Sequential Changes in Adipose Tissue Metabolism in Alloxan-Diabetic Rats

Abstract

Alloxan-diabetic rats were maintained for two weeks on Protamine Zinc Insulin (4 U./day), and changes in adipose tissue metabolism were followed over a period of ninetysix hours after termination of insulin treatment. Blood glucose and plasma FFA showed parallel increases from sixteen to ninety-six hours after the last insulin injection. Blood ketones were elevated at seventy-two and ninety-six hours and lipid accumulation in the liver was also evident at these times. FFA release from isolated fat pads was elevated at forty-eight hours and continued to rise until ninety-six hours. Insulin (0.5 mU./ml.) added in vitro moderated but did not abolish this release. Glucose uptake by adipose tissue was not seriously impaired until ninetysix hours after termination of insulin treatment, and the isolated tissue remained insulin responsive with respect to glucose uptake. Oxidation of C-1 of glucose was decreased only seventy-two hours and ninety-six hours after insulin withdrawal, oxidation of C-6 was not markedly changed. Incorporation of glucose carbons (both C-1 and C-6) into fatty acids by isolated adipose tissue was decreased seventytwo to ninety-six hours after termination of insulin. Addition of insulin in vitro stimulated lipogenesis from glucose, but the insulin effect was reduced in tissues from alloxandiabetic rats ninety-six hours after the last insulin treatment. No marked change in incorporation of either C-1 or C-6 of glucose into glyceride-glycerol was observed under the conditions of these experiments. Addition of insulin in vitro resulted in an increased recovery of glucose carbons in the glyceride-glycerol fraction. Metabolic changes during acute insulin deficiency have been studied in pancreatectomized rats, rats made diabetic by anti-insulin serum, and following withdrawal of insulin from alloxan-diabetic rats which had previously been controlled by injection of the hormone. Acute changes following pancreatectomy are complicated by the necessary surgical stress accompanying removal of the organ as well as nutritional defects resulting from loss of the digestive enzymes. Rats made diabetic with anti-insulin serum do not eat and in long-term studies effects of starvation are superimposed upon those of insulin insufficiency. Previous studies from this laboratory have indicated that in acute insulin deficiency induced by anti-insulin serum rapid mobilization of fatty acids from adipose tissue occurs and results in an increase in plasma free fatty acids which parallels the rise in blood sugar. Insulin deficiency of one to three hours' duration produced by injection of antibodies to insulin produces an accumulation of free fatty acids in adipose tissue and this precedes any impairment of glucose uptake or glycerideglycerol formation by the tissue in vitro. Insulin has been shown to have a direct effect on lipolysis in adipose tissue independent of its effect on glucose metabolism. In this case insulin appears to prevent the activation of lipolysis by catecholamines and other lipolytic hormones. However, in rats which have been alloxan diabetic for several weeks, glucose uptake by adipose tissue is severely impaired. Free fatty acid release from such tissue is thought to be related to reduced re-esterification which results from a lack of α-glycerolphosphate which in turn is related to reduced glucose utilization. It would appear then that in longstanding diabetes re-esterification of fatty acids rather than increased lipolysis is the major defect in adipose tissue leading to increased free fatty acid release. In order to study changes in adipose tissue metabolism which developed during insulin deficiency, alloxan-diabetic rats were maintained on daily insulin injections, which were then terminated and the resulting metabolic changes studied during the next ninety-six hours. The results of these experiments are reported in the present communication.