The effect of physical training on in vivo insulin-stimulated glucose utilization in relation to glycolysis (lactate formation) in rat peripheral tissues was investigated in 8 sedentary controls (SC) and 7 voluntary running rats (VR). We used a sequential euglycemic clamp procedure (insulin infusion rate; 6.0, 30.0 mU/kg.min) in combination with a microdialysis technique in M. quadriceps femoris, vastus lateralis, and inguinal adipose tissue. In the clamp study, glucose infusion rate (GIR) averaged over 45-75 min during the 6.0-mU/kg.min insulin infusion was significantly (p < 0.01) higher in VR (15.36 +/- 0.83 mg/kg.min, mean +/- SE) than in SC (10.41 +/- 0.88 mg/kg.min), and the lack of a significant difference in GIR between VR and SC was found during the 30.0-mU/kg.min insulin infusion. In these tissues, there was no significant difference in dialysate lactate levels between VR and SC in the basal state without insulin or glucose infusion, or at an insulin infusion rate of 30.0 mU/kg.min. However, dialysate lactate concentrations in muscle averaged over 45-75 min during the 6.0-mU/kg.min insulin clamp procedure in VR (8.51 +/- 0.71 mg/dl) were significantly (p < 0.05) higher than in SC (6.18 +/- 0.48 mg/dl). These results indicated that insulin action in skeletal muscle and adipose tissue could be evaluated in vivo by using the microdialysis technique, and that an increase in GIR in VR was, in part, explained by an increase in lactate formation in skeletal muscle.

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