Peripheral alpha 1- and alpha 2-adrenoreceptor mechanisms in blood pressure control.

Peripheral alpha-adrenoreceptors can be characterized as alpha 1 or alpha 2, depending on their affinity for agonists and antagonists. Alpha 2-adrenoreceptors, probably located postsynaptically on smooth muscle of resistance vessels, contribute to the pressor responses to intravenously given norepinephrine. Studies with the noncompetitive alpha-adrenoreceptor antagonist phenoxybenzamine suggest that there are important differences in the regulation of the two types of receptors. The slower recovery alpha 1-adrenoreceptor binding sites and the more rapid recovery of responses suggest the presence of "spare" alpha 1- but not alpha 2-adrenoreceptors.