Rat 1 fibroblasts transfected to express either the wild-type hamster alpha 1B-adrenergic receptor or a constitutively active mutant (CAM) form of this receptor resulting from the alteration of amino acid residues 288-294 to encode the equivalent region of the human beta 2-adrenergic receptor were examined. The basal level of inositol phosphate generation in cells expressing the CAM alpha 1B-adrenergic receptor was greater than for the wild-type receptor, The addition of maximally effective concentrations of phenylephrine or noradrenaline resulted in substantially greater levels of inositol phosphate generation by the CAM alpha 1B-adrenergic receptor, although this receptor was expressed at lower steady-state levels than the wild-type receptor. The potency of both phenylephrine and noradrenaline to stimulate inositol phosphate production was approx. 200-fold greater at the CAM alpha 1B-adrenergic receptor than at the wild-type receptor. In contrast, endothelin 1, acting at the endogenously expressed endothelin ETA, receptor, displayed similar potency and maximal effects in the two cell lines. The sustained presence of phenylephrine resulted in down-regulation of the alpha subunits of the phosphoinositidase C-linked, pertussis toxin-insensitive, G-proteins G9 and G11 in cells expressing either the wild-type or the CAM alpha 1B-adrenergic receptor. The degree of down-regulation achieved was substantially greater in cells expressing the CAM alpha 1B-adrenergic receptor at all concentrations of the agonist. However, in this assay phenylephrine displayed only a slightly greater potency at the CAM alpha 1B-adrenergic receptor than at the wild-type receptor. There were no detectable differences in the basal rate of G9 alpha/G11 alpha degradation between cells expressing the wild-type or the CAMalpha 1B-adrenergic receptor. In both cell lines the addition of phenylephrine substantially increased the rate of degradation of these G-proteins, with a greater effect at the CAM alpha 1B-adrenergic receptor. The enhanced capacity of agonist both to stimulate second-messenger production at the CAM alpha 1B-adrenergic receptor and to regulate cellular levels of its associated G-proteins by stimulating their rate of degradation is indicative of an enhanced stoichiometry of coupling of this form of the receptor to G9 and G11.