Age-Dependent Reductions in Mitochondrial Respiration are Exacerbated by Calcium in the Female Rat Heart

Background

Cardiovascular disease mortality increases rapidly after menopause by poorly defined mechanisms.

Objective

Because mitochondrial function and Ca²⁺ sensitivity are important regulators of cell death after myocardial ischemia, we sought to determine whether aging and/or estrogen deficiency (ovariectomy) increased mitochondrial Ca²⁺ sensitivity.

Methods

Mitochondrial respiration was measured in ventricular mitochondria isolated from adult (6 months; n = 26) and aged (24 months; n = 25), intact or ovariectomized female rats using the substrates α-ketoglutarate/malate (complex I); succinate/rotenone (complex II); ascorbate/N,N,N′,N′-tetramethyl-p-phenylenediamine/antimycin (complex IV). State 2 and 3 respiration was initiated by sequential addition of mitochondria and adenosine diphosphate. Ca²⁺ sensitivity was assessed by Ca²⁺-induced swelling of de-energized mitochondria and reduction in state 3 respiration. Propylpyrazole triol (PPT) was administered intraperitoneally 45 minutes before euthanasia to assess mitochondrial protective effects through estrogen receptor (ER) α activation.

Results

Aging decreased the respiratory control index (RCI; state 3/state 2) for complexes I and II by 12% and 8%, respectively, independent of ovary status (P < 0.05). Of interest, Ca²⁺ induced a greater decrease (18%–30%; P < 0.05) in complex I state 3 respiration in aged and ovariectomized animals, and mitochondrial swelling occurred twice as quickly in aged (vs adult) female rats (P < 0.05). Pretreatment with PPT increased RCI by 8% and 7% at complexes I and II, respectively (P < 0.05) but surprisingly increased Ca²⁺ sensitivity.

Conclusions

Age-dependent decreases in RCI and sensitization to Ca²⁺ may explain in part the age-associated reductions in female ischemic tolerance; however, protection afforded by ER agonism involves more complex mechanisms.