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Metabolic dysfunction in skeletal muscle is a major contributor to the development of type 2 diabetes. Endurance exercise training has long been established as an effective means to directly restore skeletal muscle glucose and lipid uptake and metabolism. However, in addition to the direct effects of skeletal muscle on glucose and lipids, there is renewed interest in the ability of skeletal muscle to coordinate metabolic activity of other tissues, such as adipose tissue and liver. The purpose of this study was to examine the effects of endurance exercise on the expression level of two novel muscle-derived secreted factors, or myokines, Myonectin and Fibronectin type III domain containing 5 (Fndc5), the precursor for Irisin. Methods: We used the diaphragm muscle from both the obese Zucker rat (OZR) and lean Zucker Rat (LZR) with 9 weeks of aerobic training on a motorized treadmill. We examined the gene expression of 12 commonly used reference genes and performed quantitative real-time PCR analysis on the gene expression of Myonectin and Fndc5. Results: Of the 12 commonly used PCR reference genes tested we were able to establish that Hypoxanthine phosphoribosyltransferase 1 (HPRT1) and lactate dehydrogenase A (Ldha) remained stable in the diaphragm muscle regardless of obesity or exercise training. Interestingly, we also concluded that the commonly used reference genes: beta-Actin, beta-2-microglobulin, Non-POU domain containing, octamer-binding, Peptidylprolyl isomerase H, 18S ribosomal rna, TATA box binding protein and Transferrin receptor were all found to be altered by the combination of exercise and obesity. Our study showed that the diaphragm muscle of the OZR had significantly higher expression levels of both myonectin and Fndc5. Exercise training had no effect on the expression level of Fndc5, but significantly lowered the gene expression of myonectin in both the LZR and OZR groups. Conclusion: Contrary to prior findings regarding the regulation of Fndc5 and myonectin we show that myonectin and Fndc5 expression are both increased in the OZR model of obesity. Further, long-term exercise training decreases myonectin levels, which is opposite, the effect reported with short-term exercise. However, this report confirms earlier work showing that Fndc5 gene expression is not altered by chronic exercise.
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