Studies on Rat Adipose Tissue in Vitro

Abstract

In the preceding report from this laboratory (1) insulin added in vitro has been shown to produce a striking and consistent stimulation of glucose uptake, glucose oxidation, and glucose conversion to lipide by the epididymal fat pad of the male rat. Furthermore, although insulin added in vitro did not stimulate lipogenesis from pyruvate or acetate when glucose was absent from the medium, the hormone consistently stimulated lipogenesis from t,hese substrates when glucose was also present. These observations suggested that the stimulation of lipogenesis produced by insulin in isolated adipose tissue is dependent upon the occurence of increased glucose metabolism as a result of insulin action. Milstein (2) has studied the oxidation of glucose-l-W and glucose-6-04 bv this same tissue in vitro. From the relative amounts of C1402 produced from these differentially labeled glucose molecules he concluded that both a glycolytic (EmdenMeyerhof) and a nonglycolytic pathway for glucose utilization operate in rat adipose tissue. From observations on tissues removed from alloxan-diabetic and insulin-treated, allosan-diabetic rats, Milstein also concluded that in the diabetic state the oxidation of glucose by the nonglycolytic pathway is selectively depressed, and that insulin treatment restores and enhances both the total oxidative capacity of the tissue and the relative importance of the nonglycolytic pathway. The observation of Langdon (3, 4) that the reduction of crotony1 coenzyme A is dependent on triphosphopyridine nucleotide has stimulated interest in the phosphogluconate oxidative pathway as a possible source of reduced triphosphopyridine nucleotide for the synthesis of fatty acids. In liver slices, Felts et al. (5) observed that the increased lipogenesis from glucose, which resulted from insulin injection, occurred via the phosphogluconate oxidative pathway as well as via glycolysis and indeed that the phosphogluconate oxidative pathway appeared to be stimulated more markedly than the Emden-Meyerhof pathway. In rat mammary gland slices, Abraham et al. (6) observed that insulin added in vitro stimulated C’*O2 production from glucose-l-C14, whereas the yield of C1402 from glucose-6-C14 was decreased in most cases. However, in this tissue, the proportions of fatty acids synthesized from acetyl coenzyme A derived from glucose via the Emden-Meyerhof and via alternate pathways were not