Interactions between noradrenaline and α₂‐adrenoceptor agonists in the superior mesenteric arterial bed of the rat

Interactions between alpha 2-agonists and noradrenaline vasoconstrictor responses were studied in the superior mesenteric arterial bed of the rat by use of perfusion both in situ with blood and in vitro with Krebs-Henseleit solution. Xylazine (1.9 X 10(-6) mol) administered into the perfusion circuit reduced the maximum response to noradrenaline in the in situ preparation by 35% and decreased the pD50 for noradrenaline from 8.5 +/- 0.01 to 7.9 +/- 0.13 (n = 7). Yohimbine (1 mg kg-1, i.v.) gave a small parallel shift in the noradrenaline log dose-response curve and prevented the reduction in the maximum response by subsequent administration of xylazine. In vitro, xylazine (1.9 X 10(-6) mol) also gave a long-lasting reduction of 37% in the maximum response but did not affect the mid-point sensitivity to noradrenaline. Yohimbine (10(-6) M) did not change either of these effects. Clonidine (1.9 X 10(-6) mol) did not affect the maximum response to noradrenaline in vitro but did reduce the pD50 from 7.72 +/- 0.17 to 6.9 +/- 0.17 (n = 6). Yohimbine did not change these effects. Guanfacine (1.8 X 10(-6) mol) had no effect on the sensitivity of the in vitro preparation to noradrenaline but did reduce the maximum response by 20%. Yohimbine (10(-6)M) prevented the depression of the maximum response. It is concluded xylazine and clonidine interfere with noradrenaline induced vasoconstriction only to a limited extent through their interaction with alpha 2-adrenoceptors and that some other, as yet uncharacterized mechanism which may be activated by their aryl amidine structure, is responsible for their in vitro effects.