Cardioprotection by metabolic shut-down and gradual wake-up

Abstract

Mitochondria play a critical role in cardiac function, and are also increasingly recognized as end effectors for various cardioprotective signaling pathways. Mitochondria use oxygen as a substrate, so by default their respiration is inhibited during hypoxia/ischemia. However, at reperfusion a surge of oxygen and metabolic substrates into the cell is thought to lead to rapid reestablishment of respiration, a burst of reactive oxygen species (ROS) generation and mitochondrial Ca(2+) overload. Subsequently these events precipitate opening of the mitochondrial permeability transition (PT) pore, which leads to myocardial cell death and dysfunction. Given that mitochondrial respiration is already inhibited during hypoxia/ischemia, it is somewhat surprising that many respiratory inhibitors can improve recovery from ischemia-reperfusion (IR) injury. In addition ischemic preconditioning (IPC), in which short non-lethal cycles of IR can protect against subsequent prolonged IR injury, is known to lead to endogenous inhibition of several respiratory complexes and glycolysis. This has led to a hypothesis that the wash-out of inhibitors or reversal of endogenous inhibition at reperfusion may afford protection by facilitating a more gradual wake-up of mitochondrial function, thereby avoiding a burst of ROS and Ca(2+) overload. This paper will review the evidence in support of this hypothesis, with a focus on inhibition of each of the mitochondrial respiratory complexes.

Figure 1. Schematic representation of the mitochondrial respiratory chain showing sites of inhibition by various cardioprotective molecules

For full details, see text.

Figure 2. Theoretical principles of application of open-flow respirometry to hypoxia-reoxygenation of isolated mitochondria in a Clark O₂ electrode chamber

Mitochondria are incubated in respiration buffer + respiratory substrates & ADP, in a respiration chamber fitted with an O₂ electrode. (A): Where indicated, the lid is closed and mitochondria respire to bring [O₂] to zero (i.e. hypoxia). 1mM CN⁻ is then added to block respiration and the lid is opened. The resulting rise in [O₂] is due to O₂ diffusion into the chamber, and equilibrium with room air is eventually obtained. The first-order rate constant for O₂ diffusion (from Ln[O₂] vs. time) is the mass transfer coefficient, m. C* is the [O₂] of air saturated buffer alone (~200μM). C₁ is the [O₂] measured by the O₂ electrode (solid line). Respiration rate (Q, lower dotted trace) is zero throughout “reoxygenation”, due to the CN⁻ inhibition. (B): Mitochondria are incubated as in panel A, without CN⁻. Opening the lid results in a slower rise of [O₂] (C₁) due to the equilibrium between O₂ diffusing in and O₂ consumed by the mitochondria. The shaded gray area represents the range of possible [O₂] recovery curves, resulting from zero or maximal respiration. Respiration rates (Q) are calculated instantaneously during reoxygenation, using the equation Q=m(C*−C₁)/dC₁/dt. (C): Mitochondria are incubated as in B, but with hypoxia. Respiration (Q) recovers rapidly at reoxygenation, but Ox-Phos damage prevents later full recovery.

Figure 3. Mitochondrial respiration rate during early reoxygenation

Respiration rate upon reoxygenation was calculated every 5 s. throughout a 200 s. reoxygenation. Data are means ± SEM; N≥3.