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Exercise Restores Skeletal Muscle Glucose Delivery But Not Insulin-Mediated Glucose Transport and Phosphorylation in Obese Subjects

Turku PET Centre (L.S., V.O., K.H., N.S., J.K., P.N., P.I.) and Department of Medicine (P.N.), University of Turku, FIN-20521 Turku, Finland; and Institute of Clinical Physiology (P.I.), CNR National Research Council, 56124 Pisa, Italy

Address all correspondence and requests for reprints to: Patricia Iozzo, M.D., Ph.D., Turku PET Centre, University of Turku, P.O. Box 52, FIN-20521 Turku, Finland. E-mail: patricia.iozzo{at}ifc.cnr.it.

Context/Objective: Insulin resistance in obese subjects results in the impaired disposal of glucose by skeletal muscle. The current study examined the effects of insulin and/or exercise on glucose transport and phosphorylation in skeletal muscle and the influence of obesity on these processes.

Subjects/Methods: Seven obese and 12 lean men underwent positron emission tomography with 2-deoxy-2-[¹⁸F]fluoro-D-glucose in resting and isometrically exercising skeletal muscle during normoglycemic hyperinsulinemia. Data were analyzed by two-tissue compartmental modeling. Perfusion and oxidative capacity were measured during insulin stimulation by [¹⁵O]H₂O and [¹⁵O]O₂.

Results: Exercise increased glucose fractional uptake (K), inward transport rate (K₁), and the k₃ parameter, combining transport and intracellular phosphorylation, in lean and obese subjects. In each group, there was no statistically significant difference between plasma flow and K₁. At rest, a significant defect in K₁ (P = 0.0016), k₃ (P = 0.016), and K (P = 0.022) was found in obese subjects. Exercise restored K₁, improved but did not normalize K (P = 0.03 vs. lean), and did not ameliorate the more than 60% relative impairment in k₃ in obese individuals (P = 0.002 vs. lean). The glucose oxidative potential tended to be reduced by obesity.

Conclusions/Interpretation: The study indicates that exercise restores the impairment in insulin-mediated skeletal muscle perfusion and glucose delivery associated with obesity but does not normalize the defect involving the proximal steps regulating glucose disposal in obese individuals. Our data support the use of 2-deoxy-2-[¹⁸F]fluoro-D-glucose-positron emission tomography in the dissection between substrate supply and intrinsic tissue metabolism.

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