Neuronal nitric oxide synthase regulation of calcium cycling in ventricular cardiomyocytes is independent of Ca1.2 channel modulation under basal conditions.

Neuronal nitric oxide synthase (nNOS) is considered a regulator of Ca1.2 L-type Ca channels and downstream Ca cycling in the heart. The commonest view is that nitric oxide (NO), generated by nNOS activity in cardiomyocytes, reduces the currents through Ca1.2 channels. This gives rise to a diminished Ca release from the sarcoplasmic reticulum, and finally reduced contractility. Here, we report that nNOS inhibitor substances significantly increase intracellular Ca transients in ventricular cardiomyocytes derived from adult mouse and rat hearts. This is consistent with an inhibitory effect of nNOS/NO activity on Ca cycling and contractility. Whole cell currents through L-type Ca channels in rodent myocytes, on the other hand, were not substantially affected by the application of various NOS inhibitors, or application of a NO donor substance. Moreover, the presence of NO donors had no effect on the single-channel open probability of purified human Ca1.2 channel protein reconstituted in artificial liposomes. These results indicate that nNOS/NO activity does not directly modify Ca1.2 channel function. We conclude that-against the currently prevailing view-basal Ca1.2 channel activity in ventricular cardiomyocytes is not substantially regulated by nNOS activity and NO. Hence, nNOS/NO inhibition of Ca cycling and contractility occurs independently of direct regulation of Ca1.2 channels by NO.