P. R. Johnson

Spontaneous activity and adipose cellularity in the genetically obese Zucker rat (fafa).

The development of spontaneous activity was studied in obese and lean Zucker rats. Rats were given access to activity wheels for 3 hr/day before weaning and for 24 hr/day after weaning. Zucker obese rats are less active than lean rats. This decreased activity occurs at weaning and follows the onset of hyperphagia and obesity. At 8 wk of age exercised lean and obese rats have less total fat and fewer adipocytes than their appropriate controls. Adipose cell size is decreased only in exercised lean rats. When rats are exercised until 8 wk of age and then confined until 6 mo of age, body weight and fat is elevated in these formerly active rats compared to control rats. Adipose cell number is permanently decreased only in formerly active lean rats. Exercise has no long-term effect in decreasing cell number in obese rats.

Carbohydrate metabolism in lean and obese Zucker rats.

Carbohydrate metabolism was evaluated in lean and obese Zucker rats. Plasma glucose concentration, renal and hepatic gluconeogenesis, and hepatic glycogen content and rates of synthesis were investigated in 2-mo and 8-mo-old animals. Mild hyperglycemia was observed in obese Zucker rats compared to lean rats and was more pronounced in males than in females. Rates of glucose disappearance were normal in both female and male rats, although there was a trend toward decreased clearance in the male. Total organ hepatic and kidney PEPCK activity and kidney glucose production were elevated in obese compared to lean rats. Total organ hepatic glycogen levels and rates of glycogen synthesis were increased significantly in obese compared to lean, the increase being greater in males than females. The mild hyperglycemia present in obese Zucker rats is not associated with delayed disappearance of intravenously administered glucose, but may be due to the increased production of glucose by whole kidney and liver.

Noncompensation of adipose mass in partially lipectomized mice and rats

The effects of surgical ablation of adipose tissue were studied in normal mice and rats. It was found that: 1) restortation of adipose tissue does not occur locally in epididymal fat pads of young rats. 2) Bilateral epididymal fat pad removal in mice disrupts the testes and causes the other fat depots to accumulate excess lipid, but these effects are not sustained; After a sufficient recovery period, testes appear normal and no excess lipid is found in the remaining depots; 3) Temporary enlargement of remaining depots is probably due specifically to epididymal pad removal. It does not occur in response to inguinal depot removal, nor in response to disruption of the testes alone; 4)The quantity of lipid stored by a rapidly growing mouse depends on the number of intact depots in the mouse. These results suggest that surgical removal of fat does not lead to compensatory growth of fat. Autoregulation of adipose tissue mass, if it occurs, most likely operates through detection of adipocyte size rather than adipocyte number or total fat mass.

Adipose tissue regeneration in adult rats.

Summary Adipose tissue in subcutaneous, but not epididymal, depots of adult rats is restored following lipectomy. The restoration process proceeds in response to lipectomy alone but may be somewhat accelerated by a diet which normally promotes large increases in adipocyte number. That adult rats can both regenerate subcutaneous adipose tissue following lipectomy and increase adipocyte number when fed certain diets suggests that in at least some depots of the adult rat adipocyte precursor cells can proliferate and differentiate, and that some element of the proliferation/differentiation sequence is normally inhibited. Whether these same processes occur in man is unknown.

Relationship of Age and Cellularity to Metabolic Activity in C57B Mice

Summary Our results indicate that isolated adipose cells from older mice convert glucose to carbon dioxide as rapidly as cells from young mice. Therefore, a slowed basal glucose conversion rate in isolated cell preparations is not exclusively age-related, but depends on the number of cells in the aliquot. Our data do not substantiate an age difference in rate of glucose conversion to CO2 per cell by adipocytes in C57B mice. Decreased insulin response in adipocytes may represent cell size changes rather than direct age-related changes. The authors wish to express their deep appreciation to Dr. Anita Zorzoli at Vassar College for providing the mice for this study. We also wish to thank Mrs. Kirsten Glasser and Miss Florence Oetjen for their excellent technical assistance.

