Effect of insulin on the transport and metabolism of sorbitol by incubated rat epipidymal adipose tissue

Abstract

1. 1. The effect of insulin on the transport and metabolism of sorbitol by incubated rat adipose tissue was investigated in epipidymal fat pads and isolated adipose tissue cells. 2. 2. The production of 14CO 2 from [ 1C]sorbitol was stimulated by insulin when adipose tissue was incubated both in the presence and in the absence of gluocose. The stimulatory effect of insulin was also observed in isolated adipose tissue cells and was not therefore dependent on the presence of reticulo-endothelial or other non-adipose tissue cells. 3. 3. The purity of the [ 14C]sorbitol was confirmed by demonstrating that: 14CO 2 production was not altered by the addition of 4 mM glucose; 14CO 2 production was linear for 6 h; 14CO 2 production was quantitatively diminished by dilution with non-radioactive sorbitol; 14CO 2 production was not reduced by pretreatment of the radioactive sorbitol with glucose oxidase; 8.6% of the radioactivity in the incubation vessel was recovered as 14CO 2, 14C-labeled lipids or 14C-labeled glycogen under conditions which favored maximal sorbitol utilization yet the [ 14C]sorbitol preparation was at least 96% pure by paper radiochromatography in two separate systems. 4. 4. The oxidation of 5 mM sorbitol was inhibited by o.2 mM phloretin. Under these conditions, phloretin has been shown to be an inhibitor of the carrier system involved in sugar transport in adipose tissue but not an inhibitor of the metabolism of intracellular sugars. 5. 5. The oxidation of 5 mM sorbitol was inhibited by 40, 80, and 120 mM 3- O-methyl glucose, a sugar analogue which is not phosphorylated, but which is transported by means of the same carrier system as that involved in glucose transport. 6. 6. The distribution of sorbitol relative to the total tissue water was not altered by the presence of insulin. 7. 7. The data were interpreted in support of the view that facilitation of the transport of sorbitol across the cell membrane and into the adipose tissue cell is the rate limiting step in the metabolism of sorbitol and, therefore, the probable site of action of insulin.