A Quantitative Estimation of the Pathways of Glucose Metabolism in Rat Adipose Tissue in Vitro

Abstract

Numerous workers, on incubating adipose tissue with specifically labeled glucoses, have observed much greater COz yields from carbon 1 than from carbon 6 of glucose, and greater yields of lipid from carbon 6 than from carbon 1.This has been interpreted as evidence for an extensive contribution of the pentose cycle to glucose metabolism.Winegrad and Renold (1) estimated that 52 to 55 %, and Cahill, LeBoeuf, and Renold (2)) 43 '% of glucose metabolism occurs via this pathway in rat epididymal adipose tissue in the absence and presence of insulin.These estimations depend upon the assumption that isomerization of glucose g-phosphate to fructose 6-phosphate is irreversible, i.e. that there is no recycling (3,4), an unlikely occurrence in view of the relatively high activity of phosphohexose isomerase in adipose tissue (5).Wood and Katz (3, 4) developed methods for the estimation of pathway contributions assuming complete recycling.Katz (6) reviewed the published work on 1% yields from labeled sugars by adipose tissue and concluded that the contribution of the pentose cycle had been previously over estimated.Experimental results obtained with glucose-and fructose-land -6-14C, and presented below, indicate that under certain conditions in adipose tissue isotopic equilibration of fructose 6-phosphate with glucose B-phosphate is extensive but incomplete.This observation has in part led to the development of methods for the estimation of pathway contributions in a system requiring no assumption as to the extent of isotopic equilibration.These methods have been detailed in the preceding paper (7).They are now applied to an evaluation of the metabolism of glucose in adipose tissue in vitro in the absence and presence of insulin. EXPERIMENTAL PROCEDUREProcedure-Male Wistar rats weighing 200 to 290 g were maintained on a standard stock diet ad lib&m.They were killed by stunning followed by decapitation and exsanguination.The fat pad adhering to each epididymis was removed, trimmed of thick portions, and segmented.Approximately 1 g of tissue composed of segments from several rats was added to each of several Erlenmeyer flasks.Each flask contained 5 ml of a modified Krebs-* Supported by Grants A-4411 to B.