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

Summary Insulin hyperpolarized the membrane of frog skeletal muscle by stimulating the electrogenic Na,K-pump. At external K concentrations of 1, 2, 5 and 10mm, 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.     

Effect of amizil and arecoline on the activity of Na, K-ATP-ase and concentration of Na+ and K+ ions in rat brain]

A study was made of the activity of Na, K-ATP-ase and the Na+ and K+ content in the brain of rats with the action of arecoline and amizyl. Both arecoline and amizyl increased the Na, K-ATP-ase activity. This could be associated with the changes in the redistribution of the Na+ and K+ ions in the nerve cell. Arecoline proved to cause changes in the electrolyte distribution by the depolarization type, whereas amizyl--by the type of hyperpolarization of the nerve cell membrane.     

Ba2+ ions block K+-induced contractures by antagonizing K+-induced membrane depolarization in frog skeletal muscle fibres

The effects of Ba2+ ions on twitches, K+-induced contractures, and on intracellularly recorded membrane potentials (Em) and depolarizations of frog skeletal muscle fibres were investigated. Exposure of toe muscles to choline--Ringer's solution with 10(-3) M Ba2+ with Ca2+ (1.08 mM) eliminated or very greatly reduced contractures produced by 60 mM K+. In contrast, not only did the same concentration of Ba2+ ions fail to depress the twitch tension of isolated semitendinosus fibres when added to Ringer's with Ca2+, but it even restored twitches that had been eliminated in a zero Ca2+ Ringer's solution. The resting Em of sartorius muscle fibres in choline--Ringer's solution was reduced about 20 mV by 10(-3) M Ba2+. This Ba2+ ion concentration also antagonized the K+-induced depolarization. Thus in the presence of 1 mM Ba2+, 20 mM K+ hyperpolarized rather than depolarized the fibres and 60 or 123 mM K+ produced only very slowly developing, small depolarizations. These results suggest that the loss of the K+-induced contracture in choline-Ringer's caused by Ba2+ ions is due to an inhibition of the K+-induced depolarization. The latter result is consistent with previous findings of other workers that Ba2+ ions block membrane K+ channels.     

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

Summary 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 10mm, 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 10mm (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.     

The effect of vanadate on the electrogenic Na+/K+ pump, intracellular Na+ concentration and electrophysiological characteristics of mouse skeletal muscle fibre

The present study reports a discrepancy between the effects of vanadate on the membrane Na+-K+-ATPase and the Na+/K+ pump of the skeletal muscle. Vanadate in concentration 4 X 10(-6) mol/l which is necessary to block the enzyme Na+-K+-ATPase activity of membrane fractions failed to inhibit the electrogenic Na+/K+ pump of intact muscle cells. The effect of vanadate on the electrophysiological parameters of the muscle fibre membrane required much higher vanadate levels, but again, Na+/K+ pump was still active. Vanadate in concentrations 4 X 10(-4) and 4 X 10(-5) mol/l depolarized the membrane potential and decreased the membrane resistance [apparently in consequence of enhanced passive membrane permeability for Na+ ions]. Action potentials and the electrical excitability of the muscle fibre membrane were reduced by these vanadate concentrations.     

ATP-driven exchange of Na+ and K+ ions by Streptococcus faecalis

We describe the characterization of KtrII, a novel potassium transport system of Streptococcus faecalis, first discovered by H. Kobayashi [1982) J. Bacteriol. 150, 506-511). KtrII requires sodium ions and mediates the stoichiometric exchange of internal Na+ for external K+. Potassium accumulation is not energized by the electrochemical potentials of either H+ or Na+; the energy source is probably ATP. Two lines of evidence indicate that KtrII is a manifestation of the sodium-stimulated ATPase reported earlier (Heefner, D. L., and Harold, F. M. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2798-2802). (i) Mutants that lack the ATPase also lack KtrII, and revertants recover both in parallel. (ii) KtrII and the Na+-ATPase are induced in parallel when cells are grown on media rich in sodium, particularly under conditions that limit the generation of a proton potential. KtrII is not induced in response to K+ deprivation. We propose that the Na+-ATPase exchanges Na+ for K+ ions.     

Influence of ouabain on Na+ and K+ concentration in haemolymph of Drosophila hydei and appearance of Malpighian tubules

After injection of ouabain into the body cavity, the Malpighian tubules of Drosophila larvae show a characteristic appearance which differs from the normal. The primary urine, which is mainly found in particles, is diluted and the concretions are washed away through the proximal part of the tubule and the ureter into the hindgut. In the haemolymph the Na⁺ and K⁺ concentrations change significantly. The K⁺ concentration increases rapidly to double the normal, while later the Na⁺ concentration rises up to 2·3 times the normal. Water movements are not the cause of the concentration changes because the quotient $\text{Na}\text{K}$ varies widely. Thus primary ion regulation mechanisms are influenced in the insect body by application of g-strophanthin. This is evidence for the existence of a ouabain-sensitive ATPase, which is decisively involved in the ion transport mechanisms in the insect body.     

Activity coefficients of intracellular Na+ and K+ during development of frog oocytes

Summary The chemical activities, (a), of Na⁺ and K⁺ were determined in large mature and in small immature frog oocytes, using open-tipped micropipettes and ionselective microelectrodes. The average chemical concentrations,c, of Na⁺ and K⁺ were determined by spectrophotometry and by electron probe X-ray microanalysis. The apparent activity coefficient (^app) was calculated for each ion as the ratio,a/c.With development, (a _Na/a _K) decreased four to fivefold and (c _Na/c _K) increased six to sevenfold. In the large mature oocytes, _Na ^app was measured to be 0.08±0.02 and _K ^app lay within the range 1.15±0.03 to 1.29±0.04, constituting the smallest value for Na⁺ and largest value for K⁺, respectively, thus far reported. This intracellular value of _K ^app was substantially greater than the activity coefficient of K⁺ in the external medium (0.76). The data suggest that the inequality of _Na ^app and _K ^app in this and probably other cells reflects the development of subcellular compartmentalization of ions. Possible intracellular sites of ionic compartmentalization are considered.