Functional overload-induced muscle hypertrophy and glucose uptake occurs independent of glucose transporter 4 (GLUT4)
McMillin, Shawna; Denise Schmidt; Barbara Khan; Carol Witczak
Functional overload induces a number of adaptations in skeletal muscle that are similar to resistance exercise training, including muscle hypertrophy and glucose uptake. While numerous studies have investigated the molecular/cellular mechanisms underlying overload-induced muscle growth, little is known regarding the mechanism(s) underlying overload-induced glucose uptake. Glucose transporter 4 (GLUT4) is the predominant glucose transporter in muscle; yet surprisingly its role in overload-induced glucose uptake is currently unknown. The goal of this study was to determine whether GLUT4 regulates overload-induced muscle glucose uptake. Overload was induced in mouse plantaris muscle via unilateral synergist ablation of the distal two-thirds of the gastrocnemius and soleus. Muscle weights and ex vivo [3H]-2-deoxy-D-glucose uptake were assessed 5-days later. Overload-induced muscle glucose uptake and growth were not impaired in muscle-specific GLUT4 knockout mice, demonstrating that GLUT4 is not necessary for these processes. To determine which transporter mediates overload-induced glucose uptake, uptake was assessed +/- the sodium-dependent glucose co-transporter (SGLT) inhibitor, phloridzin, or the GLUT inhibitor, cytochalasin B. Cytochalasin B, but not phloridzin, prevented overload-induced glucose uptake demonstrating that GLUT(s) mediate this effect. To narrow down which GLUT, hexose competition experiments were performed. Based on the GLUT’s distinct affinities for different hexoses, [3H]-2-deoxy-D-glucose uptake was evaluated against L-glucose, D-glucose, D-fructose, D-galactose, or D-xylose. Overload-induced [3H]-2-deoxy-D-glucose uptake was not inhibited by D-fructose, demonstrating that the fructose-transporting GLUT2, GLUT5, GLUT8, and GLUT12, do not mediate this effect. In contrast, overload-induced [3H]-2-deoxy-D-glucose uptake was partially impaired by D-galactose and D-xylose, suggesting a role for GLUT1, GLUT3, GLUT6 and/or GLUT10. Consistent with this finding, immunoblot analyses demonstrated a 2- to 5-fold increase in GLUT1, GLUT3, GLUT6 and GLUT10 protein levels in overload muscles. Collectively, these results demonstrate that GLUT4 is not necessary for overload-induced muscle glucose uptake or growth, and suggest that GLUT1, GLUT3, GLUT6 and/or GLUT10 mediate overload-induced glucose uptake.