Effects of midazolam on intracellular Ca2+ and tension in airway smooth muscles

Abstract

Background: It has been demonstrated that a group of minor tranquilizers, benzodiazepines, are able to relax airway smooth muscles. To determine the underlying mechanisms of this phenomenon, the effects of midazolam on the intracellular Ca2+ concentration ([Ca2+](i)) and tension in airway smooth muscles were investigated. Methods: Using front-surface fluorometry and fura- 2-loaded porcine tracheal smooth muscle strips, both [Ca2+](i) and isometric tension developments were simultaneously recorded. Results: When the tracheal strips were exposed to a high external K+-solution (40 mM) or 10-7 M carbachol containing 1.25 mM Ca2+, both [Ca2+](i) and tension increased rapidly until they reached a plateau (the steady state). During steady-state contraction induced by K+-depolarization or carbachol, the cumulative application of midazolam (10-7 ~ 10-4 M) caused decreases in both [Ca2+](i) and tension, in a concentration-dependent manner. During 40 mM K+-induced depolarization, the stepwise increases in the extracellular Ca2+ concentration induced the stepwise increases in [Ca2+](i) and tension. Midazolam (3 x 10-5 M) inhibited these increases in [Ca2+](i) and tension, but had no effect on the [Ca2+](i)-tension relationship. In the presence of 3 x 10-3 M NiCl2 (a nonselective cation channel blocker), midazolam (3 x 10-5 M) did not cause any additional reduction of [Ca2+](i) or tension during the contraction induced by carbachol (10-7 M). In the absence of extracellular Ca2+, midazolam (3 x 10-5 M) had no effect on the transient increases in either [Ca2+](i) or the tension induced by carbachol (10-7 M) or caffeine (20 mM). Pretreatment with both 10-5 M flumazenil (a specific central antagonist of benzodiazepines) and 10-5 M PK11195 (a specific peripheral antagonist of benzodiazepines) did not influence the effect of 10-5 M midazolam on [Ca2+](i) or tension during the contractions induced by carbachol. Conclusions: Midazolam directly relaxes airway smooth muscles by decreasing [Ca2+](i); this can be attributed to the inhibition of the influx of extracellular Ca2+. Midazolam has no effect on the release of stored Ca2+. In addition, midazolam has no effect on Ca2+ sensitivity of the contractile apparatus. Finally, benzodiazepine antagonists, flumazenil and PK11195, have no effect on this mechanism of direct action of midazolam on airway smooth muscles.