Transient‐Outward K + Channel Inhibition Facilitates L‐Type Ca 2+ Current in Heart

Background: Transient outward current (I to ) and L‐type calcium current (I Ca ) are important repolarization currents in cardiac myocytes. These two currents often undergo disease‐related remodeling while other currents are spared, suggesting a functional coupling between them. Here, we investigated the effects of I to channel blockers, 4‐aminopyridine (4‐AP) and heteropodatoxin‐2 (HpTx2), on I Ca in cardiac ventricular myocytes. Methods and Results: I Ca was recorded in enzymatically dissociated mouse and guinea pig ventricular myocytes using the whole‐cell voltage clamp method. In mouse ventricular myocytes, 4‐AP (2 mM) significantly facilitated I Ca by increasing current amplitude and slowing inactivation. These effects were not voltage‐dependent. Similar facilitating effects were seen when equimolar Ba 2+ was substituted for external Ca 2+ , indicating that Ca 2+ influx is not required. Measurements of Ca 2+ /calmodulin‐dependent protein kinase (CaMKII) activity revealed significant increases in cells treated with 4‐AP. Pretreatment of cells with 10μM KN93, a specific inhibitor of CaMKII, abolished the effects of 4‐AP on I Ca. To test the requirement of I to , we studied guinea pig ventricular myocytes, which do not express I to channels. In these cells, 2 mM 4‐AP had no effect on I Ca amplitude or kinetics. In both cell types, Ca 2+ ‐induced I Ca facilitation, a CaMKII‐dependent process, was observed. However, 4‐AP abolished Ca 2+ ‐induced I Ca facilitation exclusively in mouse ventricular myocytes. Conclusion: 4‐AP, an I to blocker, facilitates L‐type Ca 2+ current through a mechanism involving the I to channel and CaMKII activation. These data indicate a functional association of I Ca and I to in cardiac myocytes.