ML-9, a myosin light chain kinase inhibitor, reduces intracellular Ca2+ concentration in guinea pig trachealis

We investigated the effects of ML-9 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine], a myosin light chain kinase (MLCK) inhibitor, on intracellular Ca2+ concentration (Ca2+]i), contraction induced by high K+ and an agonist, and capacitative Ca2+ entry in fura-2-loaded guinea pig tracheal smooth muscle. ML-9 inhibited both the increase in Ca2+]i and the contraction induced by 60 mM K+, 1 microM methacholine or 1 microM thapsigargin, an inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. However, another MLCK inhibitor, wortmannin (3 microM), inhibited the contraction elicited by these stimuli without affecting Ca2+]i. Under the condition that the thapsigargin-induced contraction was fully suppressed by 3 microM wortmannin, 30 microM ML-9 caused a further decrease in Ca2+]i. The inhibitory effects of ML-9 on Ca2+]i and the contraction elicited by methacholine were similar to those of SKF-96365 (1-beta-3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride), a Ca2+ channel blocker. These results indicate that ML-9 acts as a potent inhibitor of Ca2+-permeable channels independently of MLCK inhibition in tracheal smooth muscle.