Activation of nitric oxide synthase (NOS3) by mechanical activity alters contractile activity in a Ca2+-independent manner in cardiac myocytes: Role of troponin I phosphorylation

Cardiac myocytes express the calcium-responsive nitric oxide synthase (eNOS or NOS3). Activation of NOS3 by increased intracellular Ca2+ concentration , [Ca2+](i), has been demonstrated to decrease myocyte contractile responsi veness, although this appears to occur in a Ca2+-independent manner. Theref ore, the aim of this study was to examine the possibility that contractile activity could be modulated by an NO-mediated alteration in the phosphoryla tion status of troponin I, which is known to alter myofilament sensitivity to Ca2+. During pacing at 3 Hz, P-32-labeled myocytes exhibited a 59 +/- 9% increase in TnI phosphorylation compared to quiescent cells (p < 0.05), an effect that was significantly attenuated by either methylene blue or L-nit roarginine (L-NA), While exposure to methylene blue significantly increased the contractile amplitude of paced myocytes, this was not accompanied by a n alteration in intracellular Ca2+. These data indicate that the NO-mediate d effects on myocyte contraction may be elicited through an alteration in m yofilament Ca2+ sensitivity that results from an alteration in the phosphor ylation status of troponin I. (C) 1999 Academic Press.