Mitochondrial bioenergetics are not associated with myofibrillar protein synthesis rates

Abstract

Background: Mitochondria represent key organelles influencing cellular homeostasis and have been implicated in the signalling events regulating protein synthesis.

Methods: We examined whether mitochondrial bioenergetics (oxidative phosphorylation and reactive oxygen species (H₂O₂) emission, ROS) measured in vitro in permeabilized muscle fibres represent regulatory factors for integrated daily muscle protein synthesis rates and skeletal muscle mass changes across the spectrum of physical activity, including free-living and bed-rest conditions: n = 19 healthy, young men (26 ± 4 years, 23.4 ± 3.3 kg/m²) and following 12 weeks of resistance-type exercise training: n = 10 healthy older men (70 ± 3 years, 25.2 ± 2.1 kg/m²). Additionally, we evaluated the direct relationship between attenuated mitochondrial ROS emission and integrated daily myofibrillar and sarcoplasmic protein synthesis rates in genetically modified mice (mitochondrial-targeted catalase, MCAT).

Results: Neither oxidative phosphorylation nor H₂O₂ emission were associated with muscle protein synthesis rates in healthy young men under free-living conditions or following 1 week of bed rest (both P > 0.05). Greater increases in GSSG concentration were associated with greater skeletal muscle mass loss following bed rest (r = -0.49, P < 0.05). In older men, only submaximal mitochondrial oxidative phosphorylation (corrected for mitochondrial content) was positively associated with myofibrillar protein synthesis rates during exercise training (r = 0.72, P < 0.05). However, changes in oxidative phosphorylation and H₂O₂ emission were not associated with changes in skeletal muscle mass following training (both P > 0.05). Additionally, MCAT mice displayed no differences in myofibrillar (2.62 ± 0.22 vs. 2.75 ± 0.15%/day) and sarcoplasmic (3.68 ± 0.35 vs. 3.54 ± 0.35%/day) protein synthesis rates when compared with wild-type mice (both P > 0.05).

Conclusions: Mitochondrial oxidative phosphorylation and reactive oxygen emission do not seem to represent key factors regulating muscle protein synthesis or muscle mass regulation across the spectrum of physical activity.

Keywords: Aging; Muscle protein synthesis; Physical inactivity; Reactive oxygen species; Skeletal muscle.

Figure 1

Associations between daily myofibrillar protein synthesis rates (FSR, %/day) and maximal and submaximal oxidative phosphorylation and (pmol O₂/s/mg wet weight), mitochondrial specific maximal and submaximal oxidative phosphorylation (pmol O₂/s/mg wet weight/mtDNA), maximal and submaximal H₂O₂ emission (pmol H₂O₂/s/mg wet weight), mitochondrial specific maximal and submaximal H₂O₂ emission (pmol H₂O₂/s/mg wet weight/mtDNA), total glutathione (uM), GSH (uM), GSSG (uM), and GSH:GSSG in healthy young men (n = 19) at baseline (A). H₂O₂ emission and oxidative phosphorylation values were determined under the presence of 5 mM pyruvate, 1 mM malate, and 20 mM succinate, and the absence (maximal H₂O₂) or presence of ADP (25 μM for submaximal H₂O₂ and oxidative phosphorylation, 10 000 μM for maximal oxidative phosphorylation). Data were analysed using Pearson's r product moment correlations. Values in panel (A) represent the P values and correlation coefficients (r values), which are colour‐coded to represent negative (blue) and positive (red) correlations. *Significant correlations (P < 0.05) are in bold text. Selected correlations are also displayed between mitochondrial specific submaximal oxidative phosphorylation (B, pmol O₂/s/mg wet weight/mtDNA), mitochondrial specific submaximal H₂O₂ emission (C, pmol H₂O₂/s/mg wet weight/mtDNA), total glutathione (D, μM), GSH:GSSG (E) and daily myofibrillar protein synthesis rates (%/day). The solid lines represent the linear regression lines of best fit and the dashed lines represent the 95% confidence intervals.

