α1-Adrenergic and m3-Muscarinic Receptor Stimulation of Phosphatidylinositol 4,5-Bisphosphate-Specific Phospholipase C Are Independently Mediated by Gαq/11 in Rat Parotid Gland Membranes

Stimulation of m3-muscarinic cholinergic receptors (m3AChR) in the rat parotid gland increases the hydrolysis of phosphatidylinositol 4, 5-bisphosphate (PIP2) via activation of the Gαq/11 family of G-proteins (Sawaki et al. (1993) Arch. Biochem. Biophys., 546-550). Herein we report that α1-adrenergic receptor (α1-AR) stimulation of PIP2 hydrolysis is also mediated via α subunits of the Gq/11 family of G-proteins. The α1-AR agonist, epinephrine, induced a dose-dependent increase (1.5-fold maximum) of exogenously added PIP2 in the presence of guanosine 5′-O-(3-thiotriphosphate) (GTPγS), which was inhibited by the α1-AR antagonist, phentolamine, but not by the β-adrenergic receptor antagonist, propranolol. The epinephrine-stimulated component of PIP2 hydrolysis was significantly inhibited by pretreating the membranes with an antiserum against Gαq/11. When carbachol and epinephrine were present simultaneously (with GTPγS), the increase in PIP2 hydrolysis obtained was not significantly different from the sum of the increases in PIP2 hydrolysis obtained with each agonist alone. PIP2 hydrolysis stimulated in the presence of carbachol and epinephrine was inhibited by α1-AR and m3AChR antagonists, phentolamine and atropine respectively, to the level obtained with each agonist alone. Notably, in the presence of both agonists the inhibition of PIP2 hydrolysis by anti-Gαq/11 antiserum was not significantly different from the sum of the inhibitions obtained with each agonist alone. These results indicate that m3AChR and α1-AR stimulation of PIP2 hydrolysis in rat parotid gland membranes are independently mediated by the Gαq/11 family of G-proteins.