Developmental changes in the fatty (fafa) rat: evidence for defective thermogenesis preceding the hyperlipogenesis and hyperinsulinaemia

Hyperphagia: a necessary precondition to obesity?

Weight-gain is generally attributed to a caloric imbalance resulting from hyperphagia. However, this attribution is often made without observing caloric intakes during the initial accumulation of fat stores. Instead, this conclusion is drawn because many obese organisms overeat, and overconsumption is sufficient to cause weight-gain. The literature is reviewed, specifically those studies on the onset of caloric overconsumption relative to weight-gain and the accumulation of fat during food restriction, which suggests that hyperphagia is not necessary for animals prone to obesity to become fatter and heavier. It is observed that animals with genetically- and surgically-induced obesity often become fat prior to hyperphagia and continue to gain even if food restricted to subnormal levels. Overconsumption does speed this gain.

The development of obesity in animals: the role of genetic susceptibility

There are a number of animals in which obesity is genetically determined. In some the inheritance is polygenic while in others it is by a single-gene mutation. In the most widely studied single-gene mutants--the obese (ob/ob) mouse, the diabetic-obese (db/db) mouse and the Zucker (fa/fa) rat-obesity is very substantial and is initiated before the animals are weaned. Although hyperphagia is a feature of all the major obese mutants, it is not a prerequisite for the development of obesity. The initiation of the disorder during the suckling period takes place on a normal energy intake, and excess rates of energy deposition will still continue after weaning if the obese mutants are pair-fed to the ad libitum energy intake of lean siblings. The ability to become obese without hyperphagia indicates that one or more components of energy expenditure must be reduced in the obese mutants. Studies on the ob/ob mouse have demonstrated that a reduction in thermogenesis in BAT is the main way by which this is achieved. The reduction in energy expenditure in BAT is due primarily to a low activity of the sympathetic innervation to the tissue. Once hyperphagia is established, apparently as a secondary feature of the obese syndrome, the development of obesity is accelerated, the obese mutants having an impairment in the dietary stimulation of BAT thermogenesis. Studies on different types of obese animal suggest that an inability to respond to dietary stimuli is a general feature of obesity. The final syndrome presented by genetically obese animals is of considerable metabolic and endocrinological complexity. However, it is now possible to begin to integrate some of the endocrinological abnormalities within the energy balance framework, centred on BAT, that has been developed over the past few years. This is particularly evident in the case of adrenal function in the Zucker rat; adrenalectomy of this mutant has important effects on energy balance and in the normalization of the thermogenic activity of BAT. The sensitivity of BAT to insulin is also emerging as a possible factor of importance in the modulation of thermogenesis in obese animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Body temperature of huddling newborn Zucker rats

Daily mean core temperature (Tc) in huddling Zucker rat pups reared at 25 degrees C revealed the onset of a significantly lower Tc in fa/fa pups on day 6. The difference between any pup's Tc and the mean Tc of that half of the litter with the highest Tc on any given day was an early marker for genotype. Calculating the mean difference for any 2 consecutive days from day 6 onwards gave values for each fa/fa pup that were outside the 99% confidence limits for the individual observations in the Fa/- pups.

Defective brown adipose oxygen consumption in obese Zucker rats

The thermogenic capacity and morphologic characteristics of interscapular brown adipose tissue (IBAT) were assessed in 3- to 4-mo-old male, lean and obese Zucker rats. Pads from obese rats were threefold heavier and contained similar numbers of cells but an average of 50% fewer multilocular cells than pads from lean rats and 40% less mitochondrial protein per pad. The maximal number of beta-adrenoreceptor binding sites, as assessed by [125I]iodocyanopindolol binding to isolated brown adipocytes from obese rats was 50% of that in lean rats on a per cell and per pad basis. Basal and norepinephrine (NE)-stimulated in vitro oxygen consumption in isolated brown adipocytes from lean rats correlated directly with the proportion of mutilocular cells present. This correlation was not seen in cells from obese rats that had a 50% decrease in their basal respiratory rates and could not be further stimulated by excess NE or fatty acid. Electron micrographs of IBAT from obese rats revealed distorted mitochondrial shapes and cristae patterns and the presence of numerous inclusion bodies. Because NE-stimulated lipolysis had previously been shown to be normal in the obese Zucker rat, these data suggest that defective BAT thermogenesis in the obese rat is due to an inability of mitochondria to utilize free fatty acids for the production of enhanced oxygen consumption.

