Na+-sensitive component of 3-O-methylglucose uptake in frog skeletal muscle

A Na+-sensitive uptake of 3-O-methylglucose (3-O-MG), a nonmetabolized sugar, was characterized in frog skeletal muscle. A removal of Na+ from the bathing solution reduced 3-O-MG uptake, depending on the amount of Na+ removed. At a 3-O-MG concentration of 2 mM, the Na+-sensitive component of uptake in Ringer's solution was estimated to be about 26% of the total uptake. The magnitude of Na+-sensitive component sigmoidally increased with an increase of 3-O-MG in bathing solution, whereas in Na+-free Ringer's solution the uptake was proportional to the concentration. The half saturation of the Na+-sensitive component was at a 3-O-MG concentration of about 13 mM, and the Hill coefficient was 1.4 to 1.6. Phlorizin (5 mM), a potent inhibitor specific for Na+-coupled glucose transport, reduced the uptake in a solution containing Na+ to the level in Na+-free Ringer's solution. Glucose of concentrations higher than 20 mM suppressed 3-O-MG uptake to a level slightly lower than that in Na+-free Ringer's solution. These observations indicate that there are Na+-coupled sugar transport systems in frog skeletal muscle which are shared by both glucose and 3-O-MG.