Sarcolemmal cation channels and exchangers modify the increase in intracellular calcium in cardiomyocytes on inhibiting Na+-K+-ATPase

Although inhibition of the sarcolemmal (SL) Na(+)-K(+)-ATPase is known to cause an increase in the intracellular concentration of Ca(2+) (Ca(2+)](i)) by stimulating the SL Na(+)/Ca(2+) exchanger (NCX), the involvement of other SL sites in inducing this increase in Ca(2+)](i) is not fully understood. Isolated rat cardiomyocytes were treated with or without different agents that modify Ca(2+) movements by affecting various SL sites and were then exposed to ouabain. Ouabain was observed to increase the basal levels of both Ca(2+)](i) and intracellular Na(+) concentration (Na(+)](i)) as well as to augment the KCl-induced increases in both Ca(2+)](i) and Na(+)](i) in a concentration-dependent manner. The ouabain-induced changes in Na(+)](i) and Ca(2+)](i) were attenuated by treatment with inhibitors of SL Na(+)/H(+) exchanger and SL Na(+) channels. Both the ouabain-induced increase in basal Ca(2+)](i) and augmentation of the KCl response were markedly decreased when cardiomyocytes were exposed to 0-10 mM Na(+). Inhibitors of SL NCX depressed but decreasing extracellular Na(+) from 105-35 mM augmented the ouabain-induced increase in basal Ca(2+)](i) and the KCl response. Not only was the increase in Ca(2+)](i) by ouabain dependent on the extracellular Ca(2+) concentration, but it was also attenuated by inhibitors of SL L-type Ca(2+) channels and store-operated Ca(2+) channels (SOC). Unlike the SL L-type Ca(2+)-channel blocker, the blockers of SL Na(+) channel and SL SOC, when used in combination with SL NCX inhibitor, showed additive effects in reducing the ouabain-induced increase in basal Ca(2+)](i). These results support the view that in addition to SL NCX, SL L-type Ca(2+) channels and SL SOC may be involved in raising Ca(2+)](i) on inhibition of the SL Na(+)-K(+)-ATPase by ouabain. Furthermore, both SL Na(+)/H(+) exchanger and Na(+) channels play a critical role in the ouabain-induced Ca(2+) increase in cardiomyocytes.