Pinacidil, a KATP channel opener, stimulates cardiac Na+/Ca2+ exchanger function through the NO/cGMP/PKG signaling pathway in guinea pig cardiac ventricular myocytes

Pinacidil, a nonselective ATP-sensitive K+ (KATP) channel opener, has cardioprotective effects for hypertension, ischemia/reperfusion injury, and arrhythmia. This agent abolishes early afterdepolarizations, delayed afterdepolarizations (DADs), and abnormal automaticity in canine cardiac ventricular myocytes. DADs are well known to be caused by the Na+/Ca2+ exchange current (I-NCX). In this study, we used the whole-cell patch-clamp technique and Fura-2/AM (Ca2+-indicator) method to investigate the effect of pinacidil on I-NCX in isolated guinea pig cardiac ventricular myocytes. In the patch-clamp study, pinacidil enhanced I-NCX in a concentration-dependent manner. The half-maximal effective concentration values were 23.5 and 23.0 mu M for the Ca2+ entry (outward) and Ca2+ exit (inward) components of I-NCX, respectively. The pinacidil-induced I-NCX increase was blocked by L-NAME, a nitric oxide (NO) synthase inhibitor, by ODQ, a soluble guanylate cyclase inhibitor, and by KT5823, a cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG) inhibitor, but not by N-2-mercaptopropyonyl glycine (MPG), a reactive oxygen species (ROS) scavenger. Glibenclamide, a nonselective KATP channel inhibitor, blocked the pinacidil-induced I-NCX increase, while 5-HD, a selective mitochondria KATP channel inhibitor, did not. In the Fura-2/AM study pinacidil also enhanced intracellular Ca2+ concentration, which was inhibited by L-NAME, ODQ, KT5823, and glibenclamide, but not by MPG and 5-HD. Sildenafil, a phosphodiesterase 5 inhibitor, increased further the pinacidil-induced I-NCX increase. Sodium nitroprusside, a NO donor, also increased I-NCX. In conclusion, pinacidil may stimulate cardiac Na+/Ca2+ exchanger (NCX1) by opening plasma membrane KATP channels and activating the NO/cGMP/PKG signaling pathway.