Chamber-specific regulation of myofilament Ca2+ sensitivity by nNOSβ via troponin I phosphorylation: Predominance in the right ventricle

Neuronal nitric oxide (NO) synthase (nNOS) is an important regulator of excitation–contraction coupling in cardiomyocytes, yet its chamber-specific role in right ventricular cardiomyocytes (RVCMs) remains incompletely understood. In this study, we compared the effects of nNOS inhibition on contractile function, Ca2+ signaling, and myofilament-associated regulation in RVCMs and left ventricular cardiomyocytes (LVCMs). Pharmacological inhibition of nNOS with S-methyl-L-thiocitrulline (SMTC) enhanced sarcomere shortening and impaired relaxation in both cell types, with markedly greater effects observed in RVCMs. In contrast, SMTC-induced changes in Ca2+ transients and L-type Ca2+ current were more pronounced in LVCMs than in RVCMs, indicating a dissociation between Ca2+ signaling and mechanical responses in the SMTC-treated RVCMs. Analysis of dynamic Ca2+-sarcomere length coupling indicated that nNOS inhibition in RVCMs preferentially altered the relationship between intracellular Ca2+ signaling and sarcomere response, particularly during relaxation, rather than simply reflecting changes in Ca2+ magnitude. Consistent with this interpretation, inhibition of nNOS reduced cardiac troponin I (cTnI) Ser23/24 phosphorylation and was associated with altered endogenous Ca2+ buffering properties in RVCMs. Moreover, myofilament-enriched fractions from RVCMs exhibited higher levels of Ser1417-phosphorylated nNOSβ. Collectively, these data indicate that myofilament-associated nNOSβ predominantly regulates RV contractility through cTnI phosphorylation-mediated Ca2+ desensitization.