The biochemistry of uncontrolled calcium entry

The Ca2+ paradox is characterized by a rapid and uncontrolled entry of Ca2+. Whilst the consequence of the resultant gain in Ca2+ are relatively well defined, uncertainty exists concerning the route of entry. Possible routes include passive diffusion across damaged sarcolemma and intercalated discs, active transport in exchange for K+, or Na+, and entry through the voltage-activated, Ca2+-selective slow channels. The following experiments were designed to differentiate between these possibilities. Isolated spontaneously beating Sprague-Dawley rat hearts were perfused at 37 degrees C with Ca2+-free perfusion buffer for greater than 1 min before starting Ca2+ repletion. Adding 2-4 mmol l-1 Co2+ or Mn2+ before, during but not coincident with Ca2+ repletion protected against the paradox as indicated by an absence of myoglobin release and Ca2+ overload. Despite this protection marked distortion of the glycocalyx occurred, with splitting of the basement coat and blebbing. The intercalated discs, however, remained intact. It seems unlikely therefore that splitting of the basement coat necessarily results in an uncontrolled entry of Ca2+. Since Mn2+ and Co2+ block Ca2+ entry through the slow channels their failure to prevent the uncontrolled entry of Ca2+ when added at the time of Ca2+ repletion favours the view that Ca2+ entry during the paradox does not depend upon entry of Ca2+ through the slow channels. In other experiments hearts were preloaded with Na+, to alter intracellular Na+ before Ca2+ repletion. This procedure did not markedly affect the gain in Ca2+. These findings will be discussed in terms of the route(s) and consequence(s) of the uncontrolled entry of Ca2+ that occurs when Ca2+ is reintroduced after a period of Ca2+-free perfusion.