Evidence for decreased GDP binding to brown-adipose-tissue mitochondria of obese Zucker (fa/fa) rats in the very first days of life

GDP binding to brown-adipose-tissue mitochondria of obese Zucker-rat (fa/fa) pups aged 2-14 days was significantly less than in lean control rats. Scatchard analysis in 10-day-old pups suggests that there was a large decrease in GDP-binding sites. However, a significant increase in fat content in brown adipose tissue of 2-day-old pre-obese pups raised the question of the sequential order and causal relationship between these two derangements.

Effect of age and gene dosage on brown adipose tissue of Zucker obese fa/fa rats

Experiments were performed to investigate the effect of age and genotype on brown adipose tissue thermogenesis in Zucker rats. Specific [3H]GDP binding to interscapular brown adipose tissue mitochondria ( IBATM ) was reduced in 14-day-old preobese fa/fa rats and remained lower after weaning. A gene-dosage effect of the recessive fa gene was observed in 8- to 10-wk-old rats in both IBATM [3H]GDP binding and the thermic effect of a balanced meal (50 kJ Complan ), measured by indirect calorimetry. In each case the heterozygote (Fa/fa) group had a value intermediate between those of obese (fa/fa) and the homozygous lean (Fa/Fa) groups. Norepinephrine increased IBATM [3H]GDP binding to similar levels in lean (Fa/fa) and obese (fa/fa) rats and induced similar increases in oxygen consumption in Fa/Fa, Fa/fa, and fa/fa rats. It is concluded that the impaired, diet-related brown adipose tissue thermogenesis is closely related to the primary gene defect in the obese rat. This defect may result from misregulation of the autonomic nervous system.

The relationship between energy expenditure and environmental temperature in congenitally obese and non-obese Zucker rats

The energy expenditure of normal and congenitally obese adult female Zucker rats has been measured by continuous indirect calorimetry for periods of 3-10 days at ambient temperatures varied from 30 to 5 degrees C. Rectal temperatures were also recorded. Exposure to cold caused no ill-effects in normal or obese rats. The rectal temperatures of obese rats were about 1 degree C lower than those of normal rats. The rectal temperatures of normal rats did not change measurably with ambient temperature; in obese rats rectal temperature rose slightly as ambient temperature fell. In normal and obese rats, energy expenditure showed a smooth, steeply sloping, negative relationship to ambient temperature. Energy expenditure per rat was higher in obese than in normal rats at all temperatures. The two slightly curvilinear regressions were nearly 'parallel', with a separation of about 40 kJ/day per rat at the mid-point. This study therefore does not confirm suggestions that obese Zucker rats suffer from a defect in the level of energy expenditure, or in their capacity to increase it when exposed to cold. It is suggested that in both normal and obese rats the level of energy expenditure was determined by thermoregulatory control. The greater heat production of obese rats may have been a response to their lower core temperature. A steady state in which greater heat production is associated with lower core temperature implies lower insulation between body core and surface. This could be due to greater blood flow.

Lipoprotein lipase activity and cellularity in brown and white adipose tissue in Zucker obese rats

The genetically obese adult Zucker rat (fafa) exhibits reduced thermogenesis when stimulated by physiological agents (cold, catecholamines). Recent evidence suggests that this thermogenic defect may be important in the manifestation of the animal's obesity and that it reflects a reduced thermogenic contribution from brown adipose tissue, the major nonshivering thermogenic site in many mammals. The present study describes the effects of the obese genotype on brown (and white) adipocyte size, number, and lipid content and tissue lipoprotein lipase (LPL) activity. In the obese rats, brown fat depots were increased in mass. This increase could be accounted for by brown fat hypertrophy (due primarily to an increase in the amount of triglyceride present in each cell) rather than hyperplasia (there being no increase in the number of brown fat cells). In addition, unlike the situation in white fat, the brown fat from the obese rats did not exhibit higher LPL activity than did the brown fat from their lean littermates. This absence of an increased capacity for triglyceride uptake, coupled with the greater amount of triglyceride per brown adipocyte, is consistent with a reduction of triglyceride oxidation (and, thus, heat production) in the cells from the obese (v the lean) rats.

