Regulation of the Transport and Metabolism of Amino Acids in Isolated Fat Cells

We investigated the effects of insulin on ammo acid transport, protein synthesis, and protein breakdown in fat cells isolated from rat epididymal adipose tissue.Insulin stimulated the accumulation of 'F-labeled proteins in fat cells incubated with l*C-labeled L-leucine, Lalanine, glycine, and r.-serine.With L-leucine-l-14C as a model amino acid, we observed that the effects of insulin did not require the addition of glucose or other carbohydrate to the incubation medium, and were produced by insulin concentrations comparable to those required to accelerate glucose transport.The accumulation of 14C-labeled proteins was inhibited by adrenocorticotropic hormone (ACTH) and theophylline at concentrations known to increase adenosine 3',5'-monophosphate (cyclic AMP) in isolated fat cells.The inhibitory effects of ACTH were reversed by insulin, while the inhibitory effects of theophylline and ACTH plus theophylline were only partially reversed.High concentrations of N6, 2l-O-dibutyryl cyclic AMP duplicated the effects of ACTH and theophylline.Insulin had only slight effects on the inhibition produced by dibutyryl cyclic AMP.Ammo acid transport was assessed by measuring the intracellular accumulation of 14C-labeled Lu-aminoisobutyric acid (AIB), 1-aminocyclopentane-1-carboxylic acid, and certain natural amino acids.The labeled ammo acids achieved intracellular to extracellular concentration ratios which were similar to the concentration ratios of the natural ammo acids measured in fresh adipose tissue.AIB transport was Na+dependent and temperature-sensitive and inhibited by a 20: 1 molar excess of L-leucine, but was glucose-independent and not affected by ACTH, theophylline, or N6, 2l-O-dibutyryl cyclic AMP.Insulin did not change the initial rate of AIB uptake, the initial rate of AIB ei%.rx, or the equilibrium concentration ratios of AIB and cycloleucine.When protein synthesis was blocked by lo-' M puromycin, insulin had a slight inhibitory effect on protein breakdown.