Adaptation of the cardiac muscle sodium pump to chronic potassium deficiency.

During chronic dietary K+ depletion in rabbits there is a significant reduction of intracellular K+ concentration [( K+]i) in skeletal muscle, but not in the heart. After 1 week of subsequent K+ repletion both plasma [K+] and skeletal muscle [K+]i had returned to control levels. Cardiac muscle on the other hand showed a significant increase in [K+]i above control. It has been suggested that the heart is protected during K+ depletion by some modification to the sarcolemmal Na+ pump. The Na+ pump density was estimated in membrane preparations from normal and K+ depleted rabbits by the specific binding of 3H-ouabain. 3H-ouabain binding in cardiac muscle preparations was increased significantly after 2 weeks of K+ depletion, and remained so for a further 2 weeks of depletion. There was no significant change in 3H-ouabain binding to skeletal muscle preparations after 4 weeks of K+ depletion. After 1 week of subsequent K+ repletion 3H-ouabain binding was still increased in cardiac muscle preparations, corresponding to the overshoot in cardiac muscle [K+]i. These results support the hypothesis that cardiac muscle is protected against K+ loss during chronic K+ depletion by an adaptive increase in Na+ pump density.