Effects of the intraluminal Ca load on the kinetics of 45Ca uptake and efflux in brain microsomes.

Effects of increasing intraluminal Ca ([Ca]i) on the kinetics of rat brain microsomal uptake and efflux are reported here. Isolated rat brain microsomes accumulated 45Ca in an extravesicular free Ca ([Ca]o)- and ATP-dependent manner. Increased microsomal Ca load resulted in a decreased initial rate of 45Ca uptake and an increased tau, time to reach 63% of steady-state accumulation. Isolated rate brain microsomes lost 45Ca in a temperature- and [Ca]i-dependent manner. Whether preloaded with tracer 45Ca and either < or = 0.5 or 25 microM [Ca]o, the time constant of efflux was larger at 4 degrees C as compared with 37 degrees C. Additionally, increased microsomal Ca load resulted in a decreased time constant of 45Ca efflux. This shorter efflux time constant cannot explain the effect of [Ca]i on tau during uptake which was in fact longer for preloaded microsomes. Rather, these data suggest that, as Ca accumulates into unloaded microsomes, a steadily increasing [Ca]i slows unidirectional Ca influx (presumably by inhibiting the endoplasmic reticulum Ca pump) and enhances unidirectional Ca efflux, and that these combined effects ultimately shorten the time needed to achieve steady-state luminal [Ca]i.