Impaired skeletal muscle beta-adrenergic activation and lipolysis are associated with whole-body insulin resistance in rats bred for low intrinsic exercise capacity

Rats selectively bred for high endurance running capacity (HCR) have higher insulin sensitivity and improved metabolic health compared with those bred for low endurance capacity (LCR). We investigated several skeletal muscle characteristics, in vitro and in vivo, that could contribute to the metabolic phenotypes observed in sedentary LCR and HCR rats. After 16 generations of selective breeding, HCR had approximately 400% higher running capacity (P < 0.001), improved insulin sensitivity (P < 0.001), and lower fasting plasma glucose and triglycerides (P < 0.05) compared with LCR. Skeletal muscle ceramide and diacylglycerol content, basal AMP-activated protein kinase (AMPK) activity, and basal lipolysis were similar between LCR and HCR. However, the stimulation of lipolysis in response to 10 μm isoproterenol was 70% higher in HCR (P = 0.004). Impaired isoproterenol sensitivity in LCR was associated with lower basal triacylglycerol lipase activity, Ser660 phosphorylation of HSL, and β2-adrenergic receptor protein content in skeletal muscle. Expression of the orphan nuclear receptor Nur77, which is induced by β-adrenergic signaling and is associated with insulin sensitivity, was lower in LCR (P < 0.05). Muscle protein content of Nur77 target genes, including uncoupling protein 3, fatty acid translocase/CD36, and the AMPK γ3 subunit were also lower in LCR (P < 0.05). Our investigation associates whole-body insulin resistance with impaired β-adrenergic response and reduced expression of genes that are critical regulators of glucose and lipid metabolism in skeletal muscle. We identify impaired β-adrenergic signal transduction as a potential mechanism for impaired metabolic health after artificial selection for low intrinsic exercise capacity.