Insulin stimulates GLUT4 translocation in the semitendinosus muscle of Shetland ponies.

Glucose homeostasis depends on insulin-regulated glucose uptake in the skeletal muscles and fat tissues via glucose transporter (GLUT) 4 translocation into cellular plasma membranes. The present study sought to elucidate GLUT4 expression, GLUT1 and GLUT4 translocation and glucose uptake in the skeletal muscles of Shetland ponies. Semitendinosus muscle explants were removed by open muscle biopsy from six Shetland pony geldings under general anaesthesia. The expression of GLUT4 was analysed by measuring muscle crude membrane (CM) GLUT4 protein contents. To determine the insulin-stimulated GLUT translocation, GLUT1 and GLUT4 concentrations were measured in partially purified plasma membranes (PM) and cytoplasmic vesicles (CV). GLUT contents were determined semi-quantitatively by Western blotting. Insulin-stimulated glucose uptake was analysed using 3-O-d-methyl[(3)H]glucose uptake. Incubation of semitendinosus muscle strips with 0.1 and 20mIU/mL insulin significantly increased GLUT4 translocation (PM GLUT4 contents), but had no significant effect on GLUT4 expression (CM GLUT4 concentrations) or PM GLUT1. The uptake of myocyte 3-O-Methylglucose was not significantly increased following insulin stimulation. The sub-cellular fractionation technique proved to be an appropriate tool for determining insulin-stimulated GLUT4 translocation in equine skeletal muscle. GLUT4 translocation in equines is insulin-dependent, as has been described in rodents and farm animals, but insulin-stimulated GLUT4 activation in ponies is lower than reported for pigs and cows under the same experimental conditions. Poor insulin-activated GLUT4 translocation may account for insulin resistance in ponies in previous euglycaemic, hyperinsulinaemic clamp tests.

Mechanisms of insulin-dependent glucose transport into porcine and bovine skeletal muscle.

Euglycemic, hyperinsulinemic clamp tests have shown that adult ruminants are less insulin-sensitive than monogastric omnivores. The present study was carried out to elucidate possible cellular mechanisms contributing to this impaired insulin sensitivity of ruminants. Western blotting was used to measure glucose transporters 1 and 4 (GLUT1, GLUT4) in oxidative (musculus masseter and diaphragm) and glycolytic (musculus longissimus dorsi and semitendinosus) skeletal muscle in the crude membranes of pigs and cows. Muscles were characterized biochemically. To determine insulin-stimulated 3-O-D-[(3)H]-methylglucose (3-O-MG) uptake and GLUT4 translocation, porcine and bovine musculus semitendinosus strips were removed by open muscle biopsy and incubated without and with 0.1 or 20 mIU insulin/ml. GLUT4 translocation was analyzed using subcellular fractionation techniques to isolate partially purified plasma membranes and cytoplasmic vesicles and using Western blotting. GLUT4 protein contents were significantly higher in oxidative than in glycolytic muscles in pigs and cows. GLUT1 protein contents were significantly higher in glycolytic than in oxidative muscles in bovines but not in porcines. The 3-O-MG uptake into musculus semitendinosus was similar in both species. Maximum insulin-induced GLUT4 translocation into musculus semitendinosus plasma membrane was significantly lower in bovines than in porcines. These results indicate that GLUT1 is the predominant glucose transporter in bovine glycolytic muscles and that a reinforced insulin-independent glucose uptake via GLUT1 may compensate for the impaired insulin-stimulated GLUT4 translocation, resulting in a similar 3-O-MG uptake in bovine and porcine musculus semitendinosus. These findings may explain at least in part the impaired in vivo insulin sensitivity of adult ruminants compared with that of omnivorous monogastric animals.