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

Abstract

Activated receptors for galanin and norepinephrine, and for several other agonists, inhibit insulin release from pancreatic beta-cells via pertussis toxin-sensitive Gi- and Go-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 beta-cell. Therefore, the G-protein interactions of two activated receptors, those for norepinephrine and galanin, were compared in the same beta-cell membranes. Measurements were made of the rates of receptor-G-protein interaction (by GTPgammaS 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 alpha-subunits of Gi and Go by cholera toxin (CTX). Such CTX-induced ADP ribosylation of Gi and Go 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 alpha2-adrenergic receptor was found to be markedly more effective than the galanin receptor in activating G-proteins.

Masked excitatory action of noradrenaline on rat islet ?-cells via activation of phospholipase C

“…injection [22], whilst we applied it directly to the islets or single cells. Over-night treatment of islets with direct application of PTX is known to inhibit the G i proteinlinked pathway [11]. NE elevates [Ca 2+ ] i in β-cells via the PLC pathway NE elevated [Ca 2+ ] i in islet β-cells independently of PTX (Fig.…”

“…This inhibitory process involves α 2 -adrenoceptors, leading to activation of pertussis toxin (PTX)-sensitive G i proteins and inhibition of adenylate cyclase [11,22]. Coupling of G i proteins, inhibition of adenylate cyclase and the subsequent inhibition of cAMP production has been investigated extensively using galanin, which is contained in and released from the sympathetic nerve terminal [9] and binds to its G i protein-linked receptor [7,24].…”