CALCIUM MOVEMENTS IN ISOLATED MICROSOMES OF THE RAT PAROTID-GLAND - EFFECTS OF ADP AND AN ATP REGENERATING SYSTEM

Calcium loading of a rat parotid microsomal fraction is greatly increa sed by an ATP-regenerating system (phosphocreatine and creatine phosph okinase). This effect is neither a consequence of a rise in the ATP co ncentration nor of an increased formation of inorganic phosphate origi nating from hydrolysis of ATP or phosphocreatine. Addition of ADP to t he incubation medium provokes an inhibition of Ca2+ influx and a stimu lation of Ca2+ efflux by the microsomal fraction. These results sugges t that the stimulation of Ca2+ uptake by the ATP-regenerating system i s due, at least in part, to an increase of Ca2+ influx and a slowing d own of Ca2+ efflux as consequence of a decrease of ADP availability. i t is proposed that the effect of ADP on Ca2+ movements could account f or the action of certain agonists on intracellular Ca2+ concentration. Moreover, the InsP(3) responsive Ca2+ pool was also shown to be enlar ged by the ATP-regenerating system without modification of InsP(3) sen sitivity.