Lumped constant for [18F]fluorodeoxyglucose in skeletal muscles of obese and nonobese humans

Abstract

Quantitative 2-[18F]fluoro-2-deoxy-d-glucose ([18F]FDG) positron emission tomography (PET) has been widely used to calculate glucose utilization in skeletal muscle. FDG-PET results depend partly on the lumped constant (LC), which accounts for the differences in the transport and phosphorylation between [18F]FDG and glucose. In this study, we estimated the LC for [18F]FDG directly in normal and in insulin-resistant obese subjects by combining FDG PET with the microdialysis technique. Eight obese [age 29.4 ± 1.0 yr, body mass index (BMI) 33.6 ± 1.0 kg/m2] and eight nonobese (age 25.0 ± 1.0 yr, BMI 23.1 ± 1.0 kg/m2) males were studied during euglycemic hyperinsulinemia (1 mU · kg−1· min−1for 150 min). Muscle blood flow was measured using15O-labeled water and PET. Muscle [18F]FDG uptake (rGUFDG) was calculated with Patlak graphic analysis. Interstitial glucose concentration of the quadriceps femoris muscle was measured simultaneously with [18F]FDG scanning using microdialysis. Muscle glucose uptake (by microdialysis, rGUMD) was calculated by multiplying glucose extraction by regional muscle blood flow. A significant correlation was found between rGUMDand rGUFDG( r = 0.78, P < 0.01). The LC was determined as the ratio of the rGUFDGto the rGUMD. The LC averaged 1.16 ± 0.16 and was similar in the obese and nonobese subjects (1.15 ± 0.11 vs. 1.16 ± 0.07, respectively, not significant). In conclusion, the microdialysis technique can be reliably combined with FDG PET to measure glucose uptake in skeletal muscle. Direct measurements with these two independent techniques suggest an LC value of 1.2 for [18F]FDG in human skeletal muscle during insulin stimulation, and the LC appears not to be sensitive to insulin resistance.