C5a induced tracheal contraction: A histamine independent mechanism

C5a, a peptide derived from the fifth component of complement, caused significant prolonged smooth muscle contraction in isolated guinea pig trachea. Diphenhydramine, a histamine receptor antagonist of the H₁ type, had no effect on either the rate, amplitude or duration of C5a induced tracheal contraction, whereas it significantly inhibited the tracheal response to exogenous histamine. Diphenhydramine also caused a significant delay in the response to compound 48/80 in normal guinea pig trachea and to antigen in actively sensitized trachea, indicating that the antihistamine was capable of inhibiting tracheal contractions to endogenously released histamine. C5a induced tracheal smooth muscle contraction was also unaffected by antagonists of H₂, muscarinic cholinergic and alpha adrenergic receptors. These results indicate that C5a induced tracheal contraction is independent of histamine and is potentially a mediator of allergic bronchospasm.