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 5 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; low affinity receptors were observed in the vas deferens. The affinity for yohimbine in the 2 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 1 type of .alpha.-adrenergic receptor present. Comparison of the Kd of yohimbine in the aorta with Kd obtained from the literature for this compound in a variety of tissues containing .alpha.-1 or .alpha.-2 adrenergic receptors in the aorta is of the .alpha.-2 type. The postsynaptic .alpha.-adrenergic receptor in the portal vein appears to be .alpha.-1. The differences in postsynaptic .alpha. adrenergic receptors in the 2 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.