A dual action of histamine on isolated human pulmonary arteries

Isometric tension changes in 22 isolated segmental human pulmonary arteries obtained from 17 patients at thoracic surgery were studied after histamine challenge. Cumulative dose-response contraction curves were constructed. A slight potentiation could be found with the H2 antagonist cimetidine, whereas the H1 blocker mepyramine moved the log dose-response curve for histamine to the right indicating competitive antagonism at specific receptors. When mepyramine was added after maximal contraction with histamine, a relaxation occurred that could be blocked by adding cimetidine prior to mepyramine. The in vitro tests reveal a dual action of histamine with a dominating H1 receptor mediated contraction, but with the possibility of relaxation via specific H2 receptors in human pulmonary arteries.