Glucose metabolism in human adipose tissue studied by 13C-glucose and microdialysis

Microdialysis can be used to monitor carbohydrate metabolism and lipolysis in adipose tissue. This method, however, does not discriminate between local metabolite production and delivery from other tissues. Our aim was to study glucose metabolism by direct delivery of 13C-labelled glucose into adipose tissue by microdialysis.

Seven healthy adults were studied after an overnight fast. In three of them the effect of physical activity on glucose metabolism was tested. Microdialysis catheters were introduced into abdominal adipose tissue and 25 mM 13C-labelled glucose was added to the perfusion fluid. An extraction procedure for separating lactic acid from glucose and glycerol in the microdialysate samples was developed. After derivatization, the 13C enrichment of the compounds was analysed by gas chromatography-mass spectrometry.

13C-labelled lactate was detected in the first 15-min eluate fraction following that in which 13C-glucose had reached the microdialysis probe. In the different subjects, 22-35 % of adipose tissue lactate was produced locally. During exercise there was an increase in the lactate concentration and a decrease in 13C enrichment of lactate. Although lactate production in the adipose tissue increased during exercise, most adipose tissue lactate resulted from inflow. The administered 13C-labelled glucose also rapidly converted to 13C-glycerol. The 13C enrichment of glycerol was lower than that of lactate. During exercise the 13C enrichment of glycerol increased, indicating that newly synthesized depot fat was preferentially hydrolysed during physical activity.

Metabolism of glucose to lactate and glycerol in subcutaneous adipose tissue is a rapid process that can be monitored in vivo by administration of stable isotope labelled glucose into the microdialysis probe. In adults at rest about one-fourth of adipose tissue lactate is produced locally.