Relationship between ?1-adrenoreceptor density and functional response of rat vas deferens

Abstract

The relationship between the density of α₁-adrenoreceptors and the longitudinal contractile response of rat vas deferens was examined by using phenoxybenzamine to irreversibly decrease α₁-adrenoreceptor density. Receptor density was measured by Scatchard plots of saturation analysis of specific ¹²⁵I-BE 2254 (¹²⁵IBE) binding and compared to the potency of α-adrenoceptor agonists in causing contraction and the maximum contraction elicited by these agonists. Treatment of isolated vasa deferentia in organ baths with phenoxybenzamine caused a dose-dependent decrease in the density of ¹²⁵IBE binding sites, the potency of the full agonist phenylephrine in activating contraction, and the maximum contractile response. The percentage of functional receptors remaining after phenoxybenzamine treatment (q value) was calculated from the contraction data and compared to the percentage of ¹²⁵IBE binding sites remaining. The reduction in the number of functional receptors was much greater than the reduction in the number of ¹²⁵IBE binding sites at all doses of phenoxybenzamine. Since the magnitude of the contractile response caused by a weak partial agonist should be approximately proportional to the density of functional receptors in the tissue, partial agonists were used as an independent measure of the degree of functional receptor inactivation. Bath application of two different doses of phenoxybenzamine caused a decrease in the maximal contraction caused by the partial agonists clonidine and ephedrine which was similar to the calculated decrease in the q value, but not to the observed decrease in the density of ¹²⁵IBE binding sites. Phenylephrine also caused contraction of spiral strips prepared from vas deferens, suggesting that α₁-adrenoreceptors exist on the inner circular muscle as well as the outer longitudinal muscle of this tissue. To determine whether access barriers in the layered muscle structure of the vas deferens could result in a preferential inactivation of a particular subpopulation of receptors, phenoxybenzamine was applied more uniformly by intravenous administration to anesthetized rats. Intravenous administration of phenoxybenzamine also resulted in a dose-dependent decrease in the density of ¹²⁵IBE binding sites, the potency of phenylephrine in activating contraction, and the maximum contractile response. The degree of functional receptor inactivation after intravenous phenoxybenzamine administration (q values) agreed reasonably well with the observed reduction in ¹²⁵IBE binding sites, particularly at lower doses of phenoxybenzamine. The equilibrium constant for activation of the receptor by phenylephrine (K _act) was the same whether calculated using data from bath application of phenoxybenzamine or intravenous application of phenoxybenzamine. These data suggest that only a portion of the α₁-adrenoreceptors in rat vas deferens affect longitudinal contraction. There is, however, a large pool of spare receptors for longitudinal contraction and these receptors are preferentially inactivated by application of phenoxybenzamine to the isolated tissue in an organ bath. When phenoxybenzamine is applied intravenously, the receptors affecting longitudinal contraction are inactivated to a degree similar to that of the total pool of receptors in this tissue.