A G protein-gated K channel is activated via beta 2-adrenergic receptors and G beta gamma subunits in Xenopus oocytes
In many tissues, inwardly rectifying K channels are coupled to seven- helix receptors via the Gi/Go family of heterotrimeric G proteins. This activation proceeds at least partially via G beta gamma subunits. These experiments test the hypothesis that G beta gamma subunits activate the channel even if released from other classes of heterotrimeric G proteins. The G protein-gated K channel from rat atrium, KGA/GIRK1, was expressed in Xenopus oocytes with various receptors and G proteins. The beta 2-adrenergic receptor (beta 2AR), a Gs-linked receptor, activated large KGA currents when the alpha subunit, G alpha s, was also overexpressed. Although G alpha s augmented the coupling between beta 2AR and KGA, G alpha s also inhibited the basal, agonist-independent activity of KGA. KGA currents stimulated via beta 2AR activated, deactivated, and desensitized more slowly than currents stimulated via Gi/Go-linked receptors. There was partial occlusion between currents stimulated via beta 2AR and the m2 muscarinic receptor (a Gi/Go-linked receptor), indicating some convergence in the mechanism of activation by these two receptors. Although stimulation of beta 2AR also activates adenylyl cyclase and protein kinase A, activation of KGA via beta 2AR is not mediated by this second messenger pathway, because direct elevation of intracellular cAMP levels had no effect on KGA currents. Experiments with other coexpressed G protein alpha and beta gamma subunits showed that (a) a constitutively active G alpha s mutant did not suppress basal KGA currents and was only partially as effective as wild type G alpha s in coupling beta 2AR to KGA, and (b) beta gamma subunits increased basal KGA currents. These results reinforce present concepts that beta gamma subunits activate KGA, and also suggest that beta gamma subunits may provide a link between KGA and receptors not previously known to couple to inward rectifiers.
© 1995 by The Rockefeller University Press Original version received 26 July 1994 and accepted version received 31 October 1994. We thank Dr. A. Gilman for GαsQ227L cDNA, Dr. E. Peralta for the m2R cDNA, and Drs. E. Reuveny, P. Siesinger, A.J. Connolly, and S. R. Coughlin for G protein β1 and γ2 subunit cDNAs and J. Gollub and H. Davis for help with oocytes and Y. Zhang for participating in some of the experiments. We thank C. Chavkin, C. Doupnik, L. DiMagno, N. McCarty, S. McDonough, M. Quick, and Y. Uezono for advice. This work was supported by grants from the National Institutes of Health (including postdoctoral fellowships to N. Lim), from the US-Israel Binational Science Foundation, and from ICAgen, Inc.
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