Effects of Epinephrine, Insulin, and Corticotrophin on the Metabolism of Rat Adipose Tissue

Abstract

It has been shown that the feeding of cholesterol to rabbits causes adipose tissue to disappear from the animal despite the fact that there has been no weight loss (1,2). In an effort to understand this phenomenon, studies have been carried out in vitro on rat adipose tissue. Rat epididymal adipose tissue was chosen because of the reports of Winegrad et al. (3), Cahill et al. (4), Gordon et al. (5), White et al. (6), and Ball et al. (7), indicating that this adipose tissue is consistently responsive to various hormones such as insulin, epinephrine, and corticotrophin. These three hormones have been shown to promote the oxidation of glucose by adipose tissue. Epinephrine and corticotrophin also promote lipolysis in adipose tissue. Previous studies have indicated that cholesterol will block the uptake of fatty acids by adipose tissue as well as glucose (8). To understand these effects of cholesterol, studies on the normal metabolism of adipose tissue were begun. These studies are the subject of this report. Previous studies by Milstein (9), Winegrad et al. (3), and Cahill et al. (4), have indicated that in adipose tissue, carbon 1 of glucose is more rapidly oxidized to COz than is carbon 6. These results have been interpreted to mean that there is a very active phosphogluconate pathway in this tissue. This is not unexpected since the work of Langdon (lo), Wakil (ll), and others has indicated that reduced triphosphopyridine nucleotide is necessary for the synthesis of fat. The experiments of Winegrad and Renold (3) on rat epididyma1 adipose tissue have indicated previously that insulin stimulates the conversion of radioactive glucose to radioactive COZ and triglyceride. Experiments of Cahill et al. (4) have shown that a large fraction of the isotopic glucose was recoverable in triglyceride as glycerol. Cahill et al. (4) have further shown that glyceride-glycerol is synthesized preferentially from carbon 1 of glucose. However, glycogen was synthesized equally well from carbon 1 and from carbon 6 of glucose. The experiments of White and Engel (6) have also demonstrated a rise in tissue fatty acids upon the addition of epinephrine or corticotrophin in the absence of medium albumin. The experiments herein reported are attempts to correlate the diverse effects of hormones on the carbohydrate and fat metabolism of rat adipose tissue.