Effects of intraluminal ca load on the kinetics of 45ca uptake and
efflux in brain microsomes.
Wells, Karen M., and Ronald F. Abercrombie.
Department of Physiology, Emory University School of Medicine,
Atlanta, GA, 30322
APStracts 3:0177C, 1996.
Effects of increasing intraluminal calcium ([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
calcium ([Ca2+]e)- and ATP-dependent manner. Increased microsomal Ca
load resulted in a decreased initial rate of 45Ca uptake and an
increased 116 \f "Symbol", time to reach 63% of steady-state
accumulation. Isolated rat brain microsomes lost 45Ca in a
temperature- and [Ca]i-dependent manner. Whether pre-loaded with
tracer 45Ca and either 163 \f "Symbol"0.5 M [Ca2+]e or 25 M [Ca2+]e,
the time constant of efflux was larger at 4 176 \f "Symbol"C as
compared to 37 176 \f "Symbol"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 OL
116 \f "Symbol", which was in fact longer for pre-loaded 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 ER Ca pump) and enhances
unidirectional Ca efflux, and that these combined effects ultimately
shorten the time needed to achieve a steady-state luminal Ca level.
Received 25 March 1996; accepted in final form 20 May 1996.
APS Manuscript Number C167-6.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 17 June 96