Evidence against a role of nitric oxide in the indirect negative inotropic-effect of M-cholinoceptor stimulation in human ventricular myocardium

Nitric oxide (NO) has been reported to mediate several effects in response to muscarinic cholinergic stimulation in cardiovascular tissues. Recently, an attenuation of guinea pig cardiac myocyte contraction by NO has been described. The aim of the present study was to determine whether the indirect negative inotropic effect of M-cholinoceptor stimulation in human myocardium is in part due to an effect of endogenous NO. Therefore, the effect of carbachol was studied under control conditions and during inhibition of NO-synthase by pretreatment with N^G-monomethyl-l-arginine (NMMA). Functional experiments were performed in isolated, electrically driven (1 Hz, 37°C) left ventricular papillary muscle strips of human myocardium. Since cytokines have been reported to be increased in the serum of patients with heart failure and could induce NO-synthase activity in failing myocardium, we compared samples from nonfailing and terminally failing (classified as NYHA IV) hearts. The indirect negative inotropic effect of carbachol (10 mol/l) was studied in the presence of the \-adrenoceptor agonist isoprenaline (0.03 mol/l).After stimulation with isoprenaline, carbachol significantly (P < 0.05) reduced force of contraction. This effect was diminished in failing myocardium compared to nonfailing, probably due to the diminished inotropic response most likely due to the lower cAMP levels in response to \-adrenoceptor stimulation in the former condition. Pretreatment with NMMA (100 mol/l) altered the antiadrenergic effect of carbachol neither in nonfailing nor in failing preparations. Furthermore, inhibition of guanylyl cyclase, the target enzyme of NO, by preincubation with methylene blue (10 mol/l) for 30 min had no effect on the carbachol-induced decrease in force of contraction. Basal force of contraction, as well as the positive inotropic effect of isoprenaline remained unaffected by NMMA or methylene blue.The present study provides evidence that the indirect negative inotropic effect of M-cholinoceptor agonists is not due to an effect of NO in the human myocardium. Furthermore, the well known enhancement of cGMP in response to M-cholinoceptor stimulation appears not to be involved in this antiadrenergic effect.