Atypical (relaxant) response to histamine in cat bronchus

Histamine, 2-methylhistamine (a specific H1-receptor agonist), 4-methylhistamine (a specific H2-receptor agonist), isoprenaline, bradykinin, prostaglandin E1, E2, and F 2alpha induce relaxation of carbachol-contracted isolated cat bronchial strips and tracheal chains. Bovine SRS-A contracts bronchus but not trachea. Histamine-induced relaxation of cat bronchus is not blocked by mepyramine (a specific H1-receptor antagonist); metiamide or burimamide (specific H2-receptor antagonists); propranolol ( a beta-adrenoceptor blocker) and indomethacin (a PG-synthetase inhibitor) suggesting non-participation of H1,H2-histamine receptors, beta-adrenoceptors (catecholamine release) and prostaglandin release in histamine-induced broncho-relaxations in the cat. The existence of an atypical histamine response, resistant to both H1- and H2-receptor antagonists is thus established in cat bronchus.