Calmodulin in the regulation of calcium fluxes in cardiac sarcolemma.

Three systems mediate the fluxes of calcium across heart sarcolemma: the slow calcium channel (influx), the ATP-dependent calcium pump (efflux), and the Na+/Ca2+ exchanger (efflux, but possibly also influx). Calmodulin regulates the pumping ATPase by direct interaction and also by activating a protein kinase. The Na+/Ca2+ exchanger is modulated by calmodulin via a phosphorylation-dephosphorylation cycle. Both the kinase and the phosphatase are membrane-bound and calmodulin-sensitive. The kinase has higher Ca2+ affinity than the phosphatase.