Comparative Effects of Insulin on Adipose Tissue Segments and Isolated Fat Cells of Rat and Man*

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Abstract
In vitro metabolism of glucose-1-14C by adipose tissue into 14CO2 and total 14C lipids in rat and man was compared employing both adipose tissue segments and isolated fat cells prepared from the same donor. In the rat, the basal glucose metabolism and response to insulin decreased with increasing body weight for both adipose tissue segments and isolated cells. Regardless of age, the isolated cells exhibited a persistently higher metabolic activity. Of the parameters selected, conversion to CO2 was more pronounced than that to lipid. In contrast to the rat, in man adipose tissue segments were more active than isolated cells. In four subjects, the effect of 6, 50, and 400 μU/ml of insulin was analyzed on conversion of glucose-1-carbon to CO2, long chain fatty acids, and glycerides by adipose tissue segments only. In 17 subjects, glucose oxidation and lipid synthesis by adipose tissue segments and isolated fat cells were measured and showed a definite response to physiological doses of crystalline pork insulin. There was, however, an age dependency, and consistent effects were obtained with 6 μU/ml in children and 50 μU/ml in adults. The responsiveness of human adipose tissue to exogenous insulin in concentrations comparable to those detected in blood reemphasizes the importance of adipose tissue as a major site for fatty acid synthesis.