Cardiac Nitric Oxide Synthase 1 Regulates Basal and β-Adrenergic Contractility in Murine Ventricular Myocytes

Background — Evidence indicates that myocardial NO production can modulate contractility, but the source of NO remains uncertain. Here, we investigated the role of a type 1 NO synthase isoform (NOS1), which has been recently localized to the cardiac sarcoplasmic reticulum, in the regulation of basal and β-adrenergic myocardial contraction. Methods and Results — Contraction was assessed in left ventricular myocytes isolated from mice with NOS1 gene disruption (NOS1 −/− mice) and their littermate controls (NOS1 +/+ mice) at 3 stimulation frequencies (1, 3, and 6 Hz) in basal conditions and during β-adrenergic stimulation with isoproterenol (2 nmol/L). In addition, we examined the effects of acute specific inhibition of NOS1 with vinyl- l - N -5-(1-imino-3-butenyl)- l -ornithine (L-VNIO, 500 μmol/L). NOS1 −/− myocytes exhibited greater contraction at all frequencies (percent cell shortening at 6 Hz, 10.7±0.92% in NOS1 −/− myocytes versus 7.21±0.8% in NOS1 +/+ myocytes; P <0.05) with a flat frequency-contraction relationship. Time to 50% relaxation was increased in NOS1 −/− myocytes at all frequencies (at 6 Hz, 26.53±1.4 ms in NOS1 −/− myocytes versus 21.27±1.3 ms in NOS1 +/+ myocytes; P <0.05). L-VNIO prolonged time to 50% relaxation at all frequencies (at 6 Hz, 21.28±1.7 ms in NOS1 +/+ myocytes versus 26.45±1.4 ms in NOS1 +/+ +L-VNIO myocytes; P <0.05) but did not significantly increase basal contraction. However, both NOS1 −/− myocytes and NOS1 +/+ myocytes treated with L-VNIO showed a greatly enhanced contraction in response to β-adrenergic stimulation (percent increase in contraction at 6 Hz, 25.2±10.8 in NOS1 +/+ myocytes, 68.2±11.2 in NOS1 −/− myocytes, and 65.1±13.2 in NOS1 +/+ +L-VNIO myocytes; P <0.05). Conclusions — NOS1 disruption enhances basal contraction and the inotropic response to β-adrenergic stimulation in murine ventricular myocytes. These findings indicate that cardiac NOS1-derived NO plays a significant role in the autocrine regulation of myocardial contractility.