Plasma Insulin Assay and Adipose Tissue Metabolism

Abstract

The in vitro response of epididymal adipose tissue to insulin is conditioned by the type of diet fed. The epididymal fat pad of the rat responded to the feeding of a fat-free diet by: 1) an increased endogenous metabolism, as reflected by a 50 % increase in oxygen consumption, 2) an increased capacity for fatty acid synthesis, as reflected by a doubling of the net excess CO₂ production in the presence of excess insulin, and 3) the same sensitivity to minimal quantities of insulin (25 μU), but a much greater sensitivity to larger amounts of insulin. Approximately 10 times as much insulin was required to give a maximal in vitro insulin response when the rats were fed chow. By using the epididymal fat pad from rats fed a fatfree diet for 7 days and weighing 160–180 g, the procedure for assaying insulin was greatly improved: the magnitude of the CO₂ measurement was doubled, the response was more uniform, and the accuracy was increased by the smaller amount of insulin which was required to give a maximal response. It seems probable that any insulin assay which employs adipose tissue would be improved by the use of rats fed a fat-free diet. Serum levels of insulinlike activity in humans are reported both before and after glucose administration. The average fasting level of insulinlike activity was 207 μU/ml and this increased to 329 μU/ml after glucose administration.