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 Ca⁺² (Ca(f)⁺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.