Ischemia and ceain toxins cause an increase in cytosolic calcium concentration, initially because of the release of Ca2+ from the intracellular stores, and later resulting from increased influx across the plasma membrane.Increased cytosolic Ca2+ activates a number of enzymes, with potentially deleterious cellular effects.These enzymes include phospholipases (which cause membrane damage), proteases (which break down both membrane and cytoskeletal proteins), endonucleases (which are responsible for DNA and chromatin fragmentation), and adenosine triphosphatases (ATPases) (thereby hastening ATP depletion).( Robbins Basic Pathology, 9th edition, page 23 )