Development of pump electrogenesis in hypokalaemic rat muscle

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Abstract
In hypokalaemic rats maintained on a potassium deficient diets for 10–50 days, the isolated ‘Na-loaded’ and ‘K-depleted’ (‘Na-rich’) muscle fibers showed the membrane potential less than −115 mV in ‘fresh’ muscles of normal rats in K+-free Krebs solution. Upon adding 5 mM K+ to the K+-free medium bathing the soleus muscles, the measured potentials of ‘Na-rich’ muscles always exceeded the membrane potentials of ‘fresh’ muscles in 5 mM K+. The hyperpolarization was dependent on the amount of intracellular Na+ concentration ([Na]i) accumulated during the potassium deficiency. The electrogenic Na-pump was activated by an increase of [Na]i of less than 5 mM. Further increases in [Na]i resulted in increases in membrane potential which appeared to approach a limit at [Na]i levels higher than 65 mM.