Characterization of the Uptake of the Methylxanthines Theophylline and Caffeine in Isolated Pancreatic Islets and their Effect on D-Glucose Transport1

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Abstract
The uptake of theophylline and caffeine was determined in isolated [rat] pancreatic islets employing a dual isotope procedure with sucrose as an extracellular marker. Islets rapidly accumulated caffeine and theophylline with apparent Kd of approximately 23 and 6 mM, respectively. Theophylline inhibited the uptake of caffeine and caused displacement of caffeine from islets. These results indicated a competition by theophylline and caffeine for a common site (binding and/or transport carrier). Theophylline and caffeine inhibited D-glucose transport in a dose-dependent manner and within the limits of the experimental system, this inhibition appeared to be non-competitive. (Bu)2c[dibutyryl cyclic]AMP under similar experimental conditions exerted no effect on D-glucose transport. These results present evidence for a rapid uptake of theophylline and caffeine in pancreatic islets, which is compatible with their immediate cellular effects. These results demonstrate a direct effect by theophylline and caffeine on D-glucose transport which appears independent of their ability to alter intracellular cAMP levels.

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