Dominant role of mitochondria in clearance of large Ca2+ loads from rat adrenal chromaffin cells

Cytosolic Ca2+ (Ca-c(2+)) clearance from adrenal chromaffin cells was studied by whole-cell patch clamp and indo-1 Ca2+ photometry after influx of Ca2+ through voltage-dependent Ca2+ channels. We isolated the rates of Ca-c(2+) clearance by several mechanisms using combinations of the following agents (with their expected targets): Li+ or TEA substituted for Na+ (Na+-Ca2+ exchange), 1 mM La3+ applied after the depolarization (Na+-Ca2+ exchange and plasma membrane Ca2+-ATPase), 1 mu M thapsigargin (pumping into reticular stores), and 2 mu M carbonyl cyanide m-chlorophenylhydrazone (uptake into mitochondria). Remarkably, whenever [Ca2+](c) rose above similar to 500 nM, Ca-c(2+) clearance by mitochondria exceeded clearance by either Na+-Ca2+ exchange or the Ca2+ pumps of the plasma and reticular membranes. As [Ca2+](c) fell again, Ca2+ re-emerged from mitochondria, prolonging the final return to basal levels.