Fig. 3.

The flux of glucose from the blood to the membrane to muscle. Steps 1, 2 and 3 represent glucose delivery, membrane transport and phosphorylation steps, respectively. Hexagons labeled ‘G’ are glucose molecules; those with an associated ‘P’ are glucose 6-phosphate. Green ovals are glucose transporters. The figure illustrates the fasted, sedentary state where few transporters are in the plasma membrane. Glucose 6-phosphate inhibition of glucose phosphorylation is illustrated by a negative feedback loop. The countertransport method estimates glucose gradients across each step using radioactive glucose analogues. Transgenic mice were used to alter sites of resistance at each step.