Mitochondrial dysfunction as the cause of the failure to precondition the diabetic human myocardium

Abstract

We have shown previously that human diabetic myocardium cannot be preconditioned. Here, we have investigated the basis of this cardioprotective deficit. Right atrial sections from four patient groups-non-diabetic, insulin-dependent diabetes mellitus (IDDM), non-insulin-dependent diabetes mellitus (NIDDM) receiving glibenclamide, and NIDDM receiving metformin-were subjected to one of the following protocols: aerobic control, simulated ischemia/reoxygenation, ischemic preconditioning before ischemia, and pharmacological preconditioning with alpha 1 agonist phenylephrine, adenosine, the mito-K(ATP) channel opener diazoxide, the protein kinase C (PKC) activator phorbol-12-myristate-13-acetate (PMA), or the p38 mitogen-activated protein kinase (p38MAPK) activator anisomycin. Cellular damage was assessed using creatine kinase leakage and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) reduction. In mitochondrial preparations from non-diabetic and diabetic myocardium, mitochondrial membrane potential (Psi(m)) was assessed using JC-1 dye, and production of reactive oxygen species was determined. Preconditioning with ischemia, phenylephrine, adenosine, or diazoxide failed to protect diabetic myocardium. However, activation of PKC or p38MAPK was still protective. In isolated non-diabetic mitochondria, diazoxide partially depolarized Psi(m), an effect not seen in diabetic mitochondria. Furthermore, diazoxide increased superoxide production in non-diabetic but not in diabetic mitochondria. Our results show that the cardioprotective deficit in diabetic myocardium arises upstream of PKC and p38MAPK. We suggest that mitochondrial dysfunction in diabetic myocardium, possibly dysfunctional mito-K(ATP) channels, leads to impaired depolarization and superoxide production, and that this causes the inability to respond to preconditioning.