Occurrence of inhibitory histamine H2-receptors in isolated pulmonary blood vessels of dogs and rats.

Isolated, helically cut strips of pulmonary arteries and veins of dogs, and pulmonary arteries of rats, precontracted with norepinephrine or 5-HT exhibited potent concentration-dependent relaxations to impromidine and dimaprit (histamine H2-agonists). The occurrence of inhibitory histamine H2-receptors in the pulmonary vasculature could play a modulatory role in hypoxic pulmonary vasoconstriction.