Dynamics of Mitochondrial Redox and Energy Networks: Insights from an Experimental–Computational Synergy - Systems Biology of Metabolic and Signaling Networks - page 134

5.7 Mitochondrial Oscillations in the Intact Heart: Testing
the Consequences of Mitochondrial Criticality

We determined if the nonlinear oscillatory phenomena described in single cells can
be observed at the level of the whole heart. Studies performed in permeabilized
cardiomyocytes revealed that the critical state can be induced by partial depletion of
the GSH pool and that the reversible (IMAC-mediated) and irreversible
(PTP-mediated) depolarization of

ΔΨ m can be distinguished from the cytoplasmic
glutathione redox status. IMAC-mediated ΔΨ m oscillation was triggered at a

GSH/GSSG ratio of 150:1-100:1, whereas PTP opening is triggered at a
GSH/GSSG of 50:1 (Aon et al. 2007b ). These results pointed out that GSH and
probably also the glutathione redox potential are the main cellular variables that
determine the approach of the mitochondrial network to criticality through an
increase in oxidative stress by the overwhelming of the antioxidant defenses.
Extending the mechanistic findings in permeabilized cardiomyocytes (Aon
et al. 2007b ) to the mitochondrial ROS-dependent oscillator described in living
cardiac myocytes (Aon et al. 2003 , 2004a ), and computational models (Cortassa
et al. 2004 ) to the level of the myocardial syncytium, we showed that mitochondrial
ΔΨ m oscillations could be triggered by ischemia/reperfusion (I/R) or GSH deple-
tion in intact perfused hearts using two-photon scanning laser microscopy
(Slodzinski et al. 2008 ). These results confirmed that the appearance of oscillatory
behavior is not restricted to isolated cardiomyocytes but also happens in the
epicardium of intact hearts (either flash-triggered or GSH depletion elicited), in
both ΔΨ m and NADH (Fig. 5.13 ).

An important prediction of the percolation model utilized to explain the mecha-
nism of mitochondrial synchronization at criticality is that the global transition can
be prevented if the O 2 . concentration reaching the neighboring mitochondrion is
decreased below threshold. This can be accomplished either by decreasing O 2 .
production (inhibiting respiration), decreasing O 2 . release (inhibiting IMAC), or
increasing the local ROS scavenging capacity (increasing the GSH pool) (Aon
et al. 2004a , 2007b ; Cortassa et al. 2004 ). Within this rationale,

ΔΨ m depolarization
induced by depleting the GSH pool could induce cardiac arrhythmias even under
normoxic conditions (Aon et al. 2007b , 2009 ). Indeed, Brown et al. ( 2010 )
demonstrated that systematic oxidation of the GSH pool with diamide in
Langendorff-perfused guinea pig hearts elicited ventricular fibrillation under
normoxia (Fig. 5.14b, d ). Experimental evidence further indicating the involvement
of IMAC was noted when the arrhythmias induced by GSH depletion (Brown
et al. 2010 )orH 2 O 2 -elicited oxidative stress (Biary et al. 2011 ) were prevented
with the IMAC blocker 4'-Cl-DZP (Fig. 5.14c, e ).