Receptors couple to L-type calcium channels via distinct G(0) proteins in rat neuroendocrine cell lines

1. The present study examines the hypothesis of G protein subtype selectivity in receptor-induced inhibition of calcium channel currents (I-Ca) in the insulin-secreting RINm5F and pituitary GH(3) rat cell lines. Specificity of receptor coupling to G proteins was studied by infusion of purified G alpha isoforms into cells via a patch pipette. 2. In RINm5F cells, the neuropeptide galanin inhibited dihydropyridine (DHP)- and omega-conotoxin-sensitive components of I-Ca and slowed down their activation kinetics. In GH(3) cells, DHP-sensitive I-Ca was inhibited by galanin, as well as by somatostatin and carbachol. Agonist-induced I-Ca inhibition was suppressed by pertussis toxin (PTX) pretreatment of the cells. In PTX-pretreated cells of either cell line, the response to galanin was restored only by the G alpha(01) subunit. Following PTX treatment of GH(3) cells, only the G alpha(02) subunit restored carbachol-induced inhibition of I-Ca, whereas only the G alpha(01) subunit restored somatostatin induced inhibition of I-Ca. G(i) subtypes had no effect on I-Ca inhibition. 3. Both cell lines expressed two distinct immunoreactive G(0) proteins. Whereas in RINm5F cell membranes G(01) was found to be the predominant isoform, we detected more G(02) than G(01) in GH(3) cell membranes. Nevertheless, all agonists stimulated incorporation of the photoreactive GTP analogue [alpha-P-32]GTP azidoanilide into both G(0) isoforms. 4. The results indicate that the same G(0) subtype, i.e. G(01), mediates galanin-induced inhibition of I-Ca in both cell lines and that the G(0) subtype specificity of receptor-G protein coupling is confined to intact cells.