Justin P. Dobson

Abnormal protein turnover and anabolic resistance to exercise in sarcopenic obesity

Obesity may impair protein synthesis rates and cause anabolic resistance to growth factors, hormones, and exercise, ultimately affecting skeletal muscle mass and function. To better understand muscle wasting and anabolic resistance with obesity, we assessed protein 24‐h fractional synthesis rates (24‐h FSRs) in selected hind‐limb muscles of sedentary and resistance‐exercised lean and obese Zucker rats. Despite atrophied hind‐limb muscles (–28% vs. lean rats), 24‐h FSRs of mixed proteins were significantly higher in quadriceps (+18%) and red or white gastrocnemius (+22 or +38%, respectively) of obese animals when compared to lean littermates. Basal synthesis rates of myofibrillar (+8%) and mitochondrial proteins (–1%) in quadriceps were not different between phenotypes, while manufacture of cytosolic proteins (+12%) was moderately elevated in obese cohorts. Western blot analyses revealed a robust activation of p70S6k (+178%) and a lower expression of the endogenous mTOR inhibitor DEPTOR (–28%) in obese rats, collectively suggesting that there is an obesity‐induced increase in net protein turnover favoring degradation. Lastly, the protein synthetic response to exercise of mixed (–7%), myofibrillar (+6%), and cytosolic (+7%) quadriceps subfractions was blunted compared to the lean phenotype (+34, +40, and +17%, respectively), indicating a muscle‐ and subfraction‐specific desensitization to the anabolic stimulus of exercise in obese animals.—Nilsson, M. I., Dobson, J. P., Greene, N. P., Wiggs, M. P., Shimkus, K. L., Wudeck, E. V., Davis, A. R., Laureano, M. L., Fluckey, J. D., Abnormal protein turnover and anabolic resistance to exercise in sarcopenic obesity. FASEB J. 27, 3905–3916 (2013). www.fasebj.org

Insulin resistance syndrome blunts the mitochondrial anabolic response following resistance exercise

Metabolic risk factors associated with insulin resistance syndrome may attenuate augmentations in skeletal muscle protein anabolism following contractile activity. The purpose of this study was to investigate whether or not the anabolic response, as defined by an increase in cumulative fractional protein synthesis rates (24-h FSR) following resistance exercise (RE), is blunted in skeletal muscle of a well-established rodent model of insulin resistance syndrome. Four-month-old lean (Fa/?) and obese (fa/fa) Zucker rats engaged in four lower body RE sessions over 8 days, with the last bout occurring 16 h prior to muscle harvest. A priming dose of deuterium oxide ((2)H(2)O) and (2)H(2)O-enriched drinking water were administered 24 h prior to euthanization for assessment of cumulative FSR. Fractional synthesis rates of mixed (-5%), mitochondrial (-1%), and cytosolic (+15%), but not myofibrillar, proteins (-16%, P = 0.012) were normal or elevated in gastrocnemius muscle of unexercised obese rats. No statistical differences were found in the anabolic response of cytosolic and myofibrillar subfractions between phenotypes, but obese rats were not able to augment 24-h FSR of mitochondria to the same extent as lean rats following RE (+14% vs. +28%, respectively). We conclude that the mature obese Zucker rat exhibits a mild, myofibrillar-specific suppression in basal FSR and a blunted mitochondrial response to contractile activity in mixed gastrocnemius muscle. These findings underscore the importance of assessing synthesis rates of specific myocellular subfractions to fully elucidate perturbations in basal protein turnover rates and differential adaptations to exercise stimuli in metabolic disease.