Endothelial G Protein β-Subunits Trigger Nitric Oxide– but not Endothelium-Derived Hyperpolarizing Factor–Dependent Dilation in Rabbit Resistance Arteries

A single subtype of heterotrimeric G protein–coupled receptor controls both nitric oxide (NO) (sensitive tol -arginine analogues) and endothelium-derived hyperpolarizing factor (EDHF) (sensitive to high-external K+ and apamine) production by the vascular endothelium leading to dilation. We hypothesized that α- and βγ-subunits of the G protein serve as distinct intermediates to produce NO and EDHF. In pressurized resistance arteries, selective pinocytotic endothelial incorporation of specific antibodies (Abs) directed against αq/11 -subunits abolished acetylcholine (Ach)-mediated dilation but failed to influence oxymetazoline (Oxy, α2 -adrenergic receptor agonist)-induced dilation. In contrast, αi1-2 -subunit Abs prevented Oxy- but not Ach-induced dilation. Thus, as expected, endothelial muscarinic and α2 -adrenoceptors couple to Gq protein and Gi proteins, respectively. β-subunit Abs reduced both Ach- and Oxy-induced dilation. The β-subunit Abs abolished the nitro-l -arginine (L-NNA)-sensitive component but did not impair the high-external K+ -sensitive component of the dilation induced by Ach and Oxy. Thus, G protein β-subunits primarily accounted for NO production. Neutralization of Hsp90 and inhibition of the phospholipase C by U73122 (1 μmol/L) or intracellular Ca2+ buffering with BAPTA-AM (10 μmol/L) sharply reduced NO-dependent but not K+ -sensitive dilation. In conclusion, mobilization of the G protein β-subunit is pivotal to NO-dependent dilation triggered through muscarinic and α2 -adrenergic receptors. In contrast, receptor-operated EDHF-dependent dilation was insensitive to β-subunit Abs. Although not directly activating the NO pathway, α-subunit activation is an absolute prerequisite for receptor-operated endothelium-dependent dilation of resistance arteries.