Novel Expression of GLUT3 , GLUT6 and GLUT10 in Equine Gluteal Muscle Following Glycogen-Depleting Exercise: Impact of Dietary Starch and Fat.

Horses have a slow rate of muscle glycogen repletion relative to other species for unknown reasons. Our aim was to determine the expression of glucose transporters () and genes impacting GLUT4 expression and translocation in the gluteal muscle. Five fit Thoroughbred horses performed glycogen-depleting exercises on high-starch (HS, 2869 g starch/day) and low-starch, high-fat diets (LS-HF, 358 g starch/d) with gluteal muscle biopsies obtained before and after depletion and during repletion. Muscle glycogen declined by ≈30% on both diets with little increase during repletion on LS-HF. Transcriptomic analysis identified differential expression (DE) of only 2/12 genes impacting GLUT4 translocation (two subunits of AMP protein kinase) and only at depletion on LS-HF. Only 1/13 genes encoding proteins that promote transcription had increased DE ( at depletion LS-HF). comprised ≈30% of total mRNA expression at rest. Remarkably, by 72 h of repletion expression of , and increased to ≈25% of total mRNA. Expression of and lagged from 24 h of repletion on HS to 72 h on LS-HF. Lacking an increase in gene expression in response to glycogen-depleting exercise, equine muscle increases , and expression potentially to enhance glucose transport, resembling responses observed in resistance trained GLUT4-null mice.