Atypical (relaxant) response to histamine in cat bronchus

Abstract

Histamine, 2-methylhistamine (a specific H₁-receptor agonist), 4-methylhistamine (a specific H₂-receptor agonist), isoprenaline, bradykinin, prostaglandin E₁, E₂ and F_2α 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 antagonists); metiamide or burimamide (specific H₂-receptor antagonists); propranolol (a β-adrenoceptor blocker) and indomethacin (a PG-synthetase inhibitor) suggesting non-participation of H₁, H₂-histamine receptors, β-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.