Abstract
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.