G protein subunits and the stimulation of phospholipase C by G(s)- and G(i)-coupled receptors: Lack of receptor selectivity of G alpha(16) and evidence for a synergic interaction between G beta gamma and the alpha subunit of a receptor-activated G protein

Stimulatory guanine nucleotide binding protein (G(s))-coupled receptors activated by luteinizing hormone, vasopressin, and the catecholamine isoproterenol (luteinizing hormone receptor, type 2 vasopressin receptor, and types 1 and 2 beta-adrenergic receptors) and the G(i)-coupled M2 muscarinic receptor (M2R) were expressed transiently in COS cells, alone and in combination with G beta gamma dimers, their corresponding G alpha s (G alpha(s) or G alpha(13)) and either G alpha(q) or G alpha(16) . Phospholipase C (PLC) activity, assessed;by inositol phosphate production from preincorporated myo[H-3]inositol, was then determined to gain insight into differential coupling preferences among receptors and G proteins, The following were observed: (i) All receptors tested were able to stimulate PLC activity in response to agonist occupation, The effect of the M2R was pertussis toxin sensitive, (ii) While, as expected, expression of G alpha(q) facilitated an agonist-induced activation off PLC that varied widely from receptor to receptor (400% with type 2 vasopressin receptor and only 30% with M2R), expression of G alpha(16) facilitated about equally well the activation of PLC by any of the tested receptors and thus showed little if any discrimination for one receptor over another, (iii) G beta gamma elevated basal (agonist independent) PLC activity between 2- and 4-fold, confirming the proven ability of G beta gamma to stimulate PLC beta., (iv) Activation of expressed receptors by their respective ligands in cells coexpressing excess G beta gamma elicited agonist stimulated PLC activities, which, in the case of the M2R, was not blocked by pertussis toxin (PTX), suggesting mediation by a PTX-insensitive PLC-stimulating G beta gamma subunit, presumably, but not necessarily, of the G(q) family, (v) The effects of G beta gamma and the PTX-insensitive G alpha elicited by M2R were synergistic, suggesting the possibility that one or more forms of PLC are under conditional or dual regulation of G protein subunits such that stimulation by one sensitizes to the stimulation by the other.