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.
Wells
K M
KM
Department of Physiology, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
Abercrombie
R F
RF
D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D001921:Brain / Q000378:metabolism*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism*; D008954:Models, Biological; D051381:Rats; D017207:Rats, Sprague-Dawley; D013696:Temperature
D002132:Calcium Radioisotopes; D000001:Calcimycin; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium