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

BackgroundCardiovascular disease mortality increases rapidly after menopause by poorly defined mechanisms.ObjectiveBecause 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.MethodsMitochondrial 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.ResultsAging 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.ConclusionsAge-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.