Mechanisms of inhibition of insulin release

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Abstract
Several agonists including norepinephrine, somatostatin, galanin, and prostaglandins inhibit insulin releasse. The inhibition is sensitive to pertussis toxin, indicating the involvement of heterotrimeric Gi and/or Go proteins. Receptors for the different agonists have different selectivity for these G proteins. After G protein activation, the alpha- and beta gamma-subunits dissociate and interact with multiple targets to inhibit release. These include 1) the ATP-sensitive K+ channel and perhaps other K+ channels, 2) L-type voltage-dependent Ca2+ channels, 3) adenylyl cyclase, and 4) a “distal” site late in stimulus-secretion coupling. The latter effect, which may be exerted close to the final stage of exocytosis, is the most powerful of the individual inhibitory mechanisms. G protein action on the target molecules is determined by the individual G proteins activated and their specificity for the targets. The L-type Ca2+ channel is inhibited by G(o)-1. Adenylyl cyclase is inhibited by Gi-2 and Gi-3. The distal inhibition can be exerted by Gi-1, Gi-2, Gi-3, and G(o)-2. Thus there is both selectivity and promiscuity in G protein action in the beta-cell. These characteristics allow an inhibitory ligand to be effective at multiple targets and to act differentially from other inhibitory ligands.

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