Glucose Transport Is Rate Limiting for Skeletal Muscle Glucose Metabolism in Normal and STZ-Induced Diabetic Rats

To test the hypothesis that glucose transport is rate limiting for skeletal muscle glucose metabolism, intracellular free glucose was measured in in situ abdominal wall muscle and liver that were rapidly freeze-clamped and removed from normal and streptozocin-induced diabetic (STZ-D) Sprague-Dawley rats exposed to different concentrations of serum insulin and glucose achieved by 90-min glucose clamps. Three groups of anesthetized normal (n = 17) and three groups of STZ-D (n = 19) animals were studied. In each group, infusions were adjusted to expose rats to basal, euglycemic-hyperinsulinemic, and hyperglycemic-hyperinsulinemic conditions. Final steady-state serum glucose and insulin concentrations (mean ± SE) achieved in the six groups ranged from 112 ± 2 to 467 ± 26 mg/dl and from 5.8 ± 1.9 to 3644 ±116 μU/ml, respectively. Intracellular free-glucose content of all tissue was calculated from total tissue glucose content minus the contribution of glucose in extracellular water estimated with [¹⁴C]inulin. Intracellular free glucose was not detected in muscle of any of the experimental groups. However, intracellular free glucose was detected in liver from all normal and STZ-D rats, and the calculated intracellular concentrations of free glucose correlated with serum glucose concentrations (r = .68, P < .001). We conclude that intracellular free glucose does not accumulate, regardless of glucose or insulin concentration, in normal skeletal muscle and muscle made insulin resistant (by STZ-D), suggesting that glucose transport remains rate limiting.