Global ischemia-induced inhibition of the coupling ratio of calcium uptakeand ATP hydrolysis by rat whole brain microsomal Mg2+/Ca2+ ATPase

Ischemia is associated with a loss of cytosolic calcium homeostasis. Intrac ellular stores, particularly in endoplasmic reticulum, are critical for the maintenance of calcium homeostasis. Recent studies have shown that ischemi a significantly inhibited microsomal calcium uptake mediated by Mg2+/Ca2+ A TPase, the major mechanism of endoplasmic reticulum calcium sequestration. This study was initiated to determine whether the decreased calcium uptake caused by ischemia was the result of inhibition of Mg2+/Ca2+ ATPase activit y or an uncoupling of calcium uptake from ATP hydrolysis. The microsomal Mg 2+/Ca2+ ATPase specific inhibitor thapsigargin partially inhibited ATPase a ctivity and completely inhibited calcium uptake. ATPase inhibited by thapsi gargin was considered microsomal Mg2+/Ca2+ ATPase. Ischemia from 5 to 60 mi n had no significant effect on thapsigargin sensitive ATPase activity. Howe ver, under identical conditions, increasing ischemia from 5 to 60 min signi ficantly inhibited microsomal calcium uptake. Comparing calcium uptake to A TP hydrolysis as ischemia increased from 5 to 60 min revealed that the coup ling ratio of calcium molecules sequestered to ATP molecules hydrolyzed bec ame significantly decreased. The results demonstrated that the effect of is chemia on microsomal calcium uptake was mediated by an uncoupling of calciu m transport from Mg2+/Ca2+ ATPase activity. (C) 1999 Elsevier Science B.V. All rights reserved.