Role of nitric oxide in Ca2+ sensitivity of the slowly activating delayed rectifier K+ current in cardiac myocytes

Sarcolemmal Ca(2+)entry is a vital step for contraction of cardiomyocytes, but Ca2+ overload is harmful and may trigger arrhythmias and/or apoptosis. To maintain the amount of Ca2+ entry within an appropriate range, cardiomyocytes have feedback systems that tightly regulate ion channel activities in response to the changes in intracellular Ca2+ concentration ([Ca2+](i)), thereby regulating Ca2+ entry. In guinea pig ventricular myocytes, Ca2+ ionophore, A23187, induced suppression of the L-type Ca2+ currents (I-Ca,I-L) and enhancement of the slowly activating delayed rectifier K+ currents (I-Ks). At a low stimulation rate, I-Ca,I-L suppression and I-Ks enhancement contributed to the A23187-induced APD shortening with a similar magnitude, whereas at a high stimulation rate, I-Ks enhancement dominantly contributed to APD shortening. I-Ks enhancement induced by A23187 was attributable to actions of nitric oxide (NO), because they were inhibited by an inhibitor of NO synthase ( NOS) and by a NO scavenger. A23187-induced alterations of APD and I-Ks were strongly suppressed by a NOS3 inhibitor, but barely affected by a NOS1 inhibitor, suggesting that NOS3 was responsible for NO release in this phenomenon. Inhibition of calmodulin (CaM), but not Akt, blocked the enhancement of I-Ks by A23187. Thus, CaM-dependent NOS3 activation confers the selective Ca2+-sensitivity on I-Ks. Ca2+-induced I-Ks enhancement and resultant APD shortening potentially act as a physiological regulatory mechanism of Ca2+ recycling, because they were observed at a physiological range of [Ca2+](i) in cardiac myocytes and are induced by physiologically relevant Ca2+ loading, such as digitalis application and rise in extracellular Ca2+ concentration.