Gq protein

Gq runs a different circuit from Gs and Gi. Rather than touching the cyclic-AMP machinery, Gq throws a switch that splits a membrane lipid into two new messengers at once, and one of those releases a flood of calcium inside the cell.

When a Gq-coupled receptor is activated, the Gq alpha subunit binds GTP and stimulates phospholipase C. Phospholipase C cleaves the membrane lipid PIP2 into inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 diffuses to the endoplasmic reticulum and opens calcium channels there, raising intracellular calcium; DAG stays in the membrane and, together with the calcium, activates protein kinase C. So one Gq event delivers three signals — IP3, DAG, and calcium.

Gq-coupled receptors drive contraction and secretion: alpha-1 adrenoceptors (causing vasoconstriction, the target of prazosin), muscarinic M1 and M3 receptors, angiotensin II AT1 receptors, histamine H1 receptors, and many serotonin and peptide receptors. Drugs such as alpha-1 blockers and angiotensin receptor blockers lower blood pressure precisely by interrupting Gq-driven smooth-muscle contraction.