Application of new isotope methods: Two interventions in insulin resistant rodent models
A 2H2O labeling method is applied to the measurement of mitochondrial biogenesis, represented by mitochondrial (mt) DNA synthesis, in vivo in voluntary exercised healthy and insulin resistant rodents. A time course study revealed a transient induction of mitochondrial biogenesis in voluntary exercised female Sprague Dawley (SD) rats. Voluntary exercised female rats reached steady running distances of ∼10,000 m/day by the third week. MtDNA fractional synthesis from the red portion of the gastrocnemius was increased 3.7-fold after 1 week of voluntary wheel running, but returned to baseline after the third week. No significant increase in cytochrome C oxidase activity was observed.
The influence of biological sex on voluntary wheel running behavior and on associated mitochondrial adaptations was also investigated. Female rats ran 3.6-fold fold farther per day than age matched male rats after 2 weeks of access to running wheels; no increase in mtDNA fractional synthesis was observed in male rats. High fat feeding did not affect wheel running or the associated stimulation of mtDNA fractional synthesis in female rats. Two insulin resistant rodent models, Zucker Diabetic Fatty (ZDF) and Zucker Fatty (fa/fa) female rats, were tested for spontaneous running and associated changes in mtDNA synthesis. Female ZDF rats ran 6250 m/day during the second week of access to wheel running and mtDNA fractional synthesis was significantly elevated in the red (1.7-fold), but not white gastrocnemius muscle. In contrast, non-diabetic female Zucker Fatty (fa/fa) rats ran only 1550 m/day, with large variability in distance run. A significant correlation between distance run and mtDNA synthesis was observed in the red portion of the gastrocnemius of fa/fa rats (r2 = 0.88, p < 0.05).</p>
Rosiglitazone, a PPAR-γ agonist commonly used for the treatment of insulin resistance, was administered to male ZDF pre-diabetic rats. Four weeks of oral rosiglitazone treatment (3mg/kg/day) decreased fractional synthesis of mtDNA to 65% of the rate of the control group in both the red and white portions of the gastrocnemius. Two weeks of pre-treatment with rosiglitazone reduced spontaneous wheel running distance by 90%. The reduced wheel running behavior was sufficient to ablate the decrease in mtDNA fractional synthesis induced by rosiglitazone treatment in the red, but not the white portions of the gastrocnemius.
In summary, changes in mtDNA synthesis can be measured by D2O labeling. Voluntary wheel running causes a transient increase in mtDNA fractional synthesis and these changes in mtDNA synthesis are more sensitive than changes in mitochondrial enzyme activity. The mitochondrial biogenic response to voluntary exercise is running dependent. Animals with high innate wheel running behavior (female SD, female ZDF, and male ZDF) show increased mtDNA synthesis in response to exercise, while the response is absent in animals with low innate running behavior (male SD and female fa/fa). A PPAR-γ agonist (rosiglitazone) reduces voluntary exercise and mtDNA synthesis. We conclude that measurement of mtDNA synthesis by D2O is a very sensitive biomarker of mitochondrial plasticity in skeletal muscle in response to wheel running, and use of this approach allows subtle influences on mitochondrial biogenesis to be detected, including the effects of sex, genetic strain, distance run, and pharmacologic agents.
Anatomy & physiology
0719: Anatomy & physiology