Effect of Methylxanthines on Extracellular and Intracellular 5′-Nucleotidase Activity in Isolated Fat Cells

Abstract

Adenosine is constantly released from isolated fat cells into the incubation medium in quantities that inhibit cyclic AMP accumulation and lipolysis in response to lipolytic hormones. Adenosine is generated from 5′-AMP by a 5′-nucleotidase which is located at the extracellular face of the fat cell plasma membrane (A.C. Newby et al., Biochem. J. 146, 625–633; 1975). Therefore, the effect of several stimulators of cyclic AMP dependent lipolysis on rat fat cell 5′-nucleotidase was studied. 5′-Nucleotidase activity in intact rat fat cells was measured at substrate concentrations of 1 μM 5′-AMP and was linear for 10 min under these conditions. Noradrenaline (3μM) and adenosine deaminase (10 μg/ml) had no effects on the activity of 5′-nucleotidase. The activity was inhibited by theophylline and caffeine and 50% inhibition was observed at a concentration of 0.5 mM. The inhibition by these methylxanthines occurred also in particulate preparations of fat cells. It was independent from the fat cell concentration in the incubation medium and was not influenced by adenosine up to 100 ′M. Other phosphodiesterase inhibitors which are not able to induce lipolysis like papaverine (0,1 mM) and Ro 20–1724 (0,1 mM) had no influence on the 5′-nucleotidase activity. The results indicate that methylxanthines in addition to their effect on cyclic AMP phosphodiesterase may increase cAMP levels and lipolysis in fat cells by inhibiting the adenosine generating enzyme and thereby prevent the antilipolytic control by endogenous adenosine.