Calcium chelators enhance 45 ca2+ accumulation in permeablized
synaptosomes and microsomes.
Moore, James E., and R. F. Abercrombie.
Department of Physiology, Emory University School of Medicine,
Atlanta, Georgia 30322
APStracts 2:0302C, 1995.
The study of intracellular Ca regulation usually requires using Ca
chelators to adjust [Ca2+]. We examined the effects of these
chelators on Ca accumulation in microsomes and saponin-permeabilized
synaptosomes to assess their influence on apparent transport
properties. At a fixed free calcium of 0.6[mu]M, increasing EGTA and
total Ca enhanced ATP-dependent 45Ca sequestration in synaptosomes
and microsomes. The EGTA/Ca complex did not change the maximal
initial calcium uptake rate or steady state accumulation. Rather,
EGTA/Ca increased the apparent affinity of the microsomal transporter
for Ca2+. The presence of the organic anion transport inhibitor
probenicid (2.5mM), had no effect on 45Ca accumulation in the
presence of EGTA. Replacing part of the Ca with Ni but maintaining
[Ca2+] approximately constant reduced 45Ca uptake, suggesting that
the Ni-EGTA complex did not stimulate 45Ca transport. Our results
imply that EGTA is not actively transported across the endoplasmic
reticulum (ER) membrane, nor does the divalent ion-bound form of EGTA
change the properties of the transporter. EGTA, and other mobile Ca
chelators with similar structures, e.g., BAPTA, INDO-1 and Fluo 3,
may increase Ca uptake by delivering more calcium to its transport
site.
Received 12 April 1995; accepted in final form 11 August 1995.
APS Manuscript Number C207-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 August 1995.