Antagonist like action of synthetic alpha 2-adrenoceptor agonists on contractile response to catecholamines in smooth muscle strips isolated from rainbow trout stomach (Salmo gairdneri).

1. The effects of some synthetic alpha 2-adrenoceptor agonists on the mechanical activity and on contractile responses to catecholamines were examined in smooth muscle strips isolated from rainbow trout stomach. 2. Contractile responses to noradrenaline and adrenaline in the rainbow trout stomach strips were due to alpha 2-adrenoceptor activation. 3. Clonidine, p-aminoclonidine, naphazoline and guanabenz caused no mechanical response but concentration-dependently inhibited the contractile responses to noradrenaline and adrenaline without affecting the responses to acetylcholine, carbachol, 5-hydroxytryptamine and methionine-enkephalin. The order of potency was naphazoline greater than p-aminoclonidine greater than clonidine greater than guanabenz. 4. It is suggested that in the smooth muscle preparation of the trout stomach, some synthetic compounds (clonidine, p-aminoclonidine, naphazoline and guanabenz), which act on mammalian preparations as alpha 2-adrenoceptor agonists, show an antinoradrenaline (-adrenaline) effect; those compounds can be classified as alpha 2-adrenoceptor antagonists.