Effect of G protein heterotrimer composition on coupling of neurotransmitter receptors to N-type Ca(2+) channel modulation in sympathetic neurons

Voltage-dependent (VD) inhibition of N-type Ca(2+) channels is mediated primarily by neurotransmitter receptors that couple to pertussis toxin (PTX)-sensitive G proteins (such as G(o) and G(i)). To date, however, the composition of heterotrimeric complexes, i.e., specific Gαβγ combinations, capable of coupling receptors to N-type Ca(2+) channels has not been defined. We addressed this question by heterologously expressing identified Gαβγ combinations in PTX-treated rat sympathetic neurons and testing for reconstitution of agonist-mediated VD inhibition. The heterologously expressed Gα subunits were rendered PTX-insensitive by mutating the codon specifying the ADP ribosylation site. The following results were obtained from this approach. (i) Expression of Gα(oA), Gα(oB), and Gα(i2) (along with Gβ(1)γ(2)) reconstituted VD inhibition mediated by α(2)-adrenergic, adenosine, somatostatin, and prostaglandin E(2) receptors. Conversely, expression of Gα(i1) and Gα(i3) was ineffective at restoring coupling. (ii) Coupling efficiency, as determined from the magnitude of reconstituted Ca(2+) current inhibition, depended on both the receptor and Gα subtype. The following rank order of coupling efficiency was observed: Gα(oA) = Gα(oB) > Gα(i2) for α(2)-adrenergic receptor; Gα(i2) > Gα(oA) = Gα(oB) for adenosine and prostaglandin E(2) receptors; and Gα(oB) = Gα(i2) > Gα(oA) for the somatostatin receptor. (iii) In general, varying the Gβγ composition of Gα(oA)-containing heterotrimers had little effect on the coupling of α(2)-adrenergic receptors to the VD pathway. Taken together, these results suggest that multiple, diverse Gαβγ combinations are capable of coupling neurotransmitter receptors to VD inhibition of N-type Ca(2+) channels. Thus, if exquisite Gαβγ-coupling specificity exists in situ, it cannot arise solely from the inherent inability of other Gαβγ combinations to form functional signaling complexes.