Glucose-Insulin homeostasis and characterization of proteins involved in glucose uptake signaling in equine skeletal muscle

Abstract Text: The objective of this study was to test the hypothesis that glucose-insulin homeostasis, and activation of AMP-activated protein kinase (AMPK), the protein kinase Akt, and the Akt substrate protein of 160 kDa (AS160) in equine skeletal muscle are altered by acute, exhaustive exercise and by aging. Unconditioned aged (n=6; 22.6 ± 2.25 years) and young (n=6; 5.5 ± 2.8 years) Standardbred mares were assessed for glucose-insulin homeostasis via frequently sampled intravenous glucose tolerance test (FSIGTT). All mares underwent a single bout of submaximal exercise. Plasma insulin and glucose concentrations were measured via radioimmunoassay and enzyme-electrode interface, respectively. Mid-gluteal muscle biopsies were taken pre-exercise, and at 0, 4, 24 and 48 h post-exercise. Muscle samples were analyzed via western immunoblotting for changes in activation of AMPK, Akt and AS160. Minimal model analysis of FSIGTT and repeated measures ANOVA were utilized to analyze data. Null hypothesis was rejected when P<0.05. FSIGTT results indicated that there was no difference between young and aged mares for insulin sensitivity (SI), glucose effectiveness (SG), acute insulin response to glucose (AIRg) or disposition index (AIRg x SI) (P>0.05). Area under the curve for both insulin (AUCi) and glucose (AUCg) were not different between young and aged mares (P>0.05). In response to acute exercise, young mares displayed elevated insulin concentrations at 2 (P=0.009) and 4 (P=0.007) h while aged mares displayed elevated insulin at 30 (P<0.001) and 60 (P=0.001) minutes post-exercise. Neither age nor exercise caused a significant change in AUCi (P>0.05). Glucose concentration was elevated at 2 h post-exercise in young mares (P<0.001), while in aged mares glucose remained elevated only until 60 min post-exercise (P=0.037). Exercise caused an increase in AUCg in young (P=0.007) and aged (P=0.031) mares, however there was no age effect on AUCg (P>0.05). Neither age nor exercise altered total protein concentrations or phosphorylated protein concentrations of AMPK, Akt or AS160 (P>0.05). In conclusion, age alone is not sufficient to alter insulin sensitivity in horses, but does alter glucose-insulin dynamics in response to exercise. Also, a single bout of submaximal exercise was not sufficient to alter activation of proteins believed to be involved in glucose uptake in skeletal muscle at the timepoints measured. The comparative literature suggests that these proteins are important for endocrine- and exercise-related glucose uptake and energy homeostasis.

Keywords: Aging, Exercise, Muscle