Binding of Ca2+ and Na+ to Sarcolemmal Membranes: Relation to Control of Myocardial Contractility

Studies are presented that relate the calcium binding of isolated sarcolemmal membranes to myocardial contractility. The contractile strength of the perfused rabbit interventricular septum as a function of perfusate calcium concentration is compared with calcium bound to isolated rabbit sarcolemma at the same calcium concentrations. If the calcium-binding incubation medium contains 140 mM sodium chloride, the shapes of these two curves are identical. In another experiment, sarcolemmal calcium-binding data are compared with the results of Tillisch et al. (J. Mol. Cell. Cardiol. 11:137-148, 1979), who studied the response of rabbit papillary muscles to changes in the perfusate sodium concentration. Again a positive correlation between calcium bound and force developed is obtained. The results of experiments, using the isotope 22Na on the binding of sodium to isolated sarcolemmal membranes were subjected to Scatchard analysis. This revealed a single type of sodium receptor. The affinity constant for sodium binding is 110 M-1 and calcium appears to behave as a competitive inhibitor of sodium binding. The experimental results strongly imply a quantitative relationship between sarcolemmal calcium binding and myocardial contractility.