The possible involvement of hyperpolarizing mechanisms in histamine-induced relaxation of the rat portal vein

Abstract

The present study evaluated the effects of histamine 10 -2 M on longitudinal preparations of rat portal vein. It was observed that histamine 10 -2 M induced relaxation of rat portal vein preparations pre-contracted with phenylephrine 10 -4 M. On the other hand, no pharmacological effects were observed in preparations not pre-contracted. The observed histamine-induced relaxing effect was absent in preparations pre-contracted with KCl (120 mM) or in the presence of depolarizing nutritive solution. However, the histamine-induced relaxation was still present in the endothelium-removed preparations. The histamine-induced relaxation also was not prevented by astemizole ( M or L-NAME 10 -4 M plus indomethacin 10 -5 M also did not prevent the histamineinduced relaxation observed in rat portal vein. Thus, the histamine-induced relaxation observed in rat portal vein appears to involve a non-endothelial hyperpolarizing mechanism independent of H1, H2 and H3 receptors.