Pulmonary vasodilator action of tolazoline.

The pulmonary vasodilator action of tolazoline in newborn lambs was shown to be mediated via histamine receptors. Maximal changes in pulmonary vascular resistance, deltaPVR, were calculated as percents of the base line value, %deltaPVR. The mean %deltaPVR after tolazoline, 1 mg/kg, was -25 +/- 4% for eight lambs. Four lambs then received the histamine H1 receptor antagonist, diphenhydramine, and the mean %deltaPVR due to tolazoline was -12 +/- 4%. Four lambs received the H2 receptor antagonist, metiamide, and the mean %delta PVR due to tolazoline was -18 +/- 5%. After both H1 and H2 antagonists, the mean %deltaPVR due to tolazoline was +6 +/- 8%. Therefore, both histamine H1 and H2 receptors were involved in the vasodilator response to tolazoline.