CVIC Lab.CardioVascular-Ion Channel Laboratory

Neuronal nitric oxide synthase (nNOS) is important in cardiac protection in diseased heart. Recently, we have reported that nNOS is associated with myofilament Ca²⁺ desensitization in cardiac myocytes from hypertensive rats. So far, the effect of myofilament Ca²⁺ desensitization or nNOS on L-type Ca²⁺ channel activity (I_Ca) in cardiac myocyte is unclear. Here, we examined nNOS regulation of I_Ca in left ventricular (LV) myocytes from sham and angiotensin II (Ang II)-induced hypertensive rats.Our results showed that basal I_Ca was not different between sham and hypertension (from -60 to +40mV, 0.1Hz). S-methyl-l-thiocitrulline (SMTC), a selective nNOS inhibitor, increased peak I_Ca similarly in both groups. However, chelation of intracellular Ca²⁺ [Ca²⁺]_i with BAPTA increased I_Ca and abolished SMTC-augmentation of I_Ca only in hypertension. Myofilament Ca²⁺ desensitization with butanedione monoxime (BDM), a myosin ATPase inhibitor, decreased I_Ca in both groups but to a greater extent in hypertension. Intracellular BAPTA or nNOS inhibition reinstated I_Ca in the presence of BDM to the basal level, suggesting Ca²⁺-dependent inactivation of I_Ca by nNOS and greater vulnerability in hypertension. Increasing stimulation frequencies (2, 4 and 8Hz) attenuated myofilament Ca²⁺ sensitivity in sham and reduced peak I_Ca in both groups. Nevertheless, SMTC or BAPTA exerted no effect on I_Ca at high frequencies in either group.These results suggest that nNOS attenuates I_Ca via Ca²⁺-dependent mechanism and the vulnerability is greater in hypertension subject to myofilament Ca²⁺ desensitization. nNOS or [Ca²⁺]_i does not affect I_Ca at high stimulation frequencies. The results were recapitulated with computer simulation.