Precontraction-induced contractile response of isolated canine portal vein to alpha-2 adrenoceptor agonists.

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.