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 electrical 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 antagonists tested (0.03-10 mu M) with the following order of potency: RX 821002 = idazoxan > rauwolscine = yohimbine = BRL 44408 much greater than prazosin = ARC 239 = BRL 41992. According to the current classification, it is suggested that alpha(2)-heteroadrenoceptors involved in the modulation of duodenal cholinergic neurotransmission belong to the alpha(2D) subtype.