Effects of Lidocaine on Intracellular Ca2+ and Tension in Airway Smooth Muscle

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Abstract
Many studies have demonstrated that lidocaine directly relaxes airway smooth muscle. The underlying mechanisms, especially in relation to Ca2+ mobilization, remain to be elucidated. Using front-surface fluorometry and fura-2-loaded porcine tracheal smooth muscle strips, intracellular Ca2+ concentration ([Ca2+]i) and isometric tension were simultaneously measured. In cases of 40 mM K(+)-induced contraction and 1 microM acetylcholine (ACh)-induced contraction, the cumulative application of lidocaine (10(-6) approximately 3 x 10(-3) M) caused a concentration-dependent decrease in [Ca2+]i and tension, and almost complete relaxation. To examine the effect of lidocaine on Ca2+ sensitivity of the contractile apparatus, the [Ca2+]i-tension relationship was determined by changing the extracellular Ca2+ concentration during 40 mM K+ induced depolarization, with and without treatment with lidocaine. Although treatment with 1 mM lidocaine inhibited increases in both [Ca2+]i and tension induced by extracellular Ca2+, it had little effect on the [Ca2+]i-tension relationship. In the presence of 1 microM ACh, the [Ca2+]i-tension relationship shifted markedly to the left, thereby indicating an increase in Ca2+ sensitivity of the contractile apparatus; this shift was inhibited by 1 mM lidocaine. In the absence of extracellular Ca2+, 1 mM lidocaine inhibited the release of stored Ca2+ induced by 1 microM ACh, but not that by 20 mM caffeine. Lidocaine directly relaxes airway smooth muscle by decreasing [Ca2+]i. In addition, lidocaine inhibits the ACh-induced increase of Ca2+ sensitivity of the contractile apparatus, although it has little effect on Ca2+ sensitivity during high K+ depolarization. The decrease in [Ca2+]i is attributed to inhibition of the influx of extracellular Ca2+, as induced by high K+ depolarization and by ACh, and to the inhibition of the ACh-induced release of stored Ca2+.

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