Previous studies have shown that muscarinic inhibition of cardiac contractility is mediated by either activation of nitric oxide (NO)/guanosine 3',5'-cyclic monophosphate (cGMP) pathway or stimulation of inhibitory G protein (G(i)). However, it still remains controversial as to whether NO/cGMP pathway or G(i) protein or both mediate(s) the negative inotropic effect of muscarinic agonists in adult ventricular myocytes. In the present study that involves the use of adult rat ventricular myocytes, the muscarinic agonist, carbachol, inhibited beta-adrenergic (isoproterenol) stimulation of contractility (cell shortening) by 82% and increased cGMP levels by 49% within 6 min. Pretreatment of myocytes with soluble guanylyl cyclase inhibitor (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, ODQ) or NO synthase inhibitor (N(G)-monomethyl-L-arginine, L-NMMA) for 30 min blocked carbachol-induced increases in cGMP levels. However, neither ODQ nor L-NMMA pretreatment had any effect on carbachol inhibition of isoproterenol-induced contractility. In addition, carbachol did not attenuate increases in myocyte contractility induced by forskolin (a direct activator of adenylyl cyclase) or 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (a cell-permeable cAMP analog which activates cAMP-dependent protein kinase). Pretreatment of myocytes with G(i) protein inhibitor, pertussis toxin (PTX, 1 microg/ml), for 18-20 h abolished carbachol inhibition of isoproterenol-induced contractility. Furthermore, in ventricular myocytes isolated 3 days after in vivo treatment of rats with PTX (3 microg/100 g, i.p.), there was a complete loss of the negative inotropic effect of carbachol. These data indicate that pertussis toxin-sensitive G protein but not NO/cGMP pathway is required for muscarinic inhibition of beta-adrenoceptor-mediated increases in contractility in adult rat ventricular myocytes.