Metabolic responses to tyramine and cold in young male Sprague-Dawley and Fischer 344 rats

Significant differences in responses to intraperitoneally injected tyramine or cold exposure (-10 degrees C) were observed in young (3-4 mo) male Sprague-Dawley (S-D) and Fischer 344 (F344) rats cold-acclimated (CA) or non-cold-acclimated (non-CA). Non-CA S-D and F344 rats respond to tyramine by increased O2 uptake and elevation of colonic temperature, but, as we had reported previously, the optimal doses of tyramine required was significantly different for these two strains, i.e., 2 mg/kg for F344 vs. 20 mg/kg for S-D. The response to tyramine was significantly potentiated by cold acclimation in both strains of rats. Despite the different dosages of tyramine given to the animals, similar increases in O2 uptakes and colonic temperatures were observed. Differences between these two strains were also evident in their responses to a cold challenge (3 h, -10 degrees C). The ranking for cold tolerance (-10 degrees C) was as follows: CA F344 greater than non-CA F344 greater than CA S-D greater than non-CA S-D. These differences between the two strains suggest that interpretations regarding the response of rats to stressful conditions must be made with caution and appreciation of strain differences.

Sucrose diet induced enzymatic and hormonal responses affecting carbohydrate, lipid and energy metabolism in two species differing in insulin availability: spiny and ob/ob mice

The low-insulin responding spiny mice (Acomys cahirinus), maintained on a 50% sucrose diet vs isocaloric regular diet, responded with an impressive increase in the activity of hepatic enzymes of glycolysis and lipogenesis and in hyperlipidemia. There was no hyperinsulinemia or hyperglycemia and spiny mice did not gain weight on sucrose due to loss of adipose tissue. Serum T3 levels rose 1.8 fold and the activity of the hepatic mitochondrial FAD-glycerol-3-phosphate oxidase became induced 2.6 fold representing the enhancement of multiple, T3-dependent, energy-consuming metabolic cycles. An increased TG lipolysis in adipose tissue was also observed. C57BL/6J ob/ob mice were markedly hyperinsulinemic and gained weight on sucrose almost as much as those on regular diet, without changes in serum glucose or insulin. Serum triglyceride level decreased, whereas liver triglycerides accumulated markedly. The extent of the increase in hepatic enzyme activities related to lipogenesis was much lower both in the ob/ob mice and their lean siblings, than in spiny mice, but the basal enzyme activities in ob/ob mice were remarkably elevated. Serum T3 level was also elevated already on the regular diet and rose only slightly on sucrose. Basal glycerol phosphate oxidase activity in ob/ob mice exceeded that in spiny mice and rose only marginally on sucrose. Adipose tissue lipolysis was not increased. Thus, sucrose diet by enhancing the T3 production appeared to activate protective mechanism against weight gain in normoinsulinemic spiny mice, whereas the full expression of these mechanisms appeared to be precluded by the hyperinsulinemia of ob/ob mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of fatty acid-synthetic capacity in interscapular brown adipose tissue during suckling in genetically obese Zucker rats

The development of the lipogenic capacity in brown adipose tissue was studied in suckling lean (Fa/fa) and obese (fa/fa) Zucker pups aged from 7 to 22 days. In both lean and obese pups, activities of the two key lipogenic enzymes, fatty acid synthetase and acetyl-CoA carboxylase, and of citrate cleavage enzyme rose from the early to the late suckling period. Compared with lean pups, 7-day-old fa/fa pups showed a 35% increase in fat accumulation in interscapular brown adipose tissue and a 25% increase in fatty acid synthetase activity. By 10 days of age, fat deposition, lipogenesis in vivo (assessed by the incorporation of 3H from 3H2O into fatty acids) and fatty acid synthetase activity were 1.5-2-fold higher in pre-obese than in lean pups. Compared with lean pups, the increased lipogenesis in vivo observed in brown adipose tissue of 10-day-old pre-obese pups could not entirely account for the difference in fat deposition observed in this tissue, suggesting that additional mechanisms are at play to explain the increased fat content of this tissue.

