title: Calcium chelators enhance 45Ca accumulation in permeablized synaptosomes and in microsomes.
abstract: The study of intracellular Ca2+ regulation usually requires using calcium chelators to adjust [Ca2+]. We examined the effects of these chelators on calcium accumulation in microsomes and saponin-permeabilized synaptosomes to assess their influence on apparent transport properties. At a fixed free Ca2+ of 0.6 microM, increasing ethylene glycol-bis(beta-aminoethyl ether)-N,N,N', N'-tetraacetic acid (EGTA) and total Ca2+ enhanced ATP-dependent 45Ca sequestration in synaptosomes and microsomes. The EGTA-Ca complex did not change the maximal initial calcium uptake rate or maximal 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 (2.5 mM) had no effect on 45Ca accumulation in the presence of EGTA. Replacing part of the Ca2+ with Ni2+ 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 membrane, nor does the divalent ion-bound form of EGTA change the properties of the transporter. EGTA, and other mobile calcium chelators with similar structures, e.g., 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, indo 1, and fluo 3, may increase calcium uptake by delivering more Ca2+ to its transport site.
authors.affiliation: Department of Physiology, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
authors.forename: J E
authors.initials: JE
authors.lastname: Moore
authors.forename: R F
authors.initials: RF
authors.lastname: Abercrombie
mesh_terms: D000494:Allosteric Regulation; D000818:Animals; D001921:Brain / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D002412:Cations / Q000378:metabolism; D002614:Chelating Agents / Q000494:pharmacology*; D004533:Egtazic Acid / Q000378:metabolism / Q000493:pharmacokinetics / Q000494:pharmacology; D008297:Male; D008861:Microsomes / Q000378:metabolism*; D010539:Permeability; D051381:Rats; D017207:Rats, Sprague-Dawley; D013574:Synaptosomes / Q000378:metabolism*
chemical_list: D002132:Calcium Radioisotopes; D002412:Cations; D002614:Chelating Agents; D004533:Egtazic Acid; D002118:Calcium