Abstract 1536: The CaMKII-Ito Channel Functional Unit in Cardiomyocytes
Background: The reduction of transient outward current Ca2+/(Ito) and the increase in Calmodulin-dependent kinase II (CaMKII) activity are general features of ventricular myocytes in failing heart. We recently reported that blocking of Ito significantly facilitates L-type Ca2+ current (ICa) in ventricular myocytes which is mediated by CaMKII activation. Based on these results, we proposed a hypothesis that a significant amount of inactive CaMKII forms molecular complex with Ito channel subunits in cardiomyocytes as a CaMKII reservoir. The coupled CaMKII will be dissociated from the complex and subsequently activated when Ito channel blockers bind to the channel or when Ito channel expression is reduced, causing increased CaMKII activation. To test this hypothesis, we performed co-immunoprecipitation assay to demonstrate the functional association of Ito channel subunit Kv4.3 and CaMKII in cultured HEK293 cells and in ventricular myocytes.
Results: Co-purification of Kv4.3 and CaMKII was detected in HEK293 cell expression system and in cardiomyocytes. Moreover, we found that Ito channel blocker 4-AP induced the dissociation of CaMKII from the complex and caused a significant increase in the autonomous CaMKII activity without changing the total CaMKII expression, suggesting an activation of CaMKII that was displaced from Kv4.3 channels by 4-AP. More importantly, over-expression of Ito channel Kv4.3 in ventricular myocytes significantly reduced the autonomous CaMKII activity without changing the total CaMKII expression.
Conclusion: Our data suggest that Ito channel Kv4.3 serves as a functional CaMKII reservoir in ventricular myocytes. These findings uncovered an important mechanism that implicates Ito channel regulation in the activation of CaMKII in heart, an important molecule contributing to the development of cardiac hypertrophy, heart failure, and lethal ventricular arrhythmias.