The effects of corticosterone, cold exposure and overfeeding with sucrose on brown adipose tissue of obese Zucker rats (fa/fa)

GDP binding to brown-adipose-tissue mitochondria was decreased in obese Zucker rats. Adrenalectomy restored both GDP binding and serum tri-iodothyronine of obese rats to values observed in lean rats. The effects of adrenalectomy on GDP binding and serum tri-iodothyronine were reversed by corticosterone. Decreasing food intake had no effect on brown-adipose-tissue GDP binding in obese rats. Young (5-week-old) obese rats showed a normal increase in brown-adipose-tissue mitochondrial GDP binding after housing at 4 degrees C for 7 days, but this response was attenuated in 10-week-old obese rats. Overfeeding with sucrose increased brown-adipose-tissue thermogenesis in lean, but not in obese, rats. After adrenalectomy, overfeeding with sucrose enhanced brown-adipose-tissue mitochondrial GDP binding in obese rats.

Lipogenesis in primary cultures of adipoblasts derived from genetically obese Zucker rats

Adipocyte precursor cultures prepared from the epididymal fat pads of genetically obese (fa/fa) and lean (Fa/Fa) Zucker rats grow similarly in culture. Addition of enriched medium (EM) containing human serum, insulin, and glucose stimulated lipid filling of the adipocyte precursors in both cultures. However, [3H] H2O incorporation into total lipids, fatty acid synthetase and lipoprotein lipase activities, and cytosolic protein contents are all decreased in the fa/fa compared with the Fa/Fa cultures. Substitution of lean or obese rat serum for human serum in the enriched medium does not alter the decreased lipogenic capacity of the fa/fa adipocyte precursor cultures.

Studies of origins of abnormal sympathetic function in obese Zucker rats

To explain previously described abnormalities of sympathetic function in the genetically obese Zucker rat, the kinetics of norepinephrine (NE) metabolism were examined. Lower stress-induced levels of plasma NE in the obese rat were shown to be due to decreased release of NE because uptake of [3H]NE tracer from plasma was 45% slower in the obese compared with the lean rat. NE levels were 15, 20, and 34% lower in heart, pancreas, and interscapular brown adipose tissue (IBAT), and NE turnover rates (determined by two independent methods) were 39-48 and 43-69% lower in the pancreas and IBAT of obese compared with lean rats. In vivo synthesis of [3H]NE from [3H]dopamine by the enzyme dopamine beta-hydroxylase was 60% lower in IBAT but was equal in hearts of obese compared with lean rats. These results suggest an organ-specific decrease of NE synthesis in the obese Zucker rat, possibly due to decreased dopamine beta-hydroxylase activity in IBAT.

Regulation of hepatic tyrosine aminotransferase in genetically obese rats

The activities of hepatic tyrosine aminotransferase, tryptophan oxygenase and serine dehydratase were increased in obese rats shortly after weaning. Immunotitration experiments showed that the increase in tyrosine aminotransferase activity resulted from an increase in enzyme protein in obese rats. No increase in hepatic tyrosine aminotransferase was observed in suckling pre-obese rats. The post-weaning increase in hepatic tyrosine aminotransferase of obese rats was only observed during the light phase of the diurnal cycle, but was prevented by pair-feeding and by starvation. Tryptophan increased hepatic tyrosine aminotransferase of lean rats to obese levels but had no effect in obese rats until tyrosine aminotransferase levels were reduced by starvation or adrenalectomy. Adrenalectomy abolished the increase in hepatic tyrosine aminotransferase activity in obese rats although serum corticosterone was normal in these animals. Hepatic and brain tyrosine concentrations were decreased in obese rats but normalized after adrenalectomy. The results suggest that the corticosteroid-dependent increase in food and tryptophan intake may be the primary cause of the increased hepatic amino acid catabolism of obese rats.

