C-13 P-31 NMR-STUDIES OF GLUCOSE-TRANSPORT IN HUMAN SKELETAL-MUSCLE/

The muscle intracellular (IC) free glucose concentration and the rate of muscle glycogen synthesis were measured by using in vivo C-13 and P -31 NMR spectroscopy in normal volunteers under hyperinsulinemic (appr oximate to 300 pM) clamp conditions at the following three plasma gluc ose levels: euglycemia (approximate to 6 mM), mild (approximate to 10 mM), and high (approximate to 16 mM) hyperglycemia, In keeping with bi opsy studies, muscle IC free glucose concentration at euglycemia (-0.0 3 +/- 0.03 mmol/kg of muscle, mean +/- SEM, n = 10) was not statistica lly different from zero, A small but statistically significant amount of IC free glucose was observed during mild and high hyperglycemia: 0. 15 +/- 0.08 (n = 5) and 0.43 +/- 0.20 mmol/kg of muscle (n = 5), respe ctively, Muscle glycogen synthesis rate, in mmol per kg of muscle per min, was 111 +/- 11 at euglycemia (n = 10), 263 +/- 29 during mild hyp erglycemia (n = 5), and 338 +/- 42 during high hyperglycemia (n = 5), these three rates being significantly different from each other, As pr evious in vitro and in vivo studies, these rates suggest a K-m (concen tration at which unidirectional glucose transport reaches half-maximal rate) of the muscle glucose transport system in the 15-25 mM range un der hyperinsulinemic conditions, The low concentrations of muscle IC f ree glucose observed under hyperinsulinemic conditions were interprete d, with this estimate and in the framework of metabolic control theory , as glucose transport being the predominant step controlling muscle g lucose flux not only at euglycemia but also during hyperglycemia.