Catechol-O-methyltransferase inhibition increases the uptake of C-11-3-(3,4-dihydroxyphenyl)-L-alanine in the rat pancreas

Background: The objective of the present investigation was to evaluate the uptake and metabolism of 3-(3,4-dihydroxyphenyl-L-alanine) (L-DOPA) in the rat pancreas. Methods: The procedure included intravenous injection of the positron-emitting radiotracer L-[beta-C-11]DOPA (DOP) into unanaesthetized male Sprague-Dawley rats and evaluation of uptake of radioactivity in organs in animals only given the tracer and in animals given therapeutic doses of three different catechol-O-methyltransferase (COMT) inhibitors, OR-486, OR-611, or Ro 41-0960. Selected pancreatic were homogenized, and the chemical form bearing the radioactivity was analysed with high-performance liquid chromatography (HPLC). Results: The main finding was that the tracer uptake in the pancreas increased fourfold when the rats were pretreated with COMT inhibitors. Half maximum effect of OR-486 was found at a dose of 0.2 mg/kg. HPLC analysis showed that with COMT inhibitor, the radioactivity in the pancreas consisted of 90% DOPAC. When administering MAO-A and COMT inhibitor together, the pancreas radioactivity corresponded to dopamine. Also in the pig pancreas a significant increase of DOP was observed after COMT inhibition. Conclusions: This study has shown a high turnover of L-DOPA in the rat pancreas, which can be modulated to give enhanced levels of DOPAC or dopamine by COMT and MAO inhibition.