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 Galphabetagamma combinations, capable of coupling receptors to N-type Ca(2+) channels has not been defined. We addressed this question by heterologously expressing identified Galphabetagamma combinations in PTX-treated rat sympathetic neurons and testing for reconstitution of agonist-mediated VD inhibition. The heterologously expressed Galpha 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 Galpha(oA), Galpha(oB), and Galpha(i2) (along with Gbeta(1)gamma(2)) reconstituted VD inhibition mediated by alpha(2)-adrenergic, adenosine, somatostatin, and prostaglandin E(2) receptors. Conversely, expression of Galpha(i1) and Galpha(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 Galpha subtype. The following rank order of coupling efficiency was observed: Galpha(oA) = Galpha(oB) > Galpha(i2) for alpha(2)-adrenergic receptor; Galpha(i2) > Galpha(oA) = Galpha(oB) for adenosine and prostaglandin E(2) receptors; and Galpha(oB) = Galpha(i2) > Galpha(oA) for the somatostatin receptor. (iii) In general, varying the Gbetagamma composition of Galpha(oA)-containing heterotrimers had little effect on the coupling of alpha(2)-adrenergic receptors to the VD pathway. Taken together, these results suggest that multiple, diverse Galphabetagamma combinations are capable of coupling neurotransmitter receptors to VD inhibition of N-type Ca(2+) channels. Thus, if exquisite Galphabetagamma-coupling specificity exists in situ, it cannot arise solely from the inherent inability of other Galphabetagamma combinations to form functional signaling complexes.