"Calcium paradox" as a term describes the deleterious effects conferred to a 
heart perfused with a calcium-free solution followed by repletion, including 
loss of mechanical activity and sarcomere disruption. Given that the signaling 
mechanisms triggered by calcium paradox remain elusive, in the present study, we 
tried to investigate them in the isolated perfused heart from Rana ridibunda. 
Calcium paradox was found to markedly activate members of the MAPKs (p43-ERK, 
JNKs, p38-MAPK). In addition to lactate dehydrogenase (LDH) release in the 
perfusate (indicative of necrosis), we also confirmed the occurrence of 
apoptosis by using the TUNEL assay and identifying poly(ADP-ribose) polymerase 
(PARP) fragmentation and upregulated Bax expression. Furthermore, using MDL28170 
(a selective calpain inhibitor), a role for this protease was revealed. In 
addition, various divalent cations were shown to exert a protective effect 
against the calcium paradox. Interestingly, SB203580, a p38-MAPK inhibitor, 
alleviated calcium-paradox-conferred apoptosis. This result indicates that 
p38-MAPK plays a pro-apoptotic role, contributing to the resulting myocardial 
dysfunction and cell death. To our knowledge, this is the first time that the 
calcium paradox has been shown to induce apoptosis in amphibians, with p38-MAPK 
and calpain playing significant roles.