Hyperinsulinemia of preobese and obese fa/fa rats is partly vagus nerve mediated

In vivo glucose-induced insulin secretion was greater in preweaned preobese 17-day-old Zucker rats than in the corresponding controls. This hypersecretion of insulin was reversed to normal by acute pretreatment with atropine. A short-lived (30 s) electrical stimulation of the vagus nerve preceding a glucose load potentiated the in vivo glucose-induced insulin release in adult animals (6-9 wk) and more so in obese Zucker (fa/fa) than in lean rats. This suggested the existence of enhanced sensitivity and/or responsiveness of the B cells of obese animals to the parasympathetic system. That the parasympathetic tone was increased in adult obese Zucker (fa/fa) rats was corroborated by the observation that acute vagotomy of these animals resulted in a significant decrease in glucose-induced insulin secretion, whereas no such effect was seen in lean rats. Also, perfused pancreases from adult obese (fa/fa) rats oversecreted insulin during a stimulation by arginine when compared with controls, an oversecretion that was restored toward normal by superimposed infusion of atropine. It is concluded that a) the increased insulin secretion of preobese Zucker fa/fa rats is an early abnormality that is mediated by the vagus nerve, and b) increased secretion of insulin in adult obese fa/fa rats continues to be partly vagus-nerve mediated, although a decreased sympathetic tone and other unknown defects could conceivably play a role as well.

Brown adipose tissue in genetically obese (fa/fa) rats: response to cold and diet

Young genetically obese (fatty, fa/fa) rats (7-8 wk old) maintained on a chow diet at 28 degrees C have a relatively normal amount of brown adipose tissue (BAT) (normal protein content, normal noradrenaline content, normal or slightly reduced cytochrome oxidase content, 30% reduction in DNA content) with cells grossly hypertrophied by accumulation of lipid. The binding of purine nucleotides by BAT mitochondria is lower in fa/fa rats than in lean rats, suggesting a lesser thermogenic activation of this tissue. Acute exposure to cold (24 h at 4 degrees C) activates BAT thermogenesis (visible hyperemia, marked increase in mitochondrial binding of purine nucleotides, depletion of noradrenaline content) in fa/fa rats as in lean rats. In contrast, feeding a cafeteria diet to young fa/fa rats fails to activate BAT (no increase in mitochondrial binding of purine nucleotides) as it does in lean rats, and these rats accumulate more extra fat (increase in weight of gonadal white adipose tissue) than do cafeteria diet-fed lean rats. It is concluded that the young fa/fa rat has normal cold-induced nonshivering thermogenesis in BAT but defective diet-induced thermogenesis in BAT and that the consequent reduction in energy expenditure, coupled with hyperphagia, contributes to the development of its obesity. The most probable location for the defect is suggested to be associated with the hypothalamus.

Tissue and whole-body protein synthesis in immature Zucker rats and their relationship to protein deposition

The rates of protein synthesis in skeletal muscle, intestine, liver and in the whole body of immature (18 and 25 days old) lean and obese male Zucker rats were measured. In addition, the rate of deposition of whole-body and skeletal-muscle protein over the period 16-27 days post partum was measured by comparative slaughter and analysis of the composition of the body. At 16 days post partum, lean and obese rats had similar body protein contents, but thereafter the rate of protein deposition in the body and skeletal-muscle mass was decreased in the obese rats. The decrease was particularly marked before 21 days of age, and between 23 and 27 days post partum the fractional rate of protein deposition was the same in lean and obese rats. Of the tissues that were studied, only skeletal muscle had a lower fractional rate of protein synthesis in the obese rats. At 18 days post partum, the decrease in the absolute rate of protein synthesis in skeletal muscle accounted for at least 80% of the decline in protein synthesis in the whole body. After weaning, phenotypic differences in protein synthesis was less marked than at 18 days of age, and skeletal muscle accounted for only 50% of the difference in body protein synthesis between phenotypes. The possibility that a change in the function of the adrenal cortex contributes to differences in protein metabolism between lean and obese Zucker rats is discussed.

Evidence for a high fatty acid synthesis activity in interscapular brown adipose tissue of genetically obese Zucker rats

Obese (fa/fa) rats (30 days old) exhibited a 50% increase in the weight of interscapular brown adipose tissue compared with their lean (Fa/fa) littermates. The tissue weight increase was accounted for by an increased fat content. Lipogenesis in vivo, as assessed by the incorporation of 3H from 3H2O into lipid, was increased 5-fold in brown adipose tissue of obese as compared with lean rats. Accordingly, acetyl-CoA carboxylase, fatty acid synthetase, citrate-cleavage enzyme and malic enzyme in this tissue were 4-8 times more active in obese than in lean rats.

