Dual regulation of Ca²⁺/calmodulin-dependent kinase II activity by membrane voltage and by calcium influx

Calcium entry through voltage-gated Ca²⁺ channels is critical in cardiac excitation-contraction coupling and calcium metabolism. In this report, we demonstrate both spatially resolved and temporally distinct effects of Ca²⁺/ calmodulin-dependent protein kinase II (CaMKII) on L-type Ca²⁺ channel current (I_Ca) in rat cardiac myocytes. Either depolarization alone or calcium influx can increase the amplitude and slow the inactivation of I_Ca. The distinct voltage- and Ca²⁺-dependent effects persist with time constants of ≈1.7 sec and 9 sec, respectively. Both effects are completely abolished by a specific peptide inhibitor of CaMKII. This CaMKII inhibitor also suppresses the prolongation of I_Ca induced by depolarizing holding potentials. Furthermore, using an antibody specific for the autophosphorylated (activated) CaMKII, we find that this kinase is localized close to sarcolemmal membranes and that the profile of CaMKII activation correlates qualitatively with the changes in I_Ca under various conditions. Therefore, we conclude that the action of CaMKII on I_Ca is dually regulated by membrane depolarization and by Ca²⁺ influx; the latter directly activates CaMKII, whereas the former likely promotes the interaction between constitutive CaMKII and the membrane-channel proteins. These regulatory mechanisms provide positive-feedback control of Ca²⁺ channels and are probably important in the regulation of cardiac contractility and other intracellular Ca²⁺-regulated processes.