DETERMINATION ON FUNCTIONAL BASIS OF PRESYNAPTIC ALPHA(2)-ADRENOCEPTOR SUBTYPES IN GUINEA-PIG DUODENUM

The effects of several alpha(2)-adrenoceptor agonists and antagonists were examined on the cholinergic twitch contractions evoked by electri cal field stimulation of guinea-pig duodenum. Oxymetazoline, xylazine, noradrenaline, alpha-methyl-noradrenaline or medetomidine (0.01-30 mu M) were nearly equieffective in inhibiting duodenal twitch responses. The effects of xylazine were competitively counteracted by antagonist s tested (0.03-10 mu M) with the following order of potency: RX 821002 = idazoxan > rauwolscine = yohimbine = BRL 44408 much greater than pr azosin = ARC 239 = BRL 41992. According to the current classification, it is suggested that alpha(2)-heteroadrenoceptors involved in the mod ulation of duodenal cholinergic neurotransmission belong to the alpha( 2D) subtype.