Further studies of dog tracheobronchial smooth muscle

Isolated canine tracheal and bronchial smooth muscle contracted to carbachol, 5-HT and histamine. PGF2 alpha and bradykinin were inactive. Carbachol-contracted airway smooth muscle relaxed to PGE1, PGE2, isoproterenol, dimaprit (H2-agonist) and bradykinin (BK). Indomethacin blocked relaxations to BK. Mepyramine in low doses specifically antagonized contractions to histamine. Mepyramine-resistant contractions to histamine were blocked by atropine and burimamide. In the presence of mepyramine (after H1-blockade), relatively high doses of histamine relaxed bronchi (but not tracheas) which were submaximally contracted to carbachol. Bronchial relaxations to histamine were resistant to indomethacin and propranolol suggesting that prostaglandin and catecholamine release mechanisms were not involved. Burimamide (H2-antagonist) enhanced broncho-relaxant effects of histamine and attenuated relaxations to isoproterenol and contractions to carbachol. This investigation showed a preponderance of H1-histamine receptors in canine airways and a small population of H2 (inhibitory) histamine receptors, partially resistant to H2-antagonism.