Agonist unbinding from receptor dictates the nature of deactivation kinetics of G protein-gated K+ channels

G protein-gated inwardly rectifying K+ (Kir) channels are found in neurones, atrial myocytes, and endocrine cells and are involved in generating late inhibitory postsynaptic potentials, slowing the heart rate and inhibiting hormone release. They are activated by G protein-coupled receptors (GPCRs) via the inhibitory family of G protein, G(i/o), in a membrane-delimited fashion by the direct binding of Gbetagammadimers to the channel complex. In this study we are concerned with the kinetics of deactivation of the cloned neuronal G protein-gated K+ channel, Kir3.1 + 3.2A, after stimulation of a number of GPCRs. Termination of the channel activity on agonist removal is thought to solely depend on the intrinsic hydrolysis rate of the G protein a subunit. In this study we present data that illustrate a more complex behavior. We hypothesize that there are two processes that account for channel deactivation: agonist unbinding from the GPCR and GTP hydrolysis by the G protein a subunit. With some combinations of agonist/GPCR, the rate of agonist unbinding is slow and rate-limiting, and deactivation kinetics are not modulated by regulators of G protein-signaling proteins. In another group, channel deactivation is generally faster and limited by the hydrolysis rate of the G protein a subunit. G protein isoform and interaction with G protein-signaling proteins play a significant role with this group of GPCRs.