A voltage-independent calcium current inhibitory pathway activated by muscarinic agonists in rat sympathetic neurons requires both Gαq/11 and Gβγ

Calcium current modulation by the muscarinic cholinergic agonist oxotremorine methiodide (oxo-M) was examined in sympathetic neurons from the superior cervical ganglion of the rat. Oxo-M strongly inhibited calcium currents via voltage-dependent (VD) and voltage-independent (VI) pathways. These pathways could be separated with the use of the specific M-1 acetylcholine receptor antagonist M-1-toxin and with pertussis toxin (PTX) treatment. Expression by nuclear cDNA injection of the regulator of G-protein signaling (RGS2) or a phospholipase C beta 1 C-terminal construct (PLC beta-ct) selectively reduced VI oxo-M modulation in PTX-treated and untreated cells. Expression of the G beta gamma buffers transducin (G alpha(tr)) and a G-protein-coupled-receptor kinase (GRK3) construct (MAS-GRK3) eliminated oxo-M modulation. Activation of the heterologously expressed neurokinin type 1 receptor, a G alpha(q/11)-coupled receptor, resulted in VI calcium current modulation. This modulation was eliminated with coexpression of G alpha(tr) or MAS-GRK3. Cells expressing G beta(1)gamma(2) were tonically inhibited via the VD pathway. Application of oxo-M to these cells produced VI modulation and reduced the amount of current inhibited via the VD pathway. Together, these results confirm the requirement for G beta gamma in VD modulation and implicate G alpha(q)-GTP and G beta gamma as components in the potentially novel VI pathway.