The mechanism of the action of caffeine on sarcoplasmic reticulum.

Evidence is presented that caffeine does not act on the mitochondrial Ca uptake system and that its effect cannot be attributed to the accumulation of adenosine 3',5'-phosphate. Two distinct caffeine effects are described. At high ATP concentrations caffeine decreases the coupling between ATP hydrolysis and Ca inflow. It either inhibits inflow without any inhibition of the rate of ATP hydrolysis, or it stimulates the ATPase activity without stimulating Ca inflow. These high ATP concentrations (much higher than needed for the saturation of the transport ATPase) greatly reduce the control of the turnover rate of the transport system, by accumulated Ca. At low ATP concentrations when the transport system is under maximal control by accumulated Ca, caffeine inhibits the ATPase activity without affecting the rate of Ca inflow.