Mechanism of relaxing action of the antiasthmatic drug, azelastine, in isolated porcine tracheal smooth muscle

Abstract Azelastine (1–300 μM) inhibited contractions of isolated porcine trachea induced by high K + , carbachol and endothelin-1 (ET-1) with a decrease in [Ca 2+ ] cyt (as measured by fura-2-fluorescencc). Verapamil (0.1–10 μM) also inhibited the high K + -induced increases in [Ca 2+ ] cyt and contraction, although it only partially inhibited the responses evoked by carbachol or ET-1. In the absence of extracellular Ca 2+ (with 0.5 mM EGTA). carbachol induced a transient increase in [Ca 2+ ] cyt , and force by releasing Ca 2+ from cellular stores. Azelastine (100 μm) completely inhibited these contransient changes. In the absence of extracellular Ca 2+ , carbachol and 12-deoxyphorbol 13-isobutyrate (DPB) induced small sustained contractions without increasing [Ca 2+ ] cyt . Azelastine inhibited these contractions. In muscle permeabilized with α-toxin, Ca 2+ (0.3–3 μM) induced contraction in a concentration-dependent manner. DPB (without GTP) and carbachol or ET-1 (with GTP) enhanced the Ca 2+ -induced contraction. Azelastine partially inhibited the contraction induced by 0.3 μM Ca 2+ but not the contraction induced by 3 μM Ca 2+ , and strongly inhibited the potentiating effects of DPB, carbachol and ET-1. Azelastine had no effect on the content of cyclic AMP or cyclic GMP. These results suggest that azelastine inhibits smooth muscle contraction by (i) decreasing [Ca 2+ ] cyt , by inhibition of Ca 2+ channels, (ii) decreasing agonist-induced Ca 2+ release, and (iii) direct inhibition of contractile elements.