Receptor protection studies with phenoxybenzamine indicate that a single alpha 1-adrenoceptor may be coupled to two signal transduction processes in vascular smooth muscle.

Full alpha 1-adrenoceptor agonists, such as (-)-norepinephrine, produce vasoconstriction in the rat aorta primarily through the mobilization of intracellular stores of calcium, whereas partial alpha 1-adrenoceptor agonists, such as (-)-dobutamine, produce vasoconstriction primarily through the translocation of extracellular calcium. The different pools of calcium utilized by full and partial alpha 1-adrenoceptor agonists have been proposed to result from the activation of different alpha 1-adrenoceptor subtypes. The irreversible alpha 1-adrenoceptor antagonist, phenoxybenzamine, selectively eliminates only that component of an alpha 1-adrenoceptor-mediated response in the rat aorta that is dependent upon the mobilization of intracellular stores of calcium. In order to determine whether in the rat aorta there exist two distinct alpha 1-adrenoceptor subtypes linked separately to the mobilization of intracellular and extracellular calcium, we utilized the full and partial alpha 1-adrenoceptor agonists, (-)-norepinephrine and (-)-dobutamine, respectively, and the irreversible antagonist, phenoxy-benzamine, as pharmacologic tools in a classical receptor-protection study to probe these alpha 1-adrenoceptor-mediated vasoconstrictor process(es). Our logic was that if the intracellular and extracellular pools of calcium were coupled to different alpha 1-adrenoceptor subtypes, then only (-)-norepinephrine, and not (-)-dobutamine, would protect against alpha 1-adrenoceptor alkylation by phenoxybenzamine, since phenoxybenzamine only eliminates the process that depends on intracellular calcium. Alternatively, if both (-)-norepinephrine and (-)-dobutamine produce a similar degree of alpha 1-adrenoceptor protection against phenoxybenzamine, our results would suggest that a single alpha 1-adrenoceptor subtype exists which activates both the translocation of extracellular calcium and the mobilization of intracellular calcium. Phenoxybenzamine (30 nM) abolished contractions of the rat aorta produced by (-)-norepinephrine, as expected. Pretreatment of the tissues with either (-)-norepinephrine or (-)-dobutamine, at concentrations that produced equivalent degrees of alpha 1-adrenoceptor occupancy, resulted in equal protection against alkylation of alpha 1-adrenoceptors by phenoxybenzamine, arguing against the existence of two distinct alpha 1-adrenoceptor subtypes in the rat aorta. These results are consistent with our previous hypothesis that two different signal-transduction processes may be activated in the rat aorta by a single alpha 1-adrenoceptor population, with the intrinsic efficacy of the agonist determining which signal-transduction process is activated.