Background: Type II diabetes mellitus is a major health problem contributing to increased morbidity and mortality with associated cardiovascular diseases. Metformin, an oral antidiabetic agent, has cardioprotective properties independent of their glucose lowering effect; however, mechanisms underlying cardioprotection remain poorly defined. We hypothesized that the cardioprotective effect of metformin appears to be associated with inhibition of mitochondrial permability transition pore (mPTP) that could be beneficial in diabetic hearts with attenuated endogenous cardioprotective responses.
Purpose: The aim of the study was to determine the protective effect of metformin on mPTP opening in mitochondria from human myocardium..
Methods: Mitochondria freshly isolated from the left atrial appendage of nondiabetic patients undergoing cardiac surgery were loaded with calcium- (fluo-3) and membrane potential-sensitive (Safranin-O) fluorescent dyes and challenged with sequential pulses of Ca2+ (10 μM every 3 m) in the absence and presence of different concentrations of metformin (2.5, 5, 10 mM). The sensitivity of mitochondria toward mPTP opening was assessed by abrupt release of mitochondrial Ca2+ (fluo-5N fluorescence), with simultaneous dissipation of mitochondrial membrane potential (safranin O fluorescence) and mitochondrial swelling (decrease in light scattering).
Results: Metformin caused a dose-dependent inhibition of Ca2+-induced mPTP opening with delayed mitochondrial Ca2+ release, depolarization and swelling. The tolerance of metformin-treated mitochondria to Ca2+-induced mPTP opening when compared to controls was increased from 457±71 to 600±74 nmol/mg protein at 2.5mM, 674±10 nmol/mg protein at 5 mM and 750±77 nmol/mg protein at 10 mM. Cyclosporin A (1 μM), a known mPTP inhibitor, has marginal incremental effect on metformin-induced inhibition of mPTP opening.
Conclusion: In human cardiac mitochondria, metformin inhibited calcium-overload-mediated mPTP opening that otherwise leads to mitochondrial energetic failure and cell death. Thus, metformin may help restore attenuated cardioprotection in diabetic hearts by increasing tolerance to stress-induced mPTP opening and, thus, preventing cell death.