1029-21 Cytokines and the Nitric Oxide Synthase Inhibitor, L-NMMA, Alter Chronotropic Responsiveness of Cardiac Myocytes

We have previously reported negative inotropic effects of cytokines on papillary muscles that were blocked by the nitric oxide (NO) synthase inhibitor, L-NMMA. Others have reported an effect of NO on voltage-sensitive calcium channels (VSCC). The spontaneous beating rates of neonatal cardiac myocytes are regulated by VSCC. Accordingly, we studied the effects of these cytokines on the spontaneous beating rates of neonatal rat cardiac myocytes in the presence and absence of L-NMMASpontaneously beating neonatal rat cardiac myocytes were cultured in serum-free medium for 48 hrs alone, or in the presence of L-NMMA, tumor necrosis factor-α (TNF), interleukin-l-β (IL-l) or interleukin-6(IL-6) (N = 12, for each). Beating rates (BR) decreased from 155±4 BPM to 68±4 BPM after 48 hrs in serum-free medium alone. Treatment with L-NMMA, TNF, IL-l, or IL-6 maintained BR at 115±17, 110±4,117±17, 133±22 BPM at 48 hrs. IL-4 and IL-5 failed to similarly maintain BR at 48 hrs (63±7; 87±19). TNF, IL-l and IL-6 significantly blunted the positive chronotropic effect of the β-adrenergic agonist isoproterenol (10-9 -10-7M) (p<0.05; ANOVA; N=12). L-NMMA did not reverse these chronotropic effects of cytokines. These results support an NO-independent chronotropic effect of cytokines on neonatal cardiac myocytes. Cytokines and L-NMMA may regulate a shared signal transduction pathway involving VSCC