The flux of glucose from the blood to the membrane to muscle. 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. David H. Wasserman et al. J Exp Biol 2011;214:254-262 © 2011.