Decreased m3-Muscarinic and α1-Adrenergic Receptor Stimulation of PIP2 Hydrolysis in Parotid Gland Membranes from Aged Rats: Defect in Activation of Gαq/11

m3-Muscarinic cholinergic receptor (m3-AChR) and α1-adrenergic receptor (α1-AR) stimulation of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis (by a PIP2-specific phospholipase C, PLC) in rat parotid gland membranes is mediated via activation of α subunits of the Gq/11 family of G-proteins. This study examines m3-AChR and α1-AR stimulation of PIP2 hydrolysis in membranes isolated from parotid glands of old (24 months) and young (3 months) rats (old and young rat membranes). Old rat membranes exhibited reduced stimulation of PIP2 hydrolysis in response to the addition of guanosine-5′-O-(3-thiotrisphosphate) (GTPγS) alone or GTPγS plus either carbachol (m3-AChR agonist) or epinephrine (α1-AR agonist). This reduction in receptor-stimulated PIP2 hydrolysis was not due to a decrease in PLC activity per se since cholate-solubilized PLC activity was similar in old and young rat membranes. Additionally, these membranes exhibited comparable, immunologically detectable, levels of PLCβ3, Gαq/11, and Gβ. In the presence of 10 μM AlCl3 and 10 mM NaF, stimulation of PIP2 hydrolysis in both old and young rat membranes was similar. Preincubation of membranes from old rats with GTPγS induced a time-dependent increase in the rate of PIP2 hydrolysis and, with 20 min preincubation, the rates of hydrolysis in old and young rat membranes were not statistically different. In aggregate, these data indicate that there is a defect in the activation of Gαq/11 in parotid gland membranes from old rats.