Prolonged Action Potential and After depolarizations Are Not due to Changes in Potassium Currents in NOS3 Knockout Ventricular Myocytes

Abstract

Ventricular myocytes deficient in endothelial nitric oxide synthase (NOS3−/−) exhibit prolonged action potential (AP) duration and enhanced spontaneous activity (early and delayed afterdepolarizations) during β-adrenergic (β-AR) stimulation. Studies have shown that nitric oxide is able to regulate various K+ channels. Our objective was to examine if NOS3−/− myocytes had altered K+ currents. APs, transient outward (I to), sustained (I Ksus), and inward rectifier (I K1) K+ currents were measured in NOS3−/− and wild-type (WT) myocytes. During β-AR stimulation, AP duration (measured as 90% repolarization-APD90) was prolonged in NOS3−/− compared to WT myocytes. Nevertheless, we did not observe differences in I to, I Ksus, or I K1 between WT and NOS3−/− myocytes. Our previous work showed that NOS3−/− myocytes had a greater Ca2+ influx via L-type Ca2+ channels with β-AR stimulation. Thus, we measured β-AR-stimulated SR Ca2+ load and found a greater increase in NOS3−/− versus WT myocytes. Hence, our data suggest that the prolonged AP in NOS3−/− myocytes is not due to changes in I to, I Ksus, or I K1. Furthermore, the increase in spontaneous activity in NOS3−/− myocytes may be due to a greater increase in SR Ca2+ load. This may have important implications for heart failure patients, where arrhythmias are increased and NOS3 expression is decreased.