The goal of the current study was to determine the roles of ATP content, endoplasmic reticulum (ER) Ca(2+) stores, cytosolic free Ca(2+) (Ca(2+)(f)) and calpain activity in the signaling of rabbit renal proximal tubular (RPT) cell death (oncosis). Increasing concentrations (0.3-10 microM) of the mitochondrial inhibitor antimycin A produced rapid ATP depletion that correlated to a rapid and sustained increase in Ca(2+)(f), but not phospholipase C activation. The ER Ca(2+)-ATPase inhibitors thapsigargin (5 microM) or cyclopiazonic acid (100 microM) alone produced similar but transient increases in Ca(2+)(f). Pretreatment with thapsigargin prevented antimycin A-induced increases in Ca(2+)(f) and antimycin A pretreatment prevented thapsigargin-induced increases in Ca(2+)(f). Calpain activity increased in conjunction with ER Ca(2+) 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 Ca(2+) release, a sustained increase in Ca(2+)(f) and calpain activation. Depletion of ER Ca(2+) stores prior to toxicant exposure prevents increases in Ca(2+)(f) and oncosis.
