We have recently shown that beta-adrenergic agonists enhance the cardiac sodium current (INa) in rabbits through dual G-protein regulatory pathways. To determine if muscarinic cholinergic receptor stimulation can also modulate INa, we studied the effects of acetylcholine (ACh) and carbachol on INa in enzymatically dispersed rabbit ventricular myocytes. Whole-cell patch-clamp experiments done at room temperature using 20 mM [Na+]o showed that 100 nM isoproterenol increased INa and accelerated current decay as previously described. ACh (1 microM) or carbachol (1 microM) significantly reversed the stimulatory isoproterenol effects at test potentials throughout the INa activation range and at holding potentials negative to -80 mV. This effect was completely inhibited by atropine (1 microM) and was confirmed by studying single-channel INa from cell-attached patches. When INa was stimulated by forskolin (1 microM), carbachol (1 microM) significantly reversed the effect. The muscarinic-mediated inhibition of INa was inhibited by pertussis toxin (0.1 or 1.0 microgram/ml) incubation (12-15 hours), suggesting that the effect was inhibitory G-protein dependent. Further investigation of the ACh inhibitory mechanism revealed that ACh alone had no effect on INa and that when cells were dialyzed with cAMP (5 microM), ACh failed to inhibit INa. Furthermore, cGMP failed to inhibit the effect of isoproterenol on INa. These data suggest that ACh acts at or proximal to adenylate cyclase stimulation. Thus, rabbit cardiac Na+ channels are regulated by muscarinic agonists in a fashion similar to cardiac Ca2+ channels.