Gender modulation of Ca²⁺ uptake in cardiac mitochondria

Background: Mitochondrial calcium overload is an important factor in defining ischemia/reperfusion injury. Since pre-menopausal women are relatively protected from ischemia and heart disease, we tested the hypothesis that gender differences alter Ca²⁺ handling in rat cardiac mitochondria. Methods: Using cardiac mitochondria isolated from male, female, and ovariectomized Sprague-Dawley rats, we measured mitochondrial calcium transport, redox state, and membrane potential (Δψ_m) during exposure to a calcium bolus. Redox state was modulated using either succinate (S) or succinate and pyruvate (SP) as substrates. Results: Net Ca²⁺ uptake rates were significantly lower in female than male mitochondria using SP, substrate conditions that resulted in a lower redox state (NADH/NAD⁺). Inhibition of the mitochondrial transition pore (MTP) using cyclosporin A showed significantly lower net Ca²⁺ uptake in both substrate solutions when mitochondria from female and ovariectomized animals were compared to males, a finding consistent with gender modulation of the mitochondrial uniporter. Blockade of the Ca²⁺ uniporter by ruthenium red abolished gender or substrate solution differences in calcium release. While there were no significant differences in resting Δψ_m, or Δψ_m following Ca²⁺ addition, 80% of female samples recovered from Ca²⁺-induced depolarization compared to 57% and 43% of male and ovariectomized animals, respectively. Conclusions: Mitochondria from female hearts have lower Ca²⁺ uptake rates under physiologic substrate solutions (succinate/pyruvate) and are able to appropriately maintain ΔYm under conditions of high [Ca²⁺]. These differences are consistent with gender modulation of the Ca²⁺ uniporter and may be a mechanism by which female myocardium suffers less injury with ischemia/reperfusion.