Cardiac neuronal nitric oxide synthase isoform regulates myocardial contraction and calcium handling

A neuronal isoform of nitric oxide synthase ( nNOS) has recently been located to the cardiac sarcoplasmic reticulum (SR). Subcellular localization of a constitutive NOS in the proximity of an activating source of Ca2+ suggests that cardiac nNOS-derived NO may regulate contraction by exerting a highly specific and localized action on ion channels/transporters involved in Ca2+ cycling. To test this hypothesis, we have investigated myocardial Ca2+ handling and contractility in nNOS knockout mice ( nNOS(-/-)) and in control mice ( C) after acute nNOS inhibition with 100 mumol/L L-VNIO. nNOS gene disruption or L-VNIO increased basal contraction both in left ventricular (LV) myocytes (steady-state cell shortening 10.3 +/- 0.6% in nNOS(-/-) versus 8.1 +/- 0.5% in C; P < 0.05) and in vivo ( LV ejection fraction 53.5 +/- 2.7 in nNOS(-/-) versus 44.9 +/- 1.5% in C; P < 0.05). nNOS disruption increased I-Ca density ( in pA/pF, at 0 mV, -11.4 +/- 0.5 in nNOS(-/-) versus -9.1 +/- 0.5 in C; P < 0.05) and prolonged the slow time constant of inactivation of ICa by 38% ( P < 0.05), leading to an increased Ca2+ influx and a greater SR load in nNOS(-/-) myocytes ( in pC/pF, 0.78 +/- 0.04 in nNOS(-/-) versus 0.64 +/- 0.03 in C; P < 0.05). Consistent with these data, [Ca2+](i) transient (indo-1) peak amplitude was greater in nNOS(-/-) myocytes (410/495 ratio 0.34 +/- 0.01 in nNOS(-/-) versus 0.31 +/- 0.01 in C; P < 0.05). These findings have uncovered a novel mechanism by which intracellular Ca2+ is regulated in LV myocytes and indicate that nNOS is an important determinant of basal contractility in the mammalian myocardium. The full text of this article is available at http://www.circresaha.org.