Overexpression of β1 and β2 adrenergic receptors in rat atrial myocytes -: Differential coupling to G protein-gated inward rectifier K+ channels via Gs and Gi/o

G protein-activated inwardly rectifying K+ (GIRK) channels, expressed in atrial myocytes, various neurons, and endocrine cells, represent the paradigmatic target of beta gamma subunits released from activated heterotrimeric G proteins. These channels contribute to physiological slowing of cardiac frequency and synaptic inhibition. They are activated by beta gamma dimers released upon stimulation of receptors coupled to pertussis toxin-sensitive G proteins (G(i/o)), whereas beta gamma released from G(s) do not converge on the channel subunits. This is in conflict with the finding that dimeric combinations of various beta and gamma subunits can activate GIRK channels with little specificity. In the present study, we have overexpressed the major subtypes of cardiac beta -adrenergic receptors (beta (1)-AR and beta (2)-AR) in atrial myocytes by transient transfection. Whereas in native cells beta -adrenergic stimulation with isoproterenol failed to induce measurable GIRK current, robust currents were recorded from myocytes overexpressing either beta (1)-AR or beta (2)-AR. Whereas the beta (2)-AR-induced current showed the same sensitivity to pertussis toxin as the current evoked by the endogenous G(i/o)-coupled muscarinic M-2 receptor, isoproterenol-activated currents were insensitive to pertussis toxin treatment in beta (1)-AR-overexpressing myocytes. In contrast to a recent publication (Leaney, J. L., Milligan, G., and Tinker, A. (2000) J. Biol. Chem. 275, 921-929), sizable GIRK currents could also be activated by isoproterenol when the signaling pathway was reconstituted by transient transfection in two different standard cell lines (Chinese hamster ovary and HEK293). These results demonstrate that specificity of receptor-G protein signaling can be disrupted by overexpression of receptors. Moreover, the alpha subunit of heterotrimeric G proteins does not confer specificity to G beta gamma -mediated signaling.