Sequestration of G-Protein βγ Subunits by Different G-Protein α Subunits Blocks Voltage-Dependent Modulation of Ca2+Channels in Rat Sympathetic Neurons

摘要

The membrane-delimited and voltage-dependent inhibition of N-type Ca²⁺ channels is mediated by Gβγ subunits. Previously, exogenous excess GDP-bound GαoA has been shown to dramatically attenuate the norepinephrine (NE)-mediated Ca²⁺ current inhibition by sequestration of Gβγ subunits in rat superior cervical ganglion (SCG) neurons. In the present study, we determined whether the attenuation of NE-mediated modulation is specific to GαoA or shared by a number of closely related (Gαtr, GαoB, Gαi1, Gαi2, Gαi3, Gαz) or unrelated (Gαs, Gαq, Gα11, Gα16, Gα12, Gα13) Gα subunits. Individual Gα subunits from different subfamilies were transiently overexpressed in SCG neurons by intranuclear injection of mammalian expression vectors encoding the desired protein. Strikingly, all Gα subunits except Gαz 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 Gα subunits overexpressed in neurons. Gαs 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 Gα subunits. Taken together, these data suggest that a wide variety of Gα subunits are capable of forming heterotrimers with endogenous Gβγ subunits mediating voltage-dependent Ca²⁺ channel inhibition. In conclusion, coupling specificity in signal transduction is unlikely to arise as a result of restricted Gα/Gβγ interaction.