Pulmonary Vasodilator Action of Tolazoline

Abstract

Summary: The pulmonary vasodilator action of tolazoline in newborn lambs was shown to be mediated via histamine receptors. Maximal changes in pulmonary vascular resistance, ΔPVR, were calculated as percents of the base line value, %ΔPVR. The mean %ΔPVR after tolazoline, 1 mg/kg, was −25 ± 4% for eight lambs. Four lambs then received the histamine H1 receptor antagonist, diphen-hydramine, and the mean %ΔPVR due to tolazoline was — 12 ± 4%. Four lambs received the H2 receptor antagonist, metiamide, and the mean %ΔPVR due to tolazoline was −18 ± 5%. After both H1 and H2 antagonists, the mean %ΔPVR due to tolazoline was +6 ± 8%. Therefore, both histamine H1 and H2 receptors were involved in the vasodilator response to tolazoline. Speculation: If the pulmonary vasodilator action of tolazoline is mediated by histamine receptors in human infants, as it is in newborn lambs, it may be possible to select a more specific therapeutic agent for infants with pulmonary vasospasm. Histamine, 2-mcthylhistamine, a specific H1 agonist, or 4-methylhistaminc, a specific H2 agonist, may be such agents in certain situations.