Endoplasmic reticulum Ca²⁺ signaling and calpains mediate renal cell death

The goal of the current study was to determine the roles of ATP content, endoplasmic reticulum (ER) Ca²⁺ stores, cytosolic free Ca²⁺ (Ca²⁺ _f) and calpain activity in the signaling of rabbit renal proximal tubular (RPT) cell death (oncosis). Increasing concentrations (0.3-10 μM) of the mitochondrial inhibitor antimycin A produced rapid ATP depletion that correlated to a rapid and sustained increase in Ca²⁺ _f, but not phospholipase C activation. The ER Ca²⁺-ATPase inhibitors thapsigargin (5 μM) or cyclopiazonic acid (100 μM) alone produced similar but transient increases in Ca²⁺ _f. Pretreatment with thapsigargin prevented antimycin A-induced increases in Ca²⁺ _f and antimycin A pretreatment prevented thapsigargin-induced increases in Ca²⁺ _f. Calpain activity increased in conjunction with ER Ca²⁺ release. Pretreatment, but not post-treatment, with thapsigargin or cyclopiazonic acid prevented antimycin A-induced cell death. These data demonstrate that extensive ATP depletion signals oncosis through ER Ca²⁺ release, a sustained increase in Ca²⁺ _f and calpain activation. Depletion of ER Ca²⁺ stores prior to toxicant exposure prevents increases in Ca²⁺ _f and oncosis.