Background: In the majority of studies, metformin has been demonstrated to cardioprotect diabetic patients, the mechanism of which is unclear. We hypothesized that metformin cardioprotects the ischemic heart through the Akt-mediated inhibition of mitochondrial permeability transition pore (mPTP) opening.
Materials and methods: Isolated perfused hearts from normoglycemic Wistar or from diabetic Goto-Kakizaki (GK) rats (N > or = 6/group) were subjected to 35 min ischemia and 120 min of reperfusion. Metformin (50 micromol/l) was added for 15 min at reperfusion, alone or with LY294002 (15 micromol/l), a PI3K inhibitor. Infarct size and Akt phosphorylation were measured. Furthermore, the effect of metformin on mPTP opening in adult cardiomyocytes isolated from both strains was determined.
Results: Metformin reduced infarct size in both Wistar (35 +/- 2.7% metformin vs. 62 +/- 3.0% control: P < 0.05) and GK hearts (43 +/- 4.7% metformin vs. 60 +/- 3.8% control: P < 0.05). This protection was accompanied by a significant increase in Akt phosphorylation. LY294002 abolished the metformin-induced Akt phosphorylation and the infarct-limiting effect of metformin in Wistar (61 +/- 6.7% metformin + LY294002 vs. 35 +/- 2.7% metformin: P < 0.05) and GK rats (56 +/- 5.7% metformin + LY294002 vs. 43 +/- 4.7% metformin: P < 0.05). In addition, metformin significantly inhibited mPTP opening and subsequent rigor contracture in both Wistar and GK cardiomyocytes subjected to oxidative stress, in a LY-sensitive manner.
Conclusions: We report that metformin given at the time of reperfusion reduces myocardial infarct size in both the non-diabetic and diabetic heart and this protective effect is mediated through PI3K and is associated with Akt phosphorylation. Furthermore, cardioprotection appears to be executed through a PI3K-mediated inhibition of mPTP opening. These findings may explain in part the cardioprotective properties of metformin observed in clinical studies of diabetic patients.