INTERSTITIAL GLUCOSE AND LACTATE BALANCE IN HUMAN SKELETAL-MUSCLE AND ADIPOSE-TISSUE STUDIED BY MICRODIALYSIS

1. Microdialysis was used to gain insight into the substrate exchanges in the interstitial space of skeletal muscle and adipose tissue. Probes were inserted in the quadriceps femoris muscle and para-umbilical subcutaneous adipose tissue of thirteen subjects and microdialysis was performed at different flow rates (1-4 mul min-1) and during changes in tissue blood flow. 2. When ethanol (5 mM) is included in the perfusion solution, the ethanol clearance from the probe is a measure of tissue blood flow. Blood flow changes induced by adenosine or vasopressin perfusion, by exercise or by circulatory occlusion resulted in ethanol clearance values of 69-139 % of the basal level. The ethanol clearance was higher in skeletal muscle than in adipose tissue (32-62 %, P < 0.001), a difference compatible with a higher blood flow in muscle tissue. 3. The fraction of the interstitial glucose concentration that was recovered with the microdialysis was similar in skeletal muscle (the absolute values being 1.70 +/- 0.14 mm at 1 mul min-1 and 0.59 +/- 0.05 mm at 4 mul min-1) and adipose tissue (1.89 +/- 0.20 mm at 1 mul min-1; 0.54 +/- 0.05 mm at 4 mul min-1) and correlated inversely with the tissue ethanol clearance, both in the basal state and during changes in tissue blood flow (muscle: r = -0.56 to -0.67; adipose tissue r = -0.72 to -0.95). Coefficients of variation were 6-8 % (glucose) and 11-16 % (lactate) and were similar during isometric exercise. The reproducibility of the technique (comparison of two contralateral probes; perfusion flow rate 4 mul min-1) was 5.3-8.3 % (ethanol) and 23.9-20.8 % (glucose) in muscle (n = 6) and adipose tissue (n = 4) respectively. 4. The skeletal muscle dialysate lactate concentration (1 mul min-1: 1.16 +/- 0.2 mM) was higher than in adipose tissue (0.76 +/- 0.08 mM, P< 0.05), where the absolute amount of lactate that could be removed from the tissue (at 4 mul min-1) was only half of that in skeletal muscle (0.8 +/- 0.11 vs. 1.76 +/- 0.23 nmol min-1, P< 0.05). The dialysate lactate level was not affected in either tissue by large changes in the interstitial glucose concentration indicating that in neither tissue is blood glucose a significant source of lactate formation. 5. The blood flow effects on the dialysate glucose concentration are the likely consequence of probe glucose drainage artificially shifting the balance between the supply and consumption of interstitial glucose. The results are compatible with a lower blood flow, glucose uptake and lactate release in para-umbilical adipose tissue than in resting skeletal muscle.