Dictionary of Allergies .. G-proteins

G-proteins

The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). The transduction mechanism of all a-adrenoceptors includes the use of G-proteins. A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. Differences occur in the type of G-protein the receptors are coupled to. a1-adrenoceptors are coupled through the Gp/Gq mechanism, whereas a2-adrenoceptors are coupled through Gi/Go. Gp/Gq activates phospholipase C that phosphorylates phosphatidyl inositol to produce inositol triphosphate, and diacylglycerol.

These compounds act as second messengers and cause release of calcium from intracellular stores in the sarcoplasmic reticulum, and activation of calcium channels respectively. They produce their effects by the release of calcium. The a2-adrenoceptor G-protein, Gi/Go, has been shown to be negatively coupled to adenylate cyclase and so reduces the formation of cyclic AMP which leads to a decreased influx of calcium during the action potential - the ion responsible for transmitter release. Therefore lowered levels of calcium will correspondingly lead to a decrease in transmitter release

References

Ruffolo RR Jr, Hieble JP. Alpha-adrenoceptors. Pharmacol Ther. 1994;61(1-2):1-64.