Isoproterenol alters nonshivering thermogenesis in the Zucker obese rat (fafa)

To test nonshivering thermogenic (NST) capacity of lean and obese Zucker rats, 4 doses of isoproterenol, ranging in concentration from 0.25 to 6.0 micrograms/min . kg . 75, were intravenously infused into 16 to 18 week old male rats, and oxygen consumption was continuously monitored. Obese rats had a decreased NST response relative to lean littermates. This lowered thermogenic response of the obese rats cannot be attributed to a decreased mass of brown adipose tissue since both the cervical and interscapular depots from obese rats weighed significantly more than did those from lean rats.

Defective catecholamine metabolism in peripheral organs of genetically obese Zucker rats

Catecholamine (CA) metabolism in peripheral organs of lean and obese, 3-4 and 7-8 month (mo) old male Zucker rats was studied to define further the known abnormalities of peripheral sympatho-adrenal functions in the obese rat. Norepinephrine (NE) levels in all sympathetically innervated organs from obese rats (aorta, heart, pancreas, brown adipose tissue and white adipose tissue) were decreased to 9-55% of those in lean rats at 3-4 mo. NE turnover, measured by inhibition of tyrosine hydroxylase (TH) with a-methyl-p-tyrosine, was also decreased in these same organs. NE levels and turnover were also decreased (by 50-95%) in many, but not all organs assayed of 7-8 mo old obese rats, while there were inconsistent changes in organ dopamine and epinephrine levels at both ages. Decreased NE metabolism was associated with decreased dopamine-beta-hydroxylase (D beta H) activity in every organ assayed from obese rats at 3-4 mo and 7-8 mo of age except in the superior cervical ganglia of 7-8 mo old rats. There were no consistent changes in TH or phenylethanolamine-N-methyltransferase activities. In 3-4 mo old obese rats, decreased D beta H activity was associated with decreased immunoprecipitable enzyme protein in the hearts but not in the adrenal glands, where differences in the affinity for substrate appeared to explain the activity differences. These results suggest that the previously reported defect in stress-induced plasma NE levels in obese rats could be explained by decreased D beta H activity in nerve terminals of their sympathetic nervous system and that, in this case, D beta H may play an important role in the regulation of NE synthesis.

Onset and development of hypertriglyceridemia in the Zucker rat (fa/fa)

Triglyceridemia was studied in genetically obese Zucker rats (fa/fa) and their lean littermates aged 1-8 wk. Hypertriglyceridemia was manifest in the obese from 2 wk onwards. Hepatic triacylglycerol secretion rate (TGSR) measured after administration of Triton WR-1339, was similar in obese and lean pups aged 2 wk. At 4 wk TGSR was twice as high in the obese as in the lean. Lipoprotein lipase (LPL) activity was abnormal in the tissues of obese animals, being either increased in white adipose tissue from 1 wk onwards or decreased in brown adipose tissue and cardiac and skeletal muscle from 2 wk onwards. The simultaneous appearance in the 2-wk-old obese of a decrease in LPL activity of the latter tissues, and hypertriglyceridemia strongly suggests a cause-effect relationship particularly since TGRS is normal. After weaning, LPL capacity of white adipose tissue in the obese, although considerably increased, was apparently not high enough to compensate for both an increased TGSR and a decreased LPL activity in other tissues.

Primary cultures of fetal hepatocytes from the genetically obese Zucker rat: protein synthesis

Primary fetal hepatocytes derived from Zucker rats with expected fa gene frequencies of 0.0 and 0.75 have been established and can be used to detect early effects of the fa gene on hepatocellular metabolism. Paired incubation experiments demonstrate that protein synthesis in 0.75 fa gene cultures is significantly less than in 0.0 fa gene cultures under basal conditions. Insulin stimulates protein synthesis in 0.0 fa gene cultures but has no effect on 0.75 fa gene cultures. Cycloheximide inhibits protein synthesis in both types of culture. NH4Cl inhibits protein synthesis in 0.0 but not in 0.75 fa gene cultures. These data suggest that fetal hepatocytes bearing the fa gene have in vitro a generally sluggish anabolic capacity and a blunted capacity to respond to insulin compared to fetal hepatocytes without the fa gene. These diminished capacities may be expression of a genetic error in lysosomal function.

