Involvement of CaMKII in the modulation of I under oxidative stress in guinea pig sinoatrial node cells.

Our previous study found that Ca/calmodulin-dependent protein kinase II (CaMKII) potentiates the slow delayed rectifier K current (I) in sinoatrial node (SAN) pacemaker cells. Recently, oxidative activation of CaMKII has emerged as a major cause of SAN dysfunction; however, its correlation with I regulation remains unclear. In this study, we investigated the effect of hydrogen peroxide (HO) on I in SAN cells isolated from guinea pig heart. Whole-cell patch-clamp recordings were performed using an EGTA (5 mM) pipette solution to stabilize intracellular Ca levels (pCa 7). The results showed that 5 min of HO (100 μM) perfusion initiated an increase in I, which gradually increased to saturation (∼60.5 % enhancement from baseline to saturation) after 10 min of HO exposure. In contrast, I remained almost unchanged in the presence of catalase (1000 units mL). These observations were replicable in atrial and ventricular cardiomyocytes. HO failed to stimulate KCNQ1/KCNE1 currents in HEK and CHO cells expressing low CaMKII levels. In SAN cells, HO-induced I enhancement was strongly attenuated by intracellular dialysis with a lower Ca concentration (pCa 10) or by pretreatment with KN-93 (1 μM), suggesting that Ca/calmodulin binding to CaMKII is a prerequisite for CaMKII activation. Autocamtide-2 inhibitory peptide (AIP, 1 μM), an inhibitor of the catalytic domain of CaMKII, almost completely abolished the HO-induced potentiation of I. Taken together, these findings imply that HO enhances cardiac I through the oxidative activation of CaMKII.