Symposium: obese-hyperglycaemic mice

Introductory Remarks. Jean Mayer, Harvard School of Public Health, Boston Massachusetts. I am very grateful for the opportunity to open this Symposium on the Obese-Hyperglycaemie Mouse. I have devoted a very large par t of my professional life in the last twenty years to trying to understand the development of this syndrome; for many of these years, this was a lonely task in that , while many of my colleagues throughout the world were conversant with our work and were interested, the fact that so few of the animals were produced -as well perhaps as the lack of understanding of the broad implications of the study of this complex condition -meant that very little work on the subject was done outside of my laboratory. I am glad to say tha t this is no longer the case -in part, because of the continuing interest shown by some of my early collaborators -several of whom are here today -who have continued working on the biochemistry of the syndrome with a degree of competence which I could not match. Let me suggest some reasons why I believed in the early fifties tha t work in this syndrome would prove highly rewarding -as indeed it has. First, the introduction into the experimental l i terature -long dominated, almost to the point of excluding any other, by the model of hypothalamic obesity -of another type of experimental obesity with very different metabolic and behavioural characteristics, brought a sense of sophistication to the study of experimental 0besities which it had lacked for a long time. Secondly the study of this syndrome brought back into prominence the r61e of genetic rasters in obesity, in contrast to the emphasis in the medical l i terature on psychological hyperphagia. Thirdly these investigations have demonstrated tha t inactivi ty can be as important as hyperphagia in obesity. Last ly the extremely complicated metabolic and endocrine abnormalities in the obese hyperglycaemic mice has suggested that these factors may be implicated in obesity. The study of hypothalamic obesity and hereditary obesity in mice permit ted us to develop the concept of regulatory obesities (where the pmmary lesions are in the central nervous mechanisms regulating food intake) and metabolic obesities (where the metabolic lesions in turn cause hyperphagia). In genetically obese mice the simple mode of genetic transmission suggested tha t one basic chemical error could cause the diversity of the observed abnormalities. This has challenged us to come up with a unitary hypothesis. Our experimental results lead us to believe tha t Organization Section 9 Abstracts 49 the primary lesion could be the lack of repressor to glycerokinase normally present in white adipose tissue. The resultant accumulation of three carbon fragments inhibits glucose entry into the adipose tissue. The consequent elevation of blood glucose causes the observed progressive hyperplasia of the beta cells of the islets of Langerhans. Increased circulating insulin and glucose, in turn, cause the high rate in hepatic lipogenesis observed in these animals. Reaction of the ventromedial hypothalamus to the increased circulating insulin causes the formation of local antibodies, decreased glucose uptake and hyperphagia. Whether this hypothesis survives the test of time remains to be seen. Comparison of the Obesity Syndromes of Obese (ob/ob) and Diabetic (db/db) Mice*. D.L. Coleman, K. 1 ). Hummel. The Jackson Laboratory, Bar Harbor, Maine USA. The obesity condition caused by the muta t ion obese (oh, Linkage Group XI) is phenotypically similar to tha t caused by the muta t ion diabetes (db, Linkage Group VIII) . Both conditions are characterized by hyperphagia, obesity, hyperglycaemia, and hyperinsulinaemia associated with characteristic pancreatic lesions. Diabetes causes the more severe condition; marked hyperglyeaemia appears as early as 6 weeks and is sustained unt i l death occurs around 5 to 7 months. In contrast, obese mice have a less marked and usually transient hyperglycaemia and continue gaining weight during a nearly normal lifespan. Histological examination of the islets of Langerhans suggests that the increased insulin requirement in diabetic mice leads to fl cell overaetivity to the point of exhaustion associated with compensating neogenesis of fl ceils which become exhausted in time, unt i l the demand for insulin exceeds the synthetic or regenerative capacity of the islet tissue. In contrast, obese mice respond to a similar initial sequence of events by marked islet hypertrophy and fl cell hyperplasia and by these mechanisms increase the insulin supply sufficiently to maintain near normal blood sugar concentrations and prevent premature death. Recent studies with diabetes on different inbred backgrounds suggest tha t the different pancreatic responses in these two obesity states may be related to the action of Lmknown factors associated with the different genetic backgrounds. Thus diabetes on the C57BL/6J background (rather than the C57BL/Ks strain of origin) is phenotypieally indistinguishable from obese on the C57BL/6J background, as judged by lifespan, rate of weight gain, islet histopathology, and blood levels of insulin and glucose. Since two genes are known to be responsible for these two conditions, the implication is tha t the changes observed in all of these parameters are secondary to the pr imary defect. Having these two mutat ions on the same genetic background not only permits more detailed comparisons of the developing syndromes bu t makes feasible studies such as parabiosis requiring histocompatible stocks. Parabiosis of obese mice in combination with normal and diabetic mice indicates tha t the primary lesion in diabetic mice involves a nonfunctional satiety centre whereas in obese mice this centre functions normally and the primary lesion must be sought for elsewhere. * Supported in part by l~esearch Grant ~ AM 14461 from the National Ins t i tu te of Arthritis and Metabolic Disease. The Obese-Hyperglycaemic Syndrome in Young obob-Mice. Sighild Westman-Naeser. Histological Department, University of Uppsala, Uppsala, Sweden. Studies on the development of the obese-hyperglyeaemic syndrome in obob-mice showed significantly higher body weights in the homozygous mice at an age of 26 days compared with the lean litter mates. At this time it was also possible to identify the obob-mice by the visible obesity. A few days after the overweight became manifest, the serum concentration of glucose was raised above normal. In 3 months old obese mice the highest mean serum glucose concentration was noted. The earliest metabolic abnormali ty so far demonstrated in the honlozygous animals is the increased insulin resistance. In 19--21 days old homozygous animals the mean serum insulin concentration was significantly raised. When 27 days old the obob-mice displayed a greater tolerance towards exogenous insulin than the lean animals. At this early age, i.p. administration of glucose resulted in higher serum insulin levels in the obob-mice compared with lean controls, 109 ng/ml and 11 ng/ml respectively. The muscle cells of obob-mice have been reported to be a major site of insulin resistance and the development of the muscular resistance is currently being studied. Histological sections of muscle from obob-miee did not show signs of degeneration and the proportion of A, B and C fibres seemed to be the same as in lean controls. The effect of insulin was studied in vitro on diaphragms taken from 4 weeks or 4 months old animals, starved 18 h. A significant effect of insulin was re~orded on diaphragms from both types of mice at both ages. When diaphragms from young obese and lean animals were incubated in serum obtained from fed obob-animals, the high levels of immunologically measurable serum insulin lacked effect in vitro on the muscles. These data were interpreted to support the view that an inhibitor or inhibitors of insulin action might be present in the serum from obob-mice. Preliminary data obtained by Dr. P. Naeser at our laboratory suggest, that adrenal cortical steroids contribute to the apparent insulin resistance in obob-miee. Insulin Sensitivity of Adipose Tissue of Various Types of Genetically Obese Mice. Terence T. T. Yen, Jean A. Steinmetz. Biological Research Division, The Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana, USA. In vitro glucose metabolism in the cpididymal adipose tissue of obese (ob/ob), diabetic (db/db), and viable yellow (Avy/a) mice was generally depressed on a tissue weight basis. Furthermore, adipose tissue of oh~oh, db/db and Avy/a mice did not respond to insulin as measured by either glucose oxidation and incorporation into lipid or by inhibition of epinephrineand theophyllinestimulated lipolysis. This suggests that the insulin resistance of their adipose tissue is a general phenomenon and not glucose specific. Anti-insulin antiserum had no effect on the resistance to insulin. This indicates that if the tissue is saturated with insulin due to the high circulating insulin level in these mice, the excess insulin does not account for the insulin resistance. The tissues tha t did not respond to insulin did not inactivate insulin to a degree that could account for their lack of sensitivity. Starvation of ob/ob and db/db mice up to 72 h reduced their body weights significantly but did not improve the sensitivity to insulin of their adipose tissue. In contrast, starving Avy/a mice 72 h significantly improved the sensitivity to insulin of their adipose tissue. These observations lead us to conclude tha t : 1). Such peripheral insulin resistance is a phenomenon secondary to obesity. 2). Factor(s) intrinsic to the adipose tissue is (are) responsible for the insulin resistance. Diabe~ologia, 37oi. 8 4 50 Organization Section Abstracts Diabetologla Insulin Sensitivity in Muscle and Adipose Tissue of Obese Hyperglycaemic Mice. R.R. Abraham*, Anne Beloff-Chain. Department of Biochemistry, Imperial