Binding of cardioglobulin-C-Ca45 to cardiac muscle and release by cardioglobulin-A

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Abstract
The cardiotonic action of cardioglobulin on the frog heart is a result of the interaction of three globulins, cardioglobulin-A, -B, and -C. Cardioglobulin-B binds to the heart. It has been shown that cardioglobulin-C binds to a cardioglobulin-B heart, but not to a heart without cardioglobulin-B. Cardioglobulin-C binding has been demonstrated both by biological activity and by persistence of cardioglobulin-C-Ca45 counts on the heart. The addition of cardioglobulin-A to a cardioglobulin-B-C heart causes the characteristic cardiotonic action of the system. Addition of cardioglobulin-A is associated with release of tracer from bound cardioglobulin-C-Ca45, measured as a decrease in heart radioactivity. The correlations between biological activity and localization of Ca45 of labeled cardioglobulin-C are consistent with the hypothesis that cardioglobulin action is associated with the release of cardioglobulin-C calcium into the cell. The amount of calcium involved has been estimated to be of the order of 10–4 µm for an 80-mg frog heart.