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 resp onse 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 w hether the indirect negative inotropic effect of M-cholinoceptor stimu lation in human myocardium is in part due to an effect of endogenous N O. Therefore, the effect of carbachol was studied under control condit ions and during inhibition of NO-synthase by pretreatment with N-G-mon omethyl-L-arginine (NMMA). Functional experiments were performed in is olated, electrically driven (1 Hz, 37 degrees C) left ventricular papi llary muscle strips of human myocardium. Since cytokines have been rep orted to be increased in the serum of patients with heart failure and could induce NO-synthase activity in failing myocardium, we compared s amples from nonfailing and terminally failing (classified as NYHA IV) hearts. The indirect negative inotropic effect of carbachol (10 mu mol /l) was studied in the presence of the beta-adrenoceptor agonist isopr enaline (0.03 mu mol/l). After stimulation with isoprenaline, carbacho l significantly (P < 0.05) reduced force of contraction. This effect w as diminished in failing myocardium compared to nonfailing, probably d ue to the diminished inotropic response most likely due to the lower c AMP levels in response to beta-adrenoceptor stimulation in the former condition. Pretreatment with NMMA (100 mu mol/l) altered the antiadren ergic effect of carbachol neither in nonfailing nor in failing prepara tions. Furthermore, inhibition of guanylyl cyclase, the target enzyme of NO, by preincubation with methylene blue (10 mu mol/l) for 30 min h ad 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 pr esent study provides evidence that the indirect negative inotropic eff ect of M-cholinoceptor agonists is not due to an effect of NO in the h uman myocardium. Furthermore, the well known enhancement of cCMP in re sponse to M-cholinoceptor stimulation appears not to be involved in th is antiadrenergic effect.