GLUCOSE-TRANSPORTER (GLUT4) PROTEIN-CONTENT IN OXIDATIVE AND GLYCOLYTIC SKELETAL-MUSCLES FROM CALF AND GOAT

It is well accepted that skeletal muscle is a major glucose-utilizing tissue and that insulin is able to stimulate in vivo glucose utilizati on in ruminants as in monogastrics. In order to determine precisely ho w glucose uptake is controlled in various ruminant muscles, particular ly by insulin, this study was designed to investigate in vitro glucose transport and insulin-regulatable glucose-transporter protein (GLUT4) in muscle from calf and goat. Our data demonstrate that glucose trans port is the rate-limiting step for glucose uptake in bovine fibre stri ps, as in rat muscle. Insulin increases the rate of in vitro glucose t ransport in bovine muscle, but to a lower extent than in rat muscle. A GLUT4-like protein was detected by immunoblot assay in all insulin-re sponsive tissues from calf and goat (heart, skeletal muscle, adipose t issue) but not in liver, brain, erythrocytes and intestine. Unlike the rat, bovine and goat GLUT4 content is higher in glycolytic and oxido- glycolytic muscles than in oxidative muscles. In conclusion, using bot h a functional lest (insulin stimulation of glucose transport) and an immunological approach, this study demonstrates that ruminant muscles express GLUT4 protein. Our data also suggest that, in ruminants, gluco se is the main energy-yielding substrate for glycolytic but not for ox idative muscles, and that insulin responsiveness may be lower in oxida tive than in other skeletal muscles.