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Transport of Carbon-11-Methionine Is Enhanced by Insulin

Departments of Oncology and Radiotherapy, Clinical Chemistry and Anesthesiology, University of Turku, Turku; Turku University Cyclotron/PET Center, Turku; Radiopharmaceutical Chemistry Laboratory, Turku University, Turku, Finland

Correspondence: For correspondence or reprints contact: Sirkku Leskinen, MD, Department of Oncology and Radiotherapy, University Central Hospital, FIN-20520 Turku, Finland.

ABSTRACT

The anabolic effects of insulin are not restricted to carbohydrate and lipid metabolism but also include protein metabolism. However, the effects of insulin on protein metabolism have been difficult to demonstrate in vivo. Amino acid transport is partly regulated by insulin according to the experimental data. PET provides a way to measure fractional uptake rates of amino acids. The purpose of this study was to measure the effect of insulin on amino acid transport from the plasma to the human parotid glands. Methods: We compared the uptake of L-[methyl-¹¹C]methionine ([¹¹C]methionine) into the parotid glands and cerebellum in seven healthy volunteers during the fasting state and euglycemic insulin clamp technique (1 mil/kg per minute). Results: The fractional uptake rate of [¹¹C]methionine was increased by 31% for the right parotid gland (p = 0.003) and by 29% for the left parotid gland (p = 0.009) during insulin clamp, while the increase was 19% for the cerebellum (p = 0.01). The concentration of amino acids typical for the hormone-sensitive transport system A was 11% lower during insulin infusion than in the fasting state. Conclusion: The uptake of methionine into brain tissue does not seem to be under major control by insulin, while the transport of methionine in the parotid glands is stimulated by insulin. PET provides a sophisticated method to study the transport system of amino acids in vivo.

Key Words: PET • insulin • carbon-11-methionine • salivary glands • brain

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