Figure 2

Associations between daily myofibrillar protein synthesis rates (FSR, %/day) and maximal and submaximal oxidative phosphorylation and (pmol O₂/s/mg wet weight), mitochondrial specific maximal and submaximal oxidative phosphorylation (pmol O₂/s/mg wet weight/mtDNA), maximal and submaximal H₂O₂ emission (pmol H₂O₂/s/mg wet weight), mitochondrial specific maximal and submaximal H₂O₂ emission (pmol H₂O₂/s/mg wet weight/mtDNA), total glutathione (μM), GSH (μM), GSSG (μM), and GSH:GSSG in healthy young men (n = 19) following 1 week of strict bed rest (A). H₂O₂ emission and oxidative phosphorylation values were determined under the presence of 5 mM pyruvate, 1 mM malate, and 20 mM succinate, and the absence (maximal H₂O₂) or presence of ADP (25 μM for submaximal H₂O₂ and oxidative phosphorylation, 10,000 μM for maximal oxidative phosphorylation). Data were analysed using Pearson's r product moment correlations. Values displayed in panel (A) represent the P values and correlation coefficients (r values), which are colour‐coded to represent negative (blue) and positive (red) correlations. *Significant correlations (P < 0.05) are in bold text. Selected correlations are also displayed between mitochondrial specific submaximal oxidative phosphorylation (B, pmol O₂/s/mg wet weight/mtDNA), mitochondrial specific submaximal H₂O₂ emission (C, pmol H₂O₂/s/mg wet weight/mtDNA), total glutathione (D, μM), GSH:GSSG (E) and daily myofibrillar protein synthesis rates (%/day). The solid lines represent the linear regression lines of best fit and the dashed lines represent the 95% confidence intervals.

Figure 3

Associations between changes in quadriceps cross‐sectional area (cm²) and changes maximal and submaximal oxidative phosphorylation and (pmol O₂/s/mg wet weight), mitochondrial specific maximal and submaximal oxidative phosphorylation (pmol O₂/s/mg wet weight/mtDNA), maximal and submaximal H₂O₂ emission (pmol H₂O₂/s/mg wet weight), mitochondrial specific maximal and submaximal H₂O₂ emission (pmol H₂O₂/s/mg wet weight/mtDNA), total glutathione (μM), GSH (μM), GSSG (μM), and GSH:GSSG in young men (n = 19) following 1 week of strict bed rest (A). H₂O₂ emission and oxidative phosphorylation values were determined under the presence of 5 mM pyruvate, 1 mM malate, and 20 mM succinate, and the absence (maximal H₂O₂) or presence of ADP (25 μM for submaximal H₂O₂ and oxidative phosphorylation, 10,000 μM for maximal oxidative phosphorylation). Data were analysed using Pearson's r product moment correlations. Values displayed in panel (A) represent the P values and correlation co‐efficients (r values), which are colour‐coded to represent negative (blue) and positive (red) correlations. *Significant correlations (P < 0.05) are in bold text. Selected correlations are also displayed between changes in mitochondrial specific submaximal oxidative phosphorylation (B, pmol O₂/s/mg wet weight/mtDNA), mitochondrial specific submaximal H₂O₂ emission (C, pmol H₂O₂/s/mg wet weight/mtDNA), total glutathione (D, μM), GSH:GSSG (E) and changes in quadriceps cross‐sectional area (cm²). The solid lines represent the linear regression lines of best fit and the dashed lines represent the 95% confidence intervals. CSA, cross‐sectional area.

Figure 4

Associations between daily myofibrillar protein synthesis rates (FSR, %/day) and maximal and submaximal oxidative phosphorylation and (pmol O₂/s/mg wet weight), mitochondrial specific maximal and submaximal oxidative phosphorylation (pmol O₂/s/mg wet weight/mtDNA), maximal and submaximal H₂O₂ emission (pmol H₂O₂/s/mg wet weight), mitochondrial specific maximal and submaximal H₂O₂ emission (pmol H₂O₂/s/mg wet weight/mtDNA), total glutathione (μM), GSH (μM), GSSG (μM), and GSH:GSSG in older men (n = 10) following resistance exercise training (A). H₂O₂ emission and oxidative phosphorylation values were determined under the presence of 5 mM pyruvate, 1 mM malate, and 20 mM succinate, and the absence (maximal H₂O₂) or presence of ADP (25 μM for submaximal H₂O₂ and oxidative phosphorylation, 10,000 μM for maximal oxidative phosphorylation). Data were analysed using Pearson's r product moment correlations values displayed in panel (A) represent the P values and correlation coefficients (r values), which are colour‐coded to represent negative (blue) and positive (red) correlations. *Significant correlations (P < 0.05) are in bold text. Selected correlations are also displayed between mitochondrial specific submaximal oxidative phosphorylation (B, pmol O₂/s/mg wet weight/mtDNA), mitochondrial specific submaximal H₂O₂ emission (C, pmol H₂O₂/s/mg wet weight/mtDNA), total glutathione (D, μM), GSH:GSSG (E) and daily myofibrillar protein synthesis rates (%/day). The solid lines represent the linear regression lines of best fit and the dashed lines represent the 95% confidence intervals.

