Ca(2+) is the main trigger for
mitochondrial permeability transition pore opening, which plays a key role in cardiomyocyte death after
ischemia-reperfusion. We investigated whether a reduced accumulation of mitochondrial Ca(2+) might explain the attenuation of lethal
reperfusion injury by postconditioning. Anesthetized New Zealand White rabbits underwent 30 min of
ischemia, followed by either 240 (
infarct size protocol) or 60 (mitochondria protocol) min of reperfusion. They received either no intervention (control), preconditioning by 5-min
ischemia and 5-min reperfusion, postconditioning by four cycles of 1-min reperfusion and 1-min
ischemia at the onset of reflow, or pharmacological inhibition of the transition pore opening by N-methyl-4-isoleucine-cyclosporin (
NIM811; 5 mg/kg iv) given at reperfusion. Area at risk and
infarct size were assessed by blue
dye injection and
triphenyltetrazolium chloride staining. Mitochondria were isolated from the risk region for measurement of 1) Ca(2+) retention capacity (CRC), and 2) mitochondrial content of total (atomic absorption spectrometry) and ionized (potentiometric technique)
calcium concentration. CRC averaged 0.73 +/- 0.16 in control vs. 4.23 +/- 0.17 mug Ca(2+)/mg
proteins in shams (P < 0.05). Postconditioning, preconditioning, or
NIM811 significantly increased CRC (P < 0.05 vs. control). In the control group, total and free mitochondrial
calcium significantly increased to 2.39 +/- 0.43 and 0.61 +/- 0.10, respectively, vs. 1.42 +/- 0.09 and 0.16 +/- 0.01 mug Ca(2+)/mg in
sham (P < 0.05). Surprisingly, whereas total and ionized mitochondrial Ca(2+) decreased in preconditioning, it significantly increased in postconditioning and
NIM811 groups. These data suggest that retention of
calcium within mitochondria may explain the decreased
reperfusion injury in postconditioned (but not preconditioned) hearts.