Local anesthetics have been found to act as competitive inhibitors of caffeine in frog sartorius muscle. They block caffeine-induced rigor and the attendant increase in Ca45 influx and efflux. Increased net uptake of sodium, loss of potassium, and concurrent increase in oxygen consumption are all effectively blocked by procaine. Evidence is presented that the inhibitory effect of the local anesthetics cannot be explained by the formation of molecular complexes with caffeine. Increased efflux of Ca45 produced by changing from zero calcium Ringer's to 0.1 mM or 1 mM calcium Ringer's is inhibited by procaine and tetracaine. EDTA-stimulated calcium efflux is not affected by either local anesthetic. Caffeine rigor develops in frog muscle depolarized with KCl or rendered electrically inexcitable by sodium lack. Both the rigor and the increased calcium fluxes are inhibited by local anesthetics in depolarized muscle.

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