The calcium and the oxygen paradox: non-existent on the cellular level.

Ca2+-tolerant isolated adult heart cells can be exposed to 1 mM EGTA and then again to 1 mM CaCl2 without developing irreversible hypercontracture. Thus, they are not subject to the calcium paradox, even though they apparently become more permeable to Na+ during Ca2+-free incubation. When these cells are incubated anoxically without substrate they slowly lose their energetic reserves. The process resembles that of the arrested anoxic myocardium. The appearance of irreversibly hypercontracted cells is neither accompanied by a parallel increase in enzyme release nor by an aggravation of the anoxia-induced damage by reoxygenation. Thus these cells are not subject to the oxygen paradox. It is suggested that the differences between the myocytes' behaviour in tissue and in the isolated state are due to the fact that the isolated cells are free from mechanical interactions with adjacent cells.