(A) Basal OCR (Young, n=9 from 7 mares; Old, n=14 from 8 mares), (B) maximal OCR induced by CCCP titrations (Young, n=9 from 7 mares; Old, n=14 from 8 mares), (C) proton leak OCR induced by addition of oligomycin (Young, n=3 from 3 mares; Old, n=7 from 5 mares), (D) mtDNA (Young, n=10 from 7 mares; Old, n=13 from 8 mares), (E) basal ECAR (Young, n=9 from 7 mares; Old, n=14 from 8 mares), (F) maximal ECAR induced by oligomycin titrations (Young, n=3 from 3 mares; Old, n=7 from 5 mares), and (G) proportion of basal glycolytic rate over oxidative phosphorylation rate (Young, n=9 from 7 mares; Old, n=14 from 8 mares). Barcharts present mean ± SEM. Different superscripts indicate differences at P < 0.05 (Student’s t test).