Sequestration of G-protein beta gamma subunits by different G-protein alpha subunits blocks voltage-dependent modulation of Ca2+ channels in rat sympathetic neurons.

The membrane-delimited and voltage-dependent inhibition of N-type Ca2+ channels is mediated by Gbeta gamma subunits. Previously, exogenous excess GDP-bound GalphaoA has been shown to dramatically attenuate the norepinephrine (NE)-mediated Ca2+ current inhibition by sequestration of Gbeta gamma subunits in rat superior cervical ganglion (SCG) neurons. In the present study, we determined whether the attenuation of NE-mediated modulation is specific to GalphaoA or shared by a number of closely related (Galphatr, GalphaoB, Galphai1, Galphai2, Galphai3, Galphaz) or unrelated (Galphas, Galphaq, Galpha11, Galpha16, Galpha12, Galpha13) Galpha subunits. Individual Galpha subunits from different subfamilies were transiently overexpressed in SCG neurons by intranuclear injection of mammalian expression vectors encoding the desired protein. Strikingly, all Galpha subunits except Galphaz nearly blocked basal facilitation and NE-mediated modulation. Likewise, VIP-mediated Ca2+ current inhibition, which is mediated by cholera toxin-sensitive G-protein, was also completely suppressed by a number of Galpha subunits overexpressed in neurons. Galphas expression produced either enhancement or attenuation of the VIP-mediated modulation-an effect that seemed to depend on the expression level. The onset of the nonhydrolyzable GTP analog, guanylylimidodiphosphate-mediated facilitation was significantly delayed by overexpression of different GDP-bound Galpha subunits. Taken together, these data suggest that a wide variety of Galpha subunits are capable of forming heterotrimers with endogenous Gbeta gamma subunits mediating voltage-dependent Ca2+ channel inhibition. In conclusion, coupling specificity in signal transduction is unlikely to arise as a result of restricted Galpha/Gbeta gamma interaction.