[The influences of local anesthetics on ryanodine-induced contracture in rat skeletal muscle].

One hundred and fifty paired extensor long digital muscles were excised from Wistar rats and each muscle was prepared in Krebs-Ringer's solution (K-R solution) then gassed with a mixture of 95% O2-5% CO2. The medium for the control muscles was replaced with K-R solution containing 10(-6) M ryanodine and that for the experimental muscles was replaced with medium containing 10(-6) M ryanodine and local anesthetic (LA) (procaine, tetracaine, benzocaine, lidocaine or bupivacaine at various concentration). Isometric contracture tension was recorded throughout the experiment. The ratios of the maximal contracture tension (C-ratio) and the elapsed time (T-ratio) of the muscles treated with LA compared to those of control muscles were calculated. Tetracaine (0.125-1.0 mM) specifically reduced the C-ratio. Procaine (0.5-1.0 mM) and tetracaine (10-60 microM) increased the T-ratio. Procaine (8-16 mM), benzocaine (4-8 mM), lidocaine (0.5-4 mM) and bupivacaine (0.125-1 mM) reduced the T-ratio. The influences of LAs on ryanodine-induced contracture could be explained in terms of their effects on the Ca(2+)-induced Ca2+ release mechanism, direct Ca2+ efflux from sarcoplasmic reticulum (SR), activity of Ca2+ uptake into SR and ryanodine-receptor binding. The complexity of LA effects on ryanodine-induced contracture will affect the results of ryanodine contracture tests for malignant hyperthermia when the muscle specimen is excised under local anesthesia.