The Ca(2+) paradox represents a good model to study Ca(2+) overload injury in
ischemic heart diseases. We and others have demonstrated that
contracture and
calpain are involved in the Ca(2+) paradox-induced injury. This study aimed to elucidate their roles in this model. The Ca(2+) paradox was elicited by perfusing isolated rat hearts with Ca(2+)-free KH media for 3 min or 5 min followed by 30 min of Ca(2+) repletion. The LVDP was measured to reflect contractile function, and the LVEDP was measured to indicate
contracture. TTC staining and the quantification of LDH release were used to define cell death.
Calpain activity and
troponin I release were measured after Ca(2+) repletion. Ca(2+) repletion of the once 3-min Ca(2+) depleted hearts resulted in almost no viable tissues and the disappearance of contractile function. Compared to the effects of the
calpain inhibitor MDL28170,
KB-R7943, an inhibitor of the
Na(+)/Ca(2+) exchanger, reduced the LVEDP level to a greater extent, which was well correlated with improved contractile function recovery and tissue survival. The depletion of Ca(2+) for 5 min had the same effects on injury as the 3-min Ca(2+) depletion, except that the LVEDP in the 5-min Ca(2+) depletion group was lower than the level in the 3-min Ca(2+) depletion group.
KB-R7943 failed to reduce the level of LVEDP, with no improvement in the LVDP recovery in the hearts subjected to the 5-min Ca(2+) depletion treatment; however,
KB-R7943 preserved its protective effects in surviving tissue. Both
KB-R7943 and
MDL28170 attenuated the Ca(2+) repletion-induced increase in
calpain activity in 3 min or 5 min Ca(2+) depleted hearts. However, only
KB-R7943 reduced the release of
troponin I from the Ca(2+) paradoxic heart. These results provide evidence suggesting that
contracture is the main cause for contractile dysfunction, while activation of
calpain mediates cell death in the Ca(2+) paradox.