Adrenoceptor supersensitivity in the isolated rat vas deferens during barbital withdrawal

Long-term treatment with sodium barbital did not alter significantly the responsiveness of the isolated rat vas deferens to noradrenaline. However, three days after barbital withdrawal there was a 4.5-fold increase in the sensitivity and an enhanced maximum response to the neurotransmitter. Pharmacological analysis of the affinity of postjunctional adrenoceptors showed no alteration of the pA2 of phentolamine, a decrease in the apparent dissociation constant (KA) for noradrenaline and an increase in the fraction of adrenoceptors (q) remaining active after exposure to phenoxybenzamine. Comparison between ED50 and KA indicates the presence of spare receptors in the rat vas deferens. There was also a decrease in the blocking activity of phenoxybenzamine, in vasa deferentia obtained from rats three days after barbital withdrawal. These results suggest that an increased affinity of postjunctional adrenoceptors could be responsible, at least partially, for this supersensitivity to noradrenaline. The increased maximum response to the neurotransmitter could be the consequence of an improved coupling of adrenoceptor occupation to the final contractile response.