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 ( GLUT ) 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 GLUT4 transcription had increased DE ( PPARGC1A at depletion LS-HF). GLUT4 comprised ≈30% of total GLUT mRNA expression at rest. Remarkably, by 72 h of repletion expression of GLUT3 , GLUT6 and GLUT10 increased to ≈25% of total GLUT mRNA. Expression of GLUT6 and GLUT10 lagged from 24 h of repletion on HS to 72 h on LS-HF. Lacking an increase in GLUT4 gene expression in response to glycogen-depleting exercise, equine muscle increases GLUT3 , GLUT6 and GLUT10 expression potentially to enhance glucose transport, resembling responses observed in resistance trained GLUT4-null mice.