Studies on the effect of histamine in isolated human pulmonary arteries and veins.

The effect of histamine (0.01-200 microM) was studied in isolated human pulmonary vessels. Histamine induced concentration dependent contractions in both arteries and veins. In veins the maximal response to histamine was lower than in arteries. Histamine and 2-methyl-histamine had a dual action in both arteries and veins clearly demonstrated in vessels precontracted with potassium. In these vessels histamine and 2-methyl-histamine induced relaxation at low concentrations and contractions at high concentrations. Veins were more sensitive to the relaxant effect of histamine than arteries. Mepyramine eliminated the dual action of 2-methyl-histamine and histamine and unveiled a mepyramine resistant relaxation at the highest histamine concentrations used which was resistant to the effect of cimetidine and metiamide. The H2 receptor agonist dimaprit (10-400 microM) induced a slight relaxation in both arteries and veins that could be eliminated by metiamide (100 microM). The results show that histamine has a dual action in human pulmonary vessels which includes a contractile effect mediated via H1 receptors and a relaxant response partly mediated through H1 receptors and partly via unspecific mechanisms. However, an H2 mediated relaxant effect cannot be excluded.