Development of pump electrogenesis in hypokalaemic rat muscle

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.