Biochemical and functional evidences for a GLUT-4 homologous protein in avian skeletal muscle

The characteristics and modulation of glucose transport were investigated i n skeletal muscles of 5-wk-old Muscovy ducklings (Cairina moschata). Glucos e uptake by sarcolemmal vesicles isolated from gastrocnemius muscle followe d typical Michaelis-Menten kinetics with a K-m value (17 mM) similar to tha t described in equivalent mammalian preparations. Western blot analysis of duckling sarcolemma using antibodies directed against rat GLUT-4 transporte r revealed an immunoreactive protein of similar molecular mass (45 kDa) to that present in rats. When ducklings were killed in the postabsorptive stat e, GLUT-4 homologous protein was located predominantly (80%) in intracellul ar membranes. Insulin stimulation of a perfused leg muscle preparation in v itro led to the translocation of GLUT-4 homologous proteins from intracellu lar pools to the sarcolemma, with a subsequent increase in glucose uptake b y sarcolemmal vesicles and perfused muscles. Glucose transport was positive ly controlled by the metabolic needs of skeletal muscle as reflected by the increased glucose uptake of sarcolemmal vesicles isolated from cold-acclim ated ducklings. Present results, therefore, demonstrate, for the first time in an avian species, the existence in skeletal muscle of a glucose transpo rter showing molecular and functional homologies with the mammalian GLUT-4 transporter.