question: Which phenomenon is known as the "calcium paradox" in the isolated perfused heart?
answer: When hearts are reperfused with Ca++ after a short period of Ca++-free perfusion, irreversible loss of electrical and mechanical activity is observed. This phenomenon, first described by Zimmerman and Hulsmann, was termed the "calcium paradox". This phenomenon is concomitant with a rapid consumption of myocardial high-energy phosphate stores. The Ca(2+) paradox represents a good model to study Ca(2+) overload injury in ischemic heart diseases. The Ca(2+) paradox can be elicited by perfusing isolated hearts with Ca(2+)-free media for 3 min or 5 min followed by 30 min of Ca(2+) repletion. A possible mechanism for the 'calcium paradox' is that exposure to a calcium-free medium removes extracellular calcium rendering the sarcolemma more permeable to calcium. On calcium repletion, cell injury is triggered by calcium influx. Cardiac dysfunction due to Ca2+ -paradox may be associated with apoptosis.