Mitochondrial Utilization of Competing Fuels Is Altered in Insulin Resistant Skeletal Muscle of Non-obese Rats (Goto-Kakizaki)

Nicola Lai; Ciarán E Fealy; Chinna M Kummitha; Silvia Cabras; John P Kirwan; Charles L Hoppel

doi:10.3389/fphys.2020.00677

Mitochondrial Utilization of Competing Fuels Is Altered in Insulin Resistant Skeletal Muscle of Non-obese Rats (Goto-Kakizaki)

Front Physiol. 2020 Jun 16:11:677. doi: 10.3389/fphys.2020.00677. eCollection 2020.

Authors

Nicola Lai^{1

2

3

4

5}, Ciarán E Fealy⁶, Chinna M Kummitha⁴, Silvia Cabras⁴, John P Kirwan^{6

7

8}, Charles L Hoppel^{5

9

10}

Affiliations

¹ Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, VA, United States.
² Biomedical Engineering Institute, Old Dominion University, Norfolk, VA, United States.
³ Department of Mechanical, Chemical and Materials Engineering, University of Cagliari, Cagliari, Italy.
⁴ Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH, United States.
⁵ Center for Mitochondrial Disease, Case Western Reserve University, Cleveland, OH, United States.
⁶ Department of Pathobiology, Lerner Research Institute, Cleveland Clinic, Cleveland, OH, United States.
⁷ Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH, United States.
⁸ Pennington Biomedical Research Center, Baton Rouge, LA, United States.
⁹ Department of Pharmacology, Case Western Reserve University, Cleveland, OH, United States.
¹⁰ Department of Medicine, School of Medicine, Case Western Reserve University, Cleveland, OH, United States.

Abstract

Aim: Insulin-resistant skeletal muscle is characterized by metabolic inflexibility with associated alterations in substrate selection, mediated by peroxisome-proliferator activated receptor δ (PPARδ). Although it is established that PPARδ contributes to the alteration of energy metabolism, it is not clear whether it plays a role in mitochondrial fuel competition. While nutrient overload may impair metabolic flexibility by fuel congestion within mitochondria, in absence of obesity defects at a mitochondrial level have not yet been excluded. We sought to determine whether reduced PPARδ content in insulin-resistant rat skeletal muscle of a non-obese rat model of T2DM (Goto-Kakizaki, GK) ameliorate the inhibitory effect of fatty acid (i.e., palmitoylcarnitine) on mitochondrial carbohydrate oxidization (i.e., pyruvate) in muscle fibers.

Methods: Bioenergetic function was characterized in oxidative soleus (S) and glycolytic white gastrocnemius (WG) muscles with measurement of respiration rates in permeabilized fibers in the presence of complex I, II, IV, and fatty acid substrates. Mitochondrial content was measured by citrate synthase (CS) and succinate dehydrogenase activity (SDH). Western blot was used to determine protein expression of PPARδ, PDK isoform 2 and 4.

Results: CS and SDH activity, key markers of mitochondrial content, were reduced by ∼10-30% in diabetic vs. control, and the effect was evident in both oxidative and glycolytic muscles. PPARδ (p < 0.01), PDK2 (p < 0.01), and PDK4 (p = 0.06) protein content was reduced in GK animals compared to Wistar rats (N = 6 per group). Ex vivo respiration rates in permeabilized muscle fibers determined in the presence of complex I, II, IV, and fatty acid substrates, suggested unaltered mitochondrial bioenergetic function in T2DM muscle. Respiration in the presence of pyruvate was higher compared to palmitoylcarnitine in both animal groups and fiber types. Moreover, respiration rates in the presence of both palmitoylcarnitine and pyruvate were reduced by 25 ± 6% (S), 37 ± 6% (WG) and 63 ± 6% (S), 57 ± 8% (WG) compared to pyruvate for both controls and GK, respectively. The inhibitory effect of palmitoylcarnitine on respiration was significantly greater in GK than controls (p < 10^-3).

Conclusion: With competing fuels, the presence of fatty acids diminishes mitochondria ability to utilize carbohydrate derived substrates in insulin-resistant muscle despite reduced PPARδ content.

Keywords: bioenergetic; diabetes; fatty acid oxidation; metabolic flexibility; oxidative phosphorylation.

Abstract

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