Occupancy of alpha₁-adrenergic receptors and contraction of rat vas deferens

The interaction of agonists and antagonists with alpha₁-adrenergic receptors in rat vas deferens was examined using radioligand binding assays and contractility measurements. ¹²⁵I-Labeled BE 2254 (¹²⁵IBE) was found to bind 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 x 10⁷ l/mole-sec, the k-₁ for dissociation was 2.3 x 10^-3 sec^-1, and the K(D) 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 values for alpha₁- and alpha₂-selective antagonists competitively inhibiting contractile responses to norepinephrine, epinephrine, or phenylephrine suggested that these drugs caused their contractile effects solely through alpha₁-adrenergic receptors, and that there were no alpha₂-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 K(D) values for antagonist inhibition of ¹²⁵IBE binding in this tissue. The EC₅₀ values for partial agonists were also highly correlated with the K(D) values for inhibition of ¹²⁵IBE binding in vas deferens. However, the EC₅₀ values of full agonists in causing contraction were in general 10- to 100-fold lower than the K(D) values for inhibiting ¹²⁵IBE binding, possibly representing a substantial 'spare receptor' population in the tissue. The results, suggest that rat vas deferens contains a homogeneous population of alpha₁-adrenergic receptors mediating the contractile response to norepinephrine, that these receptors can be directly labeled with ¹²⁵IBE, and that there may be a nonlinear relationship between agonist occupancy of alpha₁-adrenergic receptors and functional response of this tissue.