title: Precontraction-induced contractile response of isolated canine portal vein to alpha-2 adrenoceptor agonists.
abstract: The responses of isolated canine portal veins, in either transverse or longitudinal strip, to alpha-2 adrenoceptor agonists were examined. B-HT920, a selective alpha-2 adrenoceptor agonist, did not elicit an appreciable response in the transverse strips of the portal vein under resting tone (0.5 g/mm width). When the preparation was partially precontracted with phenylephrine, prostaglandin F2 alpha, KCl, Bay K 8644, acetylcholine, 5-hydroxytryptamine, histamine, or A23187, B-HT920 evoked concentration-dependent contractile responses (3 x 10(-9)-10(-6) M). Maximum contractions, which depended on the precontraction levels and the precontracting substances, ranged from 10-35% of those evoked by norepinephrine 10(-5) M. Similar precontraction-dependent contractile responses were obtained in the longitudinal strips. Concentration-response curves of B-HT920 in the transverse strips precontracted with prostaglandin F2 alpha were shifted by yohimbine to the right, but not by prazosin. Schild analysis yielded a slope of unity and a pA2 of 8.79. Clonidine also showed a similar precontraction-dependent contractile response. The lower part of the concentration-response curve of clonidine was shifted by yohimbine. These results may be explained by the presence of postsynaptic alpha-2 adrenoceptors in canine portal vein, which mediate a contractile response under certain conditions.
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D001381:Azepines / Q000494:pharmacology; D000001:Calcimycin / Q000494:pharmacology; D002415:Cats; D003000:Clonidine / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects*; D009638:Norepinephrine / Q000494:pharmacology; D011169:Portal Vein / Q000187:drug effects; D011224:Prazosin / Q000494:pharmacology; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D000316:Adrenergic alpha-Agonists; D001381:Azepines; D015016:Yohimbine; D000001:Calcimycin; C024763:talipexole; D003000:Clonidine; D009638:Norepinephrine; D011224:Prazosin