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Mitochondrial oxidative phosphorylation in hearts subjected to Ca2+ depletion and Ca2+ repletion
Zhanna Makazan1, Harjot K Saini-Chohan, Naranjan S Dhalla
1Institute of Cardiovascular Sciences, St. Boniface General Hospital Research Centre, Winnipeg, MB R2H 2A6, Canada.
Insights
Calcium repletion in Ca2+-depleted hearts impairs mitochondrial function, causing cardiac dysfunction. This study reveals intracellular calcium overload, not oxidative stress, underlies these mitochondrial defects in the Ca2+ paradox.
Area of Science:
- Cardiology
- Mitochondrial Biology
- Biochemistry
Background:
- The Ca2+ paradox, characterized by cardiac dysfunction and cell damage upon calcium repletion in depleted hearts, is linked to impaired high-energy phosphate stores.
- Limited information exists on how the Ca2+ paradox affects mitochondrial oxidative phosphorylation.
Purpose of the Study:
- To investigate the impact of Ca2+ paradox on mitochondrial oxidative phosphorylation in rat hearts.
- To determine whether intracellular Ca2+ overload or oxidative stress is responsible for mitochondrial dysfunction during Ca2+ paradox.
Main Methods:
- Rat hearts were perfused with Ca2+-free medium, followed by reperfusion with varying Ca2+ concentrations.
- Mitochondrial function (state 3 and 4 respiration, respiratory control index, ADP/O ratio) was assessed.
- The effects of Ca2+ channel blockers, Na+/Ca2+ and Na+/H+ exchange inhibitors, and antioxidants were evaluated.
Main Results:
- Reperfusion with 1.25 mmol/L Ca2+ significantly depressed mitochondrial state 3 respiration, respiratory control index, ADP/O ratio, and oxidative phosphorylation rate.
- These alterations were partially prevented by reperfusion with lower Ca2+ concentrations (0.10-0.50 mmol/L).
- Inhibitors of Ca2+ channels, Na+/Ca2+ exchange, Na+/H+ exchange, and antioxidants did not prevent mitochondrial dysfunction.
Conclusions:
- Defects in mitochondrial function during the Ca2+ paradox are primarily caused by intracellular Ca2+ overload.
- Oxidative stress does not appear to be the main driver of mitochondrial dysfunction in this condition.
- Direct incubation of mitochondria with Ca2+ inhibited oxidative phosphorylation, supporting the role of Ca2+ overload.
Abstract:
Repletion of Ca2+ in the Ca2+-depleted heart has been shown to produce cardiac dysfunction, myocardial cell damage, intracellular Ca2+ overload, and defects in sarcolemmal and sarcoplasmic reticulum function (Ca2+ paradox). Although these alterations in the Ca2+-paradox heart are associated with a depression in the high-energy phosphate stores, little information regarding changes in mitochondrial oxidative phosphorylation is available. Perfusion of rat hearts with Ca2+-free medium for 5 min followed by reperfusion with a medium containing 1.25 mmol/L Ca2+ for 10 min depressed mitochondrial state 3 respiration, respiratory control index, ADP/O ratio, and rate of oxidative phosphorylation without any change in state 4 respiration. These alterations were partially prevented when the reperfusion was carried out with a medium containing low Ca2+ (0.10-0.50 mmol/L). Treatment of heart with inhibitors of sarcolemmal Ca2+ channels (verapamil and diltiazem) or inhibitors of Na+/Ca2+ exchange (KB-R7943) and Na+/H+ exchange (amiloride) failed to modify changes in mitochondrial function due to Ca2+ paradox. Likewise, antioxidants N-acetylcysteine and N-(2-mercaptopropionyl)-glycine and an oxyradical-scavenging mixture of superoxide dismutase and catalase were ineffective in preventing the mitochondrial alterations in the Ca2+-paradox heart. Incubation of mitochondria with various concentrations of Ca2+ inhibited oxidative phosphorylation; this Ca2+-induced change in mitochondrial function was not affected by different oxyradical-scavenging systems. These observations suggest that defects in mitochondrial function in the Ca2+-paradox heart may be due to the occurrence of intracellular Ca2+ overload rather than the development of oxidative stress.
