Effects of aging and exercise on insulin action in rat adipocytes are correlated with changes in fat cell volume

The effects of exercise on insulin action were evaluated in epididymal adipocytes from 9-mo and 26-mo-old rats. Exercised animals were given voluntary access to running wheels at 6 mo of age. Comparisons were made among these animals, freely eating sedentary rats, and sedentary animals maintained at body weights comparable to those of the runners. When expressed as percentage changes, the responses to insulin, in terms of stimulation of both [U-14C]glucose oxidation and 14C-labeled lipid accumulation, were largest in fat cells from the runners, followed by the paired-weight adipocytes, and the cells from the freely eating sedentary animals. However, the absolute changes in the rates of glucose oxidation and 14C-labeled lipid synthesis produced by insulin were comparable among the different groups. Basal rates of oxidation and 14C-labeled lipid accumulation were directly correlated with average cell size, independent of the treatment group, whereas the percentage increases in these processes produced by insulin were inversely correlated with cell size. Little, if any, effect of age or exercise was observed that could not be attributed to the average fat cell volume.