Question: 

Mitochondria play a key role in cardiomyocyte survival after ischemia/reperfusion, and inhibition of opening of the mitochondrial permeability transition pore (mPTP) is decisive for survival. Ischemic postconditioning (PoCo), by short repetitive re-occlusions/reperfusions at the onset of reperfusion, attenuates reperfusion injury and reduces infarct size in all species tested so far, including humans. We have now investigated whether PoCo delays mPTP opening in mitochondria from ischemic/reperfused myocardium.

Methods: 

Anesthetized pigs were subjected to 90min ischemia followed by immediate full reperfusion (IFR, n=6) or PoCo (6x20s reperfusion/re-occlusion, n=6). Tissue was harvested at 10min reperfusion from the area at risk (anterior wall; AW) and a remote control region (posterior wall; PW). Calcium retention capacity (CRC), i.e. maximal mitochondrial calcium uptake, was determined in 200mg isolated mitochondria using glutamate/malate plus ADP as substrates and calcium green 5N as indicator. Calcium (10nmol) was added every minute, until a rapid increase in extra-mitochondrial calcium reflected mPTP opening and calcium release from the mitochondria. Cyclosporine A (1mM) was used as reference to keep mPTP closed.

Results: 

CRC (mmol Ca₂/mg mitochondrial protein) with PoCo (1,46±0,012; mean±SD) was higher than with IFR (0,87±0,005; p<0.05) in AW mitochondria. In PW, CRC was not different between PoCo (1,89±0,013) and IFR (1,68±0,011) and higher than in AW. With Cyclosporine A, CRC was increased with PoCo and IFR, in both AW and PW.

Conclusions: 

CRC in mitochondria from postconditioned myocardium is increased, indicating delayed mPTP opening at reperfusion with PoCo. Protected myocardium is characterized by delayed mPTP opening, supporting a role of mitochondria in cardioprotection.