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.