Divergent skeletal muscle respiratory capacities in rats artificially selected for high and low running ability: a role for Nor1?

Abstract

Inactivity-related diseases are becoming a huge burden on Western society. While there is a major environmental contribution to metabolic health, the intrinsic properties that predispose or protect against particular health traits are harder to define. We used rat models of inborn high running capacity (HCR) and low running capacity (LCR) to determine inherent differences in mitochondrial volume and function, hypothesizing that HCR rats would have greater skeletal muscle respiratory capacity due to an increase in mitochondrial number. Additionally, we sought to determine if there was a link between the expression of the orphan nuclear receptor neuron-derived orphan receptor (Nor)1, a regulator of oxidative metabolism, and inherent skeletal muscle respiratory capacity. LCR rats were 28% heavier (P < 0.0001), and fasting serum insulin concentrations were 62% greater than in HCR rats (P = 0.02). In contrast, HCR rats had better glucose tolerance and reduced adiposity. In the primarily oxidative soleus muscle, maximal respiratory capacity was 21% greater in HCR rats (P = 0.001), for which the relative contribution of fat oxidation was 20% higher than in LCR rats (P = 0.02). This was associated with increased citrate synthase (CS; 33%, P = 0.009) and β-hydroxyacyl-CoA (β-HAD; 33%, P = 0.0003) activities. In the primarily glycolytic extensor digitum longus muscle, CS activity was 29% greater (P = 0.01) and β-HAD activity was 41% (P = 0.0004) greater in HCR rats compared with LCR rats. Mitochondrial DNA copy numbers were also elevated in the extensor digitum longus muscles of HCR rats (35%, P = 0.049) and in soleus muscles (44%, P = 0.16). Additionally, HCR rats had increased protein expression of individual mitochondrial respiratory complexes, CS, and uncoupling protein 3 in both muscle types (all P < 0.05). In both muscles, Nor1 protein was greater in HCR rats compared with LCR rats (P < 0.05). We propose that the differential expression of Nor1 may contribute to the differences in metabolic regulation between LCR and HCR phenotypes.

Fig. 1.

Glucose tolerance test. A: blood glucose responses over 120 min. B: areas under the blood glucose curve (Glucose AUC). C: serum insulin responses over 120 min. D: areas under the serum insulin curve (Insulin AUC). HCR rats, rats with high running capacity; LCR rats, rats with low running capacity. Values are means ± SE; N = 8–10. *P < 0.05.

Fig. 2.

Basal and insulin-stimulated glucose uptake in the soleus muscles of LCR and HCR rats. Values are means ± SE; N = 11. *P < 0.05, different from the basal value; †P < 0.05 different from the LCR value under the same conditions.

Fig. 3.

Mass-specific O₂ consumption in the soleus muscles of LCR and HCR rats. Oct, octanoylcarnitine; M, malate; D1, 1 mM ADP; D22.5, 2.5 mM ADP; ETF, electron-transferring flavoprotein; G, glutamate; S, succinate, D5, 5 mM ADP; ETS, electron transfer system (uncoupled respiration); CIV, respiratory complex IV (cytochrome c oxidase). Values are means ± SE; N = 11. *P < 0.05.

Fig. 4.

Enzyme activities and relative mitochondrial (mt)DNA copy numbers in both the soleus and EDL muscles of LCR and HCR rats. A: citrate synthase (CS) activity (N = 10). B: β-hydroxyacyl-CoA (β-HAD) activity (N = 10). C: mtDNA-to-nuclear (n)DNA ratio [mtDNA:nDNA; in arbitrary units (AU), N = 7]. Values are means ± SE. *P < 0.05.

Fig. 5.

Protein content of the soleus muscle from LCR and HCR rats as determined by Western blot analysis. PGC-1, peroxisome proliferator-activated receptor-γ coactivator-1; Nur77, neuron-derived clone 77; Nor1, neuron-derived orphan receptor 1; FNDC5, fibronectin type III domain-containing 5; GLUT4, glucose transporter 4; FAT/CD36, fatty acid translocase/CD36; UCP3, uncoupling protein 3; CI, complex I; CII, complex II; CIII, complex III; CIV^I, CIV subunit I; CIV^II, CIV subunit II; CIV^IV, CIV subunit IV; CV, complex V. Values are means ± SE; N = 8–12. *P < 0.05.

Fig. 6.

Protein content of the EDL muscle from LCR and HCR rats as determined by Western blot analysis. Values are means ± SE; N = 8–12. *P < 0.05.