Ischemic preconditioning improves mitochondrial tolerance to experimental calcium overload

Background. Ca²⁺ overload leads to mitochondrial uncoupling, decreased ATP synthesis, and myocardial dysfunction. Pharmacologically opening of mitochondrial K_ATP channels decreases mitochondrial Ca²⁺ uptake, improving mitochondrial function during Ca²⁺ overload. Ischemic preconditioning (IPC), by activating mitochondrial K_ATP channels, may attenuate mitochondrial Ca²⁺ overload and improve mitochondrial function during reperfusion. The purpose of these experiments was to study the effect of IPC (1) on mitochondrial function and (2) on mitochondrial tolerance to experimental Ca²⁺ overload. Methods. Rat hearts (n = 6/group) were subjected to (a) 30 min of equilibration, 25 min of ischemia, and 30 min of reperfusion (Control) or (b) two 5-min episodes of ischemic preconditioning, 25 min of ischemia, and 30 min of reperfusion (IPC). Developed pressure (DP) was measured. Heart mitochondria were isolated at end-Equilibration (end-EQ) and at end-Reperfusion (end-RP). Mitochondrial respiratory function (state 2, oxygen consumption with substrate only; state 3, oxygen consumption stimulated by ADP; state 4, oxygen consumption after cessation of ADP phosphorylation; respiratory control index (RCI, state 3/state 4); rate of oxidative phosphorylation (ADP/Δt), and ADP:O ratio) was measured with polarography using α-ketoglutarate as a substrate in the presence of different Ca²⁺ concentrations (0 to 5 × 10^-7 M) to simulate Ca²⁺ overload. Results. IPC improved DP at end-RP. IPC did not improve preischemic mitochondrial respiratory function or preischemic mitochondrial response to Ca²⁺ loading. IPC improved state 3, ADP/Δt, and RCI during RP. Low Ca²⁺ levels (0.5 and 1 × 10^-7 M) stimulated mitochondrial function in both groups predominantly in IPC. The Control group showed evidence of mitochondrial uncoupling at lower Ca²⁺ concentrations (1 × 10^-7 M). IPC preserved state 3 at high Ca²⁺ concentrations. Conclusions. The cardioprotective effect of IPC results, in part, from preserving mitochondrial function during reperfusion and increasing mitochondrial tolerance to Ca²⁺ loading at end-RP. Activation of mitochondrial K_ATP channels by IPC and their improvement in Ca²⁺ homeostasis during RP may be the mechanism underlying this protection.