Depletion of endoplasmic reticulum calcium stores protects against hypoxia- and mitochondrial inhibitor-induced cellular injury and death.

We have shown previously that intracellular Ca+2 chelation and calpain inhibitors block the influx of extracellular Ca+2 and Cl- during the late phase of cell injury in renal proximal tubules (RPT) exposed to the mitochondrial inhibitor antimycin A. Since the endoplasmic reticulum (ER) is the major intracellular Ca+2 storage site, ER Ca+2 release/depletion may mediate the Ca+2 influx and cell death. Treatment of RPT suspensions with thapsigargin, an ER Ca+2-ATPase inhibitor, increased cytosolic free Caf+2 (Ca+2) levels from 122 +/- 7 to 322 +/- 55 nM within 10 sec of addition followed by a return to control levels within 3 min. A 5-min pretreatment of RPT suspensions with thapsigargin blocked antimycin A- and hypoxia-induced influx of extracellular Ca+2 and Cl- and the resulting cell death/lysis. These data suggest that ER Ca+2 release/depletion during cell injury may trigger a signaling cascade that causes extracellular Ca+2 influx followed by Cl- influx, cell swelling, and ultimately cell death/ lysis.