Insulin and Adenosine Regulate the Phosphatidylcholine Concentration in Isolated Rat Adipocyte Plasma Membranes*

Blockade of adenosine receptors by 3-isobutyl-1-methylxanthine or deg radation of endogenous adenosine with adenosine deam inase increased the phosphatidylcholine concentration in isolated rat adipocyte plasma mem branes, an effect which was suppressed by the phosphatidylethanol-am ine m ethy ltransferase inh ib ito r, S-adenosyl-L-hom ocysteine, and reversed by the adenosine analogue, N6-(L-phenylisopropyl)-adenosine. For example, the addition of N6-(L-phenylisopropyl)-adenosine to adeno sine deam inase pretreated plasma membranes rapidly lowered the concen tration of phosphatidylcholine by 171 nmol/mg at 30 seconds compared to control. Insulin-induced stimulation of phospholipid m éthylation in m em branes treated with 3-isobutyl-1-methylxanthine or adenosine deaminase was achieved only after the addition of N6-(L-phenylisopropyl)-adenosine. These results suggest that adenosine receptor occupancy inhibits phospho lipid méthylation, is required for insulin stimulation of phospholipid m éth ylation, and may perhaps activate a phosphatidylcholine-specific phospho-lipase C or phospholipase D