Metabolite levels in human skeletal muscle and adipose tissue studied with microdialysis at low perfusion flow

To identify a perfusion flow at which the interstitial fluid completely equilibrates with the microdialysis perfusion fluid, a protocol with successively lower perfusion flows was used. A colloid was included in the perfusion fluid to make sampling possible at the lowest perfusion flows. At 0.16 microliter/min, the measured metabolites had reached a complete equilibration in both tissues, and the measured concentrations of glucose, glycerol, and urea were in good agreement with expected tissue-specific levels. The glucose concentration in adipose tissue (4.98 +/- 0.14 mM) was equal to that of plasma (5.07 +/- 0.07 mM), whereas the concentration in muscle (4.41 +/- 0.11 mM) was lower than in plasma and adipose tissue (P < 0.001). The concentration of lactate was higher (P < 0.001) in muscle (2.39 +/- 0.22 mM) than in adipose tissue (1.30 +/- 0.12 mM), whereas the glycerol concentration in adipose tissue (233 +/- 19.7 microM) was higher (P < 0.001) than in muscle (40.8 +/- 3.0 microM) and in plasma (68.7 +/- 3.97 microM). The concentration of urea was equal in the two tissues. Overall, the study indicates that microdialysis at a low perfusion flow may be a tool to continuously monitor tissue interstitial concentrations.