Postsynaptic Alpha Adrenergic Receptor Subtypes Differentiated by Yohimbine in Tissues from the Rat. Existence of Alpha-2 Adrenergic Receptors in Rat Aorta

The selective alpha-2 adrenergic receptor antagonist, yohimbine, was used to differentiate postsynaptic alpha-adrenergic receptors in five peripheral tissues of the rat. Three distinct postsynaptic receptor subtypes were observed based on the affinity of the receptors for yohimbine. Receptors with high affinity for yohimbine were detected in the aorta, whereas low affinity receptors were observed in the vas deferens. The affinity for yohimbine in these two tissues differed by over 50-fold. Receptors with intermediate affinity for yohimbine were found in the portal vein, spleen and bladder. The Schild plot for yohimbine in the bladder suggests that blockade of alpha receptors in this tissue is not competitive or that there exists more than one type of alpha-adrenergic receptor present. Comparison of the dissociation constant of yohimbine in the aorta with dissociation constants obtained from the literature for this compound in a variety of tissues containing alpha-1 or alpha-2 adrenergic receptors indicates that the postsynaptic alpha adrenergic receptor in the aorta is of the alpha-2 type. Conversely, the postsynaptic alpha adrenergic receptor in the portal vein appears to be alpha-1. The differences in postsynaptic alpha appears to be alpha-1. The differences in postsynaptic alpha adrenergic receptors in these two vascular tissues may reflect the marked differences in adrenergic innervation and the possible relative lack of neuroeffector junctional alpha adrenergic receptors in the rat aorta.