Relaxant responses of the isolated guinea-pig left pulmonary artery produced by acidic histamine solutions.

Contractile responses to histamine were examined on left pulmonary arterial rings isolated from reserpine-pretreated guinea pigs. In the presence of mepyramine, contractile responses to histamine dihydrochloride and histamine diphosphate were preceded by concentration-dependent relaxant responses which were not blocked by metiamide. Hydrochloric acid also produced relaxation of the histamine- and noradrenaline-contracted vessels. Solutions of histamine dihydrochloride and histamine free base adjusted to about pH 7 did not produce the relaxant response. Changes in tissue bath pH were correlated with relaxant responses produced by acidic histamine and hydrochloric acid solutions. Relaxation produced by histamine in this preparation is due to changes in tissue bath pH and not to activation of an atypical type of histamine receptor.