Szabo_1989_Trends.Pharmacol.Sci_Suppl_46

Muscarinic agonists open potassium-selective K(ACh) channels in cardiac myocytes of pacemaker or atrial origin. Receptor activation is coupled to channel opening by a membrane bound guanine nucleotide-binding protein (GK) through a process that does not require cytoplasmic intermediates. We have used the muscarinic potassium channel and the corresponding macroscopic current, IACh, as rapid, sensitive and specific indicators of the state of activation of GK. This approach, developed here in quantitative detail, allowed us to identify the salient kinetic processes involved in the activation and deactivation of GK in vivo. Agonist was found to act by accelerating the rate of GDP release, and the subsequent GTP uptake by GK, while deactivation was found to occur by a process that requires GTP hydrolysis. Unexpectedly, deactivation in the intact system is much more rapid than the rate of GTP hydrolysis by isolated G proteins, suggesting the presence of a GTPase-stimulating factor in intact cells.