Neuronal nitric oxide synthase regulation of calcium cycling in ventricular cardiomyocytes is independent of Ca_v1.2 channel modulation under basal conditions

Neuronal nitric oxide synthase (nNOS) is considered a regulator of Ca_v1.2 L-type Ca²⁺ channels and downstream Ca²⁺ cycling in the heart. The commonest view is that nitric oxide (NO), generated by nNOS activity in cardiomyocytes, reduces the currents through Ca_v1.2 channels. This gives rise to a diminished Ca²⁺ release from the sarcoplasmic reticulum, and finally reduced contractility. Here, we report that nNOS inhibitor substances significantly increase intracellular Ca²⁺ transients in ventricular cardiomyocytes derived from adult mouse and rat hearts. This is consistent with an inhibitory effect of nNOS/NO activity on Ca²⁺ cycling and contractility. Whole cell currents through L-type Ca²⁺ 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 Ca_v1.2 channel protein reconstituted in artificial liposomes. These results indicate that nNOS/NO activity does not directly modify Ca_v1.2 channel function. We conclude that—against the currently prevailing view—basal Ca_v1.2 channel activity in ventricular cardiomyocytes is not substantially regulated by nNOS activity and NO. Hence, nNOS/NO inhibition of Ca²⁺ cycling and contractility occurs independently of direct regulation of Ca_v1.2 channels by NO.