Impact of aging on mitochondrial respiration in various organs

Abstract

Mitochondria are considered central regulator of the aging process; however, majority of studies dealing with the impact of age on mitochondrial oxygen consumption focused on skeletal muscle concluding (although not uniformly) a general declining trend with advancing age. In addition, gender related differences in mitochondrial respiration have not been satisfactorily described yet. The aim of the present study was to evaluate mitochondrial oxygen consumption in various organs of aging male and female Fischer 344 rats at the ages of 6, 12 and 24 months. Mitochondrial respiration of homogenized (skeletal muscle, left and right heart ventricle, hippocampus, cerebellum, kidney cortex), gently mechanically permeabilized (liver) tissue or intact cells (platelets) was determined using high-resolution respirometry (oxygraphs O2k, Oroboros, Austria). The pattern of age-related changes differed in each tissue: in the skeletal muscle and kidney cortex of both sexes and in female heart, parameters of mitochondrial respiration significantly declined with age. Resting respiration of intact platelets displayed an increasing trend and it did not correlate with skeletal muscle respiratory states. In the heart of male rats and brain tissues of both sexes, respiratory states remained relatively stable over analyzed age categories with few exceptions of lower mitochondrial oxygen consumption at the age of 24 months. In the liver, OXPHOS capacity was higher in females than in males with either no difference between the ages of 6 and 24 months or even significant increase at the age of 24 months in the male rats. In conclusion, the results of our study indicate that the concept of general pattern of age-dependent decline in mitochondrial oxygen consumption across different organs and tissues could be misleading. Also, the statement of higher mitochondrial respiration in females seems to be conflicting, since the gender-related differences may vary with the tissue studied, combination of substrates used and might be better detectable at younger ages than in old animals.

Fig. 1

Oxygen consumption rate of skeletal muscle (SM; A–D) normalized to mg of tissue wet weight and platelets (PLT; E, F) normalized to 10⁷ cells in 6-, 12- and 24-month-old male (M) and female (F) Fischer 344 rats. (A) P_FAO=OXPHOS state after injection of malate (M), palmitoyl-L-carnitine (Pcar), ADP (D) and cytochrome c (c). (B) P_FAO+I=OXPHOS after M, Pcar, D, c + glutamate (G) and pyruvate (P). (C) P_FAO+I+II=OXPHOS after injection of M, Pcar, D, c, G, P and succinate (S). (D) Complex IV (C IV) capacity after Complex III inhibition and addition of TMPD with ascorbate (sodium azide-sensitive portion) and sodium azide. (E) ROUT=Routine respiration of intact platelets in mixed PBS+glucose and plasma (1:1) solution. (F) ETSC=maximum capacity of uncoupled respiration chain of platelets (after titration of uncoupler FCCP). The boxes are mean ± SD with horizontal line denoting median and whiskers showing 10 to 90 percentile range, * p<0.05.

Fig. 2

Oxygen consumption rate of the left (LV; A–C) and right ventricle (RV; D–F) normalized to mg of tissue wet weight in 6-, 12- and 24-month-old male (M) and female (F) Fischer 344 rats. (A, D) P_FAO=OXPHOS state after injection of malate (M), palmitoyl-L-carnitine (Pcar), ADP (D), and cytochrome c (c). (B, E) P_FAO+I+II=OXPHOS after injection of M, Pcar, D, c, glutamate (G), pyruvate (P) and succinate (S). (C, F) Complex IV (C IV) capacity after complex III inhibition and addition of TMPD with ascorbate (sodium azide-sensitive portion) and sodium azide. The boxes are mean ± SD with horizontal line denoting median and whiskers showing 10 to 90 percentile range, * p<0.05.

Fig. 3

Oxygen consumption rate of hippocampus (HIP; A–C) and cerebellum (CEREB; D–F) normalized to mg of tissue wet weight in 6-, 12- and 24-month-old male (M) and female (F) Fischer 344 rats. (A, D) P_I=OXPHOS state after injection of malate (M), glutamate (G), ADP (D) and cytochrome c (c). (B, E) P_I+II=OXPHOS after injection of M, G, D, c, pyruvate (P) and succinate (S). (C, F) Complex IV (C IV) capacity after complex III inhibition and addition of TMPD with ascorbate (sodium azide-sensitive portion) and sodium azide. The boxes are mean ± SD with horizontal line denoting median and whiskers showing 10 to 90 percentile range, * p<0.05.

Fig. 4

Oxygen consumption rate of the kidney cortex (KIDNEY; A–C) and liver (D–F) normalized to mg of tissue wet weight in 6-, 12- and 24-month-old male (M) and female (F) Fischer 344 rats. (A, D) P_FAO=OXPHOS state after injection of malate (M), palmitoyl-L-carnitine (Pcar), ADP (D) and cytochrome c (c). (B, E) P_FAO+I=OXPHOS after injection of M, Pcar, D, c, glutamate (G), and pyruvate (P). (C, F) P_FAO+I+II=OXPHOS after injection of M, Pcar, D, c, G, P, and succinate. The boxes are mean ± SD with horizontal line denoting median and whiskers showing 10 to 90 percentile range, * p<0.05.