Norepinephrine-induced alteration in the coupling of alpha 1-adrenergic receptor occupancy to calcium efflux in rabbit aortic smooth muscle cells.

To determine whether alpha-adrenergic desensitization of vascular smooth muscle is due to an alteration in alpha 1-adrenergic receptor coupling, we determined the relationship between receptor occupancy and maximal receptor-coupled Ca2+ efflux in cultured rabbit aortic smooth muscle cells (i) under basal conditions as defined by receptor inactivation with phenoxybenzamine and (ii) after 48 hr of exposure to several concentrations of 1-norepinephrine (NE). Neither phenoxybenzamine nor NE exposure caused a change in binding affinity for [3H]prazosin or NE. Maximal [3H]prazosin binding capacity and maximal NE-stimulated 45Ca2+ efflux decreased progressively with exposure of incubated cells to increasing concentrations of phenoxybenzamine or NE. An approximately 80% decrease in maximal [3H]prazosin binding capacity caused by either phenoxybenzamine or NE resulted in complete loss of NE-stimulated 45Ca2+ efflux, indicating that under these conditions approximately 20% of alpha 1-adrenergic receptors are not coupled to the Ca2+ efflux. Under basal conditions, the relationship between maximal [3H]prazosin binding capacity and maximal NE-stimulated 45Ca2+ efflux was markedly nonlinear, so that a near maximal response could be elicited by occupancy of only approximately 40% of the receptors. In contrast, after a 48-hr incubation of cells with NE, occupancy-response coupling was considerably less efficient, so that even full occupancy of the 35% of receptors that remained after NE exposure resulted in only approximately 20% of maximal NE-stimulated 45Ca2+ efflux. Thus, an alteration in occupancy-response coupling at a step proximal to Ca2+ mobilization and/or influx, rather than a reduction in receptor number, is of primary importance in the process of agonist-induced alpha-adrenergic receptor desensitization of vascular smooth muscle cells.