Alterations in Mitochondrial State 4→3 Transition Underlie Stress-Induced Energetic-Redox Imbalance and Myocyte Dysfunction in Diabetic Mice

Abstract

Diabetes is a syndrome in which mitochondrial alterations may participate in the dysfunction, and ultimately failure of several organs. The diabetic heart in particular can be subjected to both glycemic level oscillations and increased workload (via β-stimulation), events that may further deteriorate its mechanical properties. Here we show that when diabetic heart (db/db) cells are challenged with high levels of extracellular glucose (30 mM) and concomitant β-adrenergic stimulation via isoproterenol (ISO), intracellular GSH is greatly diminished while superoxide and H2O2 generation substantially increased. The typical enhancement in sarcomere shortening and whole Ca2+ transient driven by β-adrenergic stimulation is blunted in db/db cells, along with hampered relaxation and Ca2 reuptake. Mitochondria isolated from db/db hearts subjected to high-glucose/ISO regimen exhibit markedly decreased coupling ratios from respiratory complexes I, II, and IV while ROS emission is greatly increased, under both forward and reverse electron transport. These changes reflect a profound alteration in the crucial (energy supplying) state 4→3 transition that ultimately results in lowered ATP synthesis and in a “non-stop” reactive oxygen species (ROS) emission in the presence of ADP. Incubating high-glucose/ISO treated db/db myocytes with cell-permeable GSH restores intracellular GSH, blunts ROS production and fully rescues contractility/relaxation in these myocytes. These results show the direct role played by an oxidized redox environment in triggering the negative synergy among mitochondrial ROS and energetics leading to the E-C coupling impairment. Our study maps mitochondrial sites, i.e. state 4→3 transition, that in diabetic heart cells account, at least in part, for excess ROS emission and loss in energy supply when metabolically and energetically challenged.