Effects of azelastine on contraction of guinea pig tracheal smooth muscle

Abstract

Azelastine [4-(p-chlorobenzyl)-2-(hexahydro-1-methyl-1H-azepine-4-yl)-1(2H)-phthalazinone hydrochloride] is a new anti-asthmatic drug. We examined the mechanism of its inhibitory action on guinea pig tracheal smooth muscle contraction by measuring membrane potential and isometric force using intracellular microelectrodes and a micro-force transducer. The mean resting membrane potential of guinea pig tracheal muscle cells was −54 mV. Perfusion with 20 mM tetraethylammonium (TEA) caused membrane depolarization and elicited spontaneous action potentials. Azelastine (1–100 μM) suppressed both the amplitude and maximal rate of rise of the action potentials in a concentration-dependent manner. Complete abolition occurred at 100 μM. Similarly, azelastine (0.1–100 μM) inhibited and abolished 50 mM KCl-induced contractions. These results suggest that azelastine may inhibit voltage-dependent Ca 2+ channels. Next, pretreatment of tracheal muscle (for 15 min) with azelastine (0.01–100 μM) inhibited subsequent acetylcholine (ACh) (0.01–100 μM)-induced contractions. Azelastine, 100 μM, completely abolished the ACh-induced contractions. In contrast, high concentrations of Ca 2+ channel antagonists diltiazem (10–100 μM) or nifedipine (20 μM), and Ca 2+-free solution, only partially depressed the ACh contractions suggesting that azelastine has an additional effect on intracellular Ca 2+ release. In Ca 2+-free solution (containing 0.5 nM EGTA), azelastine (1–100 μM) depressed and abolished the transient contractions induced by 10 μM ACh. We conclude that azelastine inhibits airway constriction by inhibiting both voltage-sensitive Ca 2+ slow channels on the cell membrane and Ca 2+ release from a intracellular storage site.