Calcium transport in microsomes isolated from rat parotid gland. Effects of ADP and an ATP-regenerating system

Calcium loading of a rat parotid microsomal fraction is greatly increased by an ATP-regenerating system (phosphocreatine and creatine phosphokinase). This effect is neither a consequence of a rise in the ATP concentration nor of an increased formation of inorganic phosphate originating from hydrolysis of ATP or phosphocreatine. Addition of ADP to the incubation medium provokes an inhibition of Ca2+ influx and a stimulation of Ca2+ efflux by the microsomal fraction. These results suggest that the stimulation of Ca2+ uptake by the ATP-regenerating system is due, at least in part, to an increase of Ca2+ influx and a slowing down of Ca2+ efflux as consequence of a decrease of ADP availability. It is proposed that the effect of ADP on Ca2+ movements could account for the action of certain agonists on intracellular Ca2+ concentration. Moreover, the InsP3 responsive Ca2+ pool was also shown to be enlarged by the ATP-regenerating system without modification of InsP3 sensitivity.