Calcium chelators enhance ⁴⁵Ca accumulation in permeablized synaptosomes and in microsomes

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 μM, increasing ethylene glycol-bis(β-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.