Figure 5

Associations between changes in quadriceps cross‐sectional area (cm²) and changes in maximal and submaximal oxidative phosphorylation and (pmol O₂/s/mg wet weight), mitochondrial specific maximal and submaximal oxidative phosphorylation (pmol O₂/s/mg wet weight/mtDNA), maximal and submaximal H₂O₂ emission (pmol H₂O₂/s/mg wet weight), mitochondrial specific maximal and submaximal H₂O₂ emission (pmol H₂O₂/s/mg wet weight/mtDNA), total glutathione (μM), GSH (μM), GSSG (μM), and GSH:GSSG in older men (n = 10) following 12 weeks of progressive resistance exercise training (A). H₂O₂ emission and oxidative phosphorylation values were determined under the presence of 5 mM pyruvate, 1 mM malate, and 20 mM succinate, and the absence (maximal H₂O₂) or presence of ADP (25 μM for submaximal H₂O₂ and oxidative phosphorylation, 10,000 μM for maximal oxidative phosphorylation). Data were analysed using Pearson's r product moment correlations. Values displayed in panel (A) represent the P values and correlation coefficients (r values), which are colour‐coded to represent negative (blue) and positive (red) correlations. *significant correlations (P < 0.05) are in bold text. Selected correlations are also displayed between changes in mitochondrial specific submaximal oxidative phosphorylation (B, pmol O₂/s/mg wet weight/mtDNA), mitochondrial specific submaximal H₂O₂ emission (C, pmol H₂O₂/s/mg wet weight/mtDNA), total glutathione (D, μM), GSH:GSSG (E) and changes in quadriceps cross‐sectional area (cm²). The solid lines represent the linear regression lines of best fit and the dashed lines represent the 95% confidence intervals. CSA, cross‐sectional area.

Figure 6

Resting maximal oxidative phosphorylation (A, pmol O₂/min/mg dry weight), maximal (B) and submaximal (C) mitochondrial H₂O₂ emission (pmol H₂O₂/min/mg dry weight) together with myofibrillar (D) and sarcoplasmic (E) protein synthesis rates (FSR, %/day) assessed over a 7‐day period using deuterium oxide in C57BL/6NJ (wild‐type, WT, black dots, n = 8) and mitochondrial catalase transgenic (MCAT, white dots, n = 8) mice. H₂O₂ emission and oxidative phosphorylation values were determined under the presence of 5 mM pyruvate, 1 mM malate, and 20 mM succinate, and the absence (maximal H₂O₂) or presence of ADP (100 uM for submaximal H₂O₂ and oxidative phosphorylation, 10,000 μM for maximal oxidative phosphorylation). Values represent means + SDs. Data were analysed using an unpaired Student's t test. *Significantly different from WT (P < 0.05).

Figure 7

Associations between resting maximal oxidative phosphorylation (A, pmol O₂/min/mg dry weight), submaximal (B), and maximal (C) H₂O₂ emission (pmol H₂O₂/min/mg dry weight) and daily myofibrillar protein synthesis rates (FSR, %/day) assessed over a 7‐day period using deuterium oxide in C57BL/6NJ and mitochondrial catalase transgenic mice (n = 16). Associations between resting maximal oxidative phosphorylation (D, pmol O₂/min/mg dry weight), submaximal (E), and maximal (F) H₂O₂ emission (pmol H₂O₂/min/mg dry weight) and daily sarcoplasmic protein synthesis rates (FSR, %/day). Data were analysed using Pearson's r product moment correlations. The solid line represents the linear regression line of best fit and the dashed lines represent the 95% confidence interval.