The relationship between caffeine contracture of intact muscle and the effect of caffeine on reticulum.

At concentrations between 1 to 10 mM, caffeine reduced the Ca-accumulating capacity of fragmented reticulum obtained from frog and rabbit muscle. With 8 mM caffeine enough Ca was released from frog reticulum to account for the force of the contracture. Caffeine did not affect all reticulum membranes equally. The fraction which was spun down at 2000 g was more sensitive than the lighter fractions. The percentage of the total accumulated Ca released by caffeine decreased with decreasing Ca content of the reticulum. In parallel with their known effects on the caffeine contracture, a drop in temperature increased the caffeine-induced Ca release while procaine inhibited it. Caffeine also inhibited the rate of Ca uptake, which may in part account for the prolongation of the active state caused by caffeine.