Occurrence of inhibitory histamine H2-receptors in bovine pulmonary arteries.

The spirally cut strips of bovine pulmonary artery responded with contractions to 5HT greater than histamine and relaxed to isoproterenol, impromidine and greater than dimaprit. After complete H1-blockade with mepyramine, histamine caused weak relaxation of the tissues contracted with 5HT. Mepyramine competitively antagonized histamine-induced contractions while the relaxant responses of pulmonary arteries to impromidine and dimaprit were sensitive to cimetidine. Impromidine was approximately 1000 - 5000 X more potent and 3 X more efficacious than dimaprit. Mepyramine neither potentiated nor inhibited impromidine-induced relaxations. The results suggest that while excitatory H1-receptors predominate; a small population of inhibitory H2-receptors also exist in bovine pulmonary arteries. The greater potency and efficacy of impromidine compared to dimaprit confirm that impromidine is a potent and specific H2-agonist.