Effects of midazolam on intracellular Ca2+ and tension in airway smooth muscles.
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) approximately 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.
D000818:Animals; D014151:Anti-Anxiety Agents / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D004347:Drug Interactions; D005442:Flumazenil / Q000494:pharmacology; D005470:Fluorometry; D018757:GABA Modulators / Q000494:pharmacology; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D017136:Ion Transport; D007546:Isoquinolines / Q000494:pharmacology; D008874:Midazolam / Q000494:pharmacology*; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects* / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D013552:Swine; D014132:Trachea / Q000187:drug effects
D014151:Anti-Anxiety Agents; D018757:GABA Modulators; D007546:Isoquinolines; D005442:Flumazenil; D008874:Midazolam; D011188:Potassium; D002118:Calcium; C037850:PK 11195