The α₂-Adrenergic Receptor Is More Effective Than the Galanin Receptor in Activating G-Proteins in RINm5F β-cell Membranes

Activated receptors for galanin and norepinephrine, and for several other agonists, inhibit insulin release from pancreatic β-cells via pertussis toxin–sensitive G_i- and G_o-proteins and by acting on at least four cellular mechanisms. These mechanisms include repolarization via activation of the ATP-sensitive potassium (K_ATP) channel, inhibition of adenylyl cyclase, and inhibition by unknown mechanism at a “distal” site. For norepinephrine and galanin there is also inhibition of the L-type Ca²⁺ channel. Consequently, during simultaneous activation by multiple agonists, the effectiveness with which a receptor interacts with the G-proteins will, to some extent, determine the responses. This could have important consequences for the β-cell. Therefore, the G-protein interactions of two activated receptors, those for norepinephrine and galanin, were compared in the same β-cell membranes. Measurements were made of the rates of receptor–G-protein interaction (by GTPγS binding) and of the rates of turnover of G-proteins (by GTPase activity). A comparison was also made of the ability of norepinephrine and galanin to facilitate ADP ribosylation of the α-subunits of G_i and G_o by cholera toxin (CTX). Such CTX-induced ADP ribosylation of G_i and G_o occurs during G-protein interaction with an activated receptor. By measurement of the number of receptors in the membrane preparation used, the relative effectiveness of the two receptors was assessed. The α₂adrenergic receptor was found to be markedly more effective than the galanin receptor in activating G-proteins.