Ventricular Myocytes

We have recently shown that J3-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+L] showed that 100 nM isoproterenol increased INa and accelerated current decay as previously described. ACh (1 ,uM) or carbachol (1 ,uM) 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 uM) and was confirmed by studying singlechannel INa from cell-attached patches. When INa was stimulated by forskolin (1 ,M), carbachol (1 ,uM) significantly reversed the effect. The muscarinic-mediated inhibition Of INa was inhibited by pertussis toxin (0.1 or 1.0 gg/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