The Regulation of the Na+ ‐Ca2+ Exchanger of Heart Sarcolemma

Abstract

The Na+/Ca 2+ ‐exchange of calf‐heart sarcolemma is activated by a treatment with ATP, Mg 2+, and Ca 2+, and deactivated by a treatment with phosphorylase phosphatase. The effect of th e latter can be substituted by a treatment with Mg 2+, Ca 2+, and calmodulin. the activating treatment does not require added calmodulin, but is inhibited by calmodulin antagonists. Evidently, engdogenous calmodulin is required and sufficient. Activation is half‐maximal at about 2μM Ca 2+. Added clamodulin, however, decreases the Km (Ca 2+) of the activating process to about 0.8 μM. Deactivation is half‐maximal, at optimal calmodulin concentrations, at about 1.5 μM Ca 2+. Experiments with adenosine 5′‐[γ‐thio]triphosphate have shown that the activating treatment is mediated by a kinase and the deactivating treatment by a phosphatase. The concerted operation of the two enzymes is made possible by their different Ca 2+ affinity. At saturating Ca 2+ concentrations, the level of ATP may also influence the balance of the two enzymes.