Interaction of sodium with the sarcolemmal calcium system

Abstract

Sarcolemma isolated from guinea pig heart binds Ca in an ATP-dependent manner. Na ions decrease the total amount of Ca bound by the membranes. ATP-dependent Ca binding is more sensitive to Na than the non-ATP-dependent Ca binding. The ATPase active during Ca binding is affected by Na ions to the same extent as the ATP-dependent Ca binding process. The inhibition of the Ca binding process and of ATPase activity by Na was more pronounced when the membranes were preincubated with Na. The effect of Na on Ca binding is dependent on the time of contact between Na and the membranes and the concentration of Na. The effect of Na on the Ca binding system in the sarcolemma may be a link between the inhibition of Na+K+-ATPase (EC 3.6.1.3) by cardiac glycosides and the subsequent increase in intracellular Ca.