Interstitial glycerol concentrations in human skeletal muscle and adipose tissue during graded exercise

Aim: It is not clear how lipolysis changes in skeletal muscle and adipose tissue during exercise of different intensities. We aimed at estimating this by microdialysis and muscle biopsy techniques. Methods: Nine healthy, young men were kicking with both legs at 25% of maximal power (W-max) for 45 min and then simultaneously with one leg at 65% and the other leg at 85%W-max for 35 min. Results: Glycerol concentrations in skeletal muscle and adipose tissue interstitial fluid and in arterial plasma increased (P < 0.001) during low intensity exercise and increased (P < 0.05) even more during moderate intensity exercise. The difference between interstitial muscle and arterial plasma water glycerol concentration, which indicates the direction of the glycerol flux, was positive (P < 0.05) at rest (21 +/- 9 mum) and during exercise at 25%W-max (18 +/- 6 mum). The difference decreased (P < 0.05) with increasing exercise intensity and was not significantly different from zero during exercise at 65% (-11 +/- 17 muM) and 85% (-12 +/- 13 mum) W-max. In adipose tissue, the difference between interstitial and arterial plasma water glycerol increased (P < 0.001) with increasing intensity. The net triacylglycerol breakdown, measured chemically from the biopsy, did not differ significantly from zero at any exercise intensity although directional changes were similar to microdialysis changes. Conclusions: Skeletal muscle releases glycerol at rest and at low exercise intensity but not at higher intensities. This can be interpreted as skeletal muscle lipolysis peaking at low exercise intensities but could also indicate that glycerol is taken up in skeletal muscle at a rate which is increasing with exercise intensity.