Abstract

In mammalian heart, vagal stimulation or the direct application of acetylcholine produces profound direct effects on the electro-physiologic characteristics of atrial myocytes. At the tissue level, these effects are observed as shortening of atrial action potential duration. Despite anatomic, biochemical, and physiologic evidence for significant vagal input to the mammalian ventricle, similar direct effects of acetylcholine on the ventricular action potential have been difficult to demonstrate. Chronic denervation via cervical vagotomy is one method that has been shown to render previously unresponsive ventricular myocytes sensitive to acetylcholine, but the molecular mechanism has not been defined. In the experiments described, selective cardiac para-sympathectomy was performed on mongrel dogs. Five to seven days after parasympathectomy, the dogs were sacrificed, electrophysiologic responses to acetylcholine were measured, and sarcolemmal vesicles were prepared. After parasympathectomy, ventricular myocytes were responsive to the effects of acetylcholine, manifested as shortening of the action potential duration. A quantitative and functional assessment of the transmembrane signalling mechanisms of the muscarinic receptor was carried out. After parasympathectomy, the density of muscarinic receptors in the sarcolemma was increased, compared with control ventricles. After parasympathectomy, ventricular sarcolemma displayed significant increases in both basal and oxotremorine-stimulated GTPase activity. ADP-ribosylation revealed significantly increased quantities of the pertussis toxin substrates Gi and Go. The quantity of ADP ribose incorporated was correlated with the increased level of GTPase activity in control and oxotremorine-stimulated membranes. Quantitation of the alpha and beta gamma subunits of the guanine nucleotide-binding proteins by immunoblot confirmed the increase in density of inhibitory guanine nucleotide-binding proteins following parasympathectomy. The results offer new insights into possible mechanisms of altered electrophysiologic responsiveness to acetylcholine following cardiac parasympathectomy.