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.