Global ischemia-induced inhibition of the coupling ratio of calcium uptake and ATP hydrolysis by rat whole brain microsomal Mg 2+/Ca 2+ ATPase

Abstract

Ischemia is associated with a loss of cytosolic calcium homeostasis. Intracellular stores, particularly in endoplasmic reticulum, are critical for the maintenance of calcium homeostasis. Recent studies have shown that ischemia significantly inhibited microsomal calcium uptake mediated by Mg 2+/Ca 2+ ATPase, 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 Mg 2+/Ca 2+ ATPase activity or an uncoupling of calcium uptake from ATP hydrolysis. The microsomal Mg 2+/Ca 2+ ATPase specific inhibitor thapsigargin partially inhibited ATPase activity and completely inhibited calcium uptake. ATPase inhibited by thapsigargin was considered microsomal Mg 2+/Ca 2+ ATPase. Ischemia from 5 to 60 min had no significant effect on thapsigargin sensitive ATPase activity. However, under identical conditions, increasing ischemia from 5 to 60 min significantly inhibited microsomal calcium uptake. Comparing calcium uptake to ATP hydrolysis as ischemia increased from 5 to 60 min revealed that the coupling ratio of calcium molecules sequestered to ATP molecules hydrolyzed became significantly decreased. The results demonstrated that the effect of ischemia on microsomal calcium uptake was mediated by an uncoupling of calcium transport from Mg 2+/Ca 2+ ATPase activity.