Interaction with histamine H1-receptors and bronchospasmolytic effects of urapidil.

The antihypertensive drug, urapidil, competitively antagonized the histamine-induced contractions in guinea pig isolated tracheal and ileal preparations. Its affinity to histamine H1-receptors was 3-fold higher than that of histamine but 10- and 30-fold weaker than that of diphenhydramine and indoramin, respectively. Urapidil did not inhibit contractions induced by muscarinic agonists in both organs. Investigations in spontaneously breathing guinea pigs showed a greater efficacy of urapidil than that of diphenhydramine in protecting the animals against histamine-induced bronchospasms, whereas acetylcholine-induced spasms were only moderately inhibited. Theophylline, at a 100-fold higher dosage than urapidil, protected the animals against both histamine and acetylcholine challenges. This experimentally observed effect of urapidil supports clinical trials in hypertensive patients suffering also from obstructive lung disease and/or allergic illness.