Effects of histamine H2-receptor agonists and antagonists on isolated guinea-pig airway muscles.

The effects of histamine H2-receptor agonists and antagonists on isolated guinea-pig lung parenchymal and tracheal strips were investigated. Dimaprit, 10(-7) to 10(-3) M, produced dose-dependent relaxation of parenchymal strips while at 10(-3) M, tracheal strips relaxed only partially. Pretreatment of parenchymal strips with cimetidine or YM-11170, H2-receptor antagonists, resulted in a parallel shift of the dimaprit dose-response curve to the right. 4-Methylhistamine produced relaxation of parenchymal strips in a bell-shaped dose-related manner; at high concentrations, it effected the contraction of tracheal strips. In mepyramine-pretreated parenchymal and tracheal strips, 4-methylhistamine induced dose-dependent relaxation, indicating that at high concentrations, it stimulates two functionally opposite receptors, H1 and H2. These results suggest that there are H2-receptors in guinea-pig airways, predominantly in the periphery.