J Pharmacobiodyn 1982 Nov 01;511:877-85.

Difference in the mechanism of the action of caffeine and thymol on the calcium sequestering property of sarcoplasmic reticulum of frog skinned muscle fibers.

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The effects of caffeine and thymol on the release of calcium from sarcoplasmic reticulum (SR) of mechanically skinned fibers isolated from semitendinosus muscle of frog, Xenopus laevis, are studied. Calcium release caused by caffeine is composed of two processes, a fast and a slow ones. The fast process is observable only when the amount of calcium stored in SR is above a "threshold" value while the slow one detectable only below the threshold value. The rate of these two processes does not depend on the concentration of caffeine so strongly, but the threshold value decreased as caffeine concentration is increased. On the contrary to this, calcium release produced by thymol is observed as a single rate process. More thymol results in increase in the rate of this process. The effects of these two kinds of drug on the uptake of calcium to SR are also studied. These two drugs give different effects on the calcium uptake, which are consistent with those actions of drugs on the calcium release. Procaine weakens the effects of these drugs. A simple model is given to explain the action of these two drugs on SR. The mechanisms of the drug action are also discussed in relation to the results obtained with the spin-label study.

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