Estimates of the external [K+] that achieves a half-maximal pump rate (Km) range from 0.9 to 6.3 mM depending on the preparation and method of estimation. Here we show that much of the variability in the estimates of the Km can be eliminated when saturation of the internal Na+ pump site is taken into account. If the half-activation concentration for saturation of this Na+ site is sufficiently high (greater than 20 mM), removal of intracellular Na+ in response to a Na+ load will approximate first-order kinetics. Under these conditions however, Na+ saturation will nevertheless cause large systematic errors in estimates of the K+ dependence of pump activity.

Lithium ions increase action potential duration of mammalian neurons

During Lucifer Yellow staining of mammalian neurons, intracellular recording revealed a prolongation of the action potential that is probably the result of leakage of lithium ions into the intracellular fluid. In experiments on dorsal root ganglion neurons intracellular iontophoresis of lithium ions also broadened the action potential. Cadmium, a calcium channel blocker, shortens the lithium-evoked wide actions potentials. The present experiments do not reveal whether lithium directly enhances inward calcium current, or whether a block of outward rectification allows calcium currents to increase the action potential duration.