The vas deferens as a suitable preparation for the study of alpha-adrenoreceptor molecular mechanisms

The responses of the isolated guinea pig vas deferens to phenylephrine in the absence or presence of phenoxybenzamine were studied. The occurrence of a homogenous receptor population was suggested by the finding that the dissociation constant (KA) values for the agonist did not change following varying degrees of receptor inactivation by different concentrations of phenoxybenzamine in the organ bath. In addition, constant pD'2 values were obtained for the antagonistic effect of phenoxybenzamine in these circumstances. Different values for KA and median effective concentration indicated a full agonistic effect for phenylephrine. The presence of spare receptors was revealed by comparing theoretical curves for receptor occupancy and concentration-effect curves for phenylephrine. It is suggested that the isolated guinea pig vas deferens may be a suitable preparation for the study of alpha-adrenergic mechanisms and their regulation.