Regulation of the Transient Outward K Current by Ca/Calmodulin-Dependent Protein Kinases II in Human Atrial Myocytes

Ca/calmodulin-dependent protein kinases II (CaMKII) have important functions in regulating cardiac excitability and contractility. In the present study, we examined whether CaMKII regulated the transient outward K current (Ito) in whole-cell patch-clamped human atrial myocytes. We found that a specific CaMKII inhibitor, KN-93 (20 mmol/L), but not its inactive analog, KN-92, accelerated the inactivation of Ito (tfast: 66.964.4 versus 43.064.4 ms, n535; P,0.0001) and inhibited its maintained component (at 160 mV, 4.960.4 versus 2.860.4 pA/pF, n535; P,0.0001), leading to an increase in the extent of its inactivation. Similar effects were observed by dialyzing cells with a peptide corresponding to CaMKII residues 281 to 309 or with autocamtide-2–related inhibitory peptide and by external application of the calmodulin inhibitor calmidazolium, which also suppressed the effects of KN-93. Furthermore, the phosphatase inhibitor okadaic acid (500 nmol/L) slowed Ito inactivation, increased Isus, and inhibited the effects of KN-93. Changes in [Ca]i by dialyzing cells with '30 nmol/L Ca 21 or by using the fast Ca buffer BAPTA had opposite effects on Ito. In BAPTA-loaded myocytes, Ito was less sensitive to KN-93. In myocytes from patients in chronic atrial fibrillation, characterized by a prominent Isus, KN-93 still increased the extent of inactivation of Ito. Western blot analysis of atrial samples showed that d-CaMKII expression was enhanced during chronic atrial fibrillation. In conclusion, CaMKII control the extent of inactivation of Ito in human atrial myocytes, a process that could contribute to Ito alterations observed during chronic atrial fibrillation. (Circ Res. 1999;85:810-819.)