OCCUPANCY OF ALPHA1-ADRENERGIC RECEPTORS AND CONTRACTION OF RAT VAS-DEFERENS

Abstract

The interaction of agonists and antagonists with .alpha.1-adrenergic receptors in rat vas deferens was examined using radioligand binding assays and contractility measurements. 125I-Labeled BE 2254 [2-[.beta.-(4-hydroxyphenyl)ethylaminomethyl]-tetralone] (125IBE) bound rapidly and reversibly to a single class of high-affinity binding sites in homogenates of rat vas deferens. The k1 for association was 3.8 .times. 107 l/mol-s, the k-1 for dissociation was 2.3 .times. 10-3 s-1, and the Kd was 105 pM. The order of potency for antagonists inhibiting 125IBE binding was prazosin > indoramin > phentolamine > yohimbine. Norepinephrine, phenylephrine and other .alpha.-adrenergic agonists produced dose-dependent contractions of whole vas deferens in vitro. This contractile response was competitively inhibited by .alpha.-adrenergic blocking drugs with the same potency order observed for inhibition of specific 125IBE binding. Comparison of pA2 [competitive antagonistic activity] values for .alpha.- and .alpha.-selective antagonists competitively inhibiting contractile responses to norepinephrine, epinephrine or phenylephrine suggested that these drugs caused their contractile effects solely through .alpha.-adrenergic receptors, and that there were no .alpha.-adrenergic receptors mediating contraction in this tissue. The pA2 values for antagonist inhibition of .alpha.-adrenergic receptor-mediated contractile responses were highly correlated (r = 0.995) with the Kd values for antagonist inhibition of 125IBE binding in this tissue. The EC50 [median effective concentration] values for partial agonists were highly correlated with the Kd values for inhibition of 125IBE binding in vas deferens. The EC50 values of full agonists in causing contraction were in general 10- to 100-fold lower than the Kd values for inhibiting 125IBE binding, possibly representing a substantial spare receptor population in this tissue. Apparently, rat vas deferens contains a homogeneous population of .alpha.-adrenergic receptors mediating the contractile response to norepinephrine. These receptors may be directly labeled with 125IBE and there may be a nonlinear relationship between agonist occupancy of .alpha.-adrenergic receptors and the functional response of this tissue.