Metabolic and sympatho-adrenal abnormalities in the obese Zucker rat: effect of chronic phenoxybenzamine treatment

The obese Zucker rat manifests a number of physiologic and metabolic abnormalities which are controlled or modulated by the sympatho-adrenal system. The interrelationship of these was examined by subjecting 3-4 month old male, homozygous lean and obese Zucker rats to various stresses which are known to activate the sympatho-adrenal system, and by chronic (16-19 days) phenoxybenzamine (PBZ) treatment to block alpha-adrenergic receptors. Both obese and lean PBZ treated rats gained only 1% and 10% of the body weight of their respective control rats during the treatment period, while only the lean rats had a significant reduction (20%) in food intake. Control obese rats failed to maintain rectal temperature after 4 hr at 7 degrees C and their relative output of plasma catecholamines (CA) to cold stress, as measured from indwelling atrial cannulae, was decreased. PBZ treatment did not alter this rectal temperature response although it was associated with increased baseline norepinephrine levels (at ambient temperature 21-22 degrees C) and relative output of CA in the obese rats, suggesting tat sympathetic neural activity was increased under these circumstances. No abnormalities of sympatho-adrenal function, as reflected in plasma CA levels, were found in treated or control obese rats immobilization for 1 hr followed by decapitation. Simultaneously obtained baseline plasma glucose levels were similar in untreated lean and obese rats, but insulin and glycerol levels in the obese rat were 1350% and 213% of lean values, respectively. During sequential stresses, the obese rats became markedly hyperglycemic and hyperglycerolemic compared to the lean rats, while insulin levels were decreased more in the obese than lean rats (12-15% versus 34-35% of controls, respectively). PBZ affected insulin levels only in the obese rats, reducing their baseline levels by 4-fold and stressed induced levels to those seen in the lean control rats. These results suggest that some of the metabolic and physiologic abnormalities of the obese Zucker rat which are modulated by the sympatho-adrenal system can be normalized by procedures which increase sympatho-adrenal activity.

Feeding responses of Zucker fatty rat to 2-deoxy-D-glucose, norepinephrine, and insulin

From the pathophysiological viewpoint, feeding responses to various stimuli were examined in Zucker fatty rats and their lean littermates. Intraventricular administration of norepinephrine (NE, 10 micrograms/rat) stimulated food intake in both rats. Intraventricular administration of 2-deoxy-D-glucose (2DG, 3.8 mg/rat) induced hyperphagia and concomitant hyperglycemia in lean rats. However, in fatty rats, the blood glucose was elevated but food intake was unaltered after 2DG administration. Subcutaneous administration of insulin (2 or 8 U/kg) stimulated food intake of both rats. Streptozotocin (STZ)-induced hypoinsulinemia produced transient reduction of food intake followed by sustained diabetic hyperphagia in lean rats. In fatty rats, the experimental hypoinsulinemia caused transient aphagia but not sustained diabetic hyperphagia. Daily injection of insulin (5 U/rat) restored energy assimilation in both diabetic rats. An increase in food intake due to insulin injection was remarkable only in diabetic fatty rats. From these findings, the regulatory system of food intake in fatty rats appears to be sensitive to changes in the circulating insulin level but insensitive to either glucoprivation or changes in body storage of energy.

Abnormal sympatho- adrenal function and plasma catecholamines in obese Zucker rats

The functional integrity of the peripheral sympathetic nervous system and adrenal medulla was assessed in homozygous, lean and obese, 7--8 month old male Zucker rats by the changes in plasma catecholamines during cold and immobilization stresses. Five of eight obese, but no lean rats died during a 24 hr cold stress (4--7 degrees C) from hypothermia. While both lean and obese rats had decreased rectal temperatures after 4 hr of cold stress, the obese had lower temperatures, relatively less of an increase of plasma norepinephrine (NE) and epinephrine (E) than the lean rats, and were unable to consistently maintain their temperatures even during intravenous NE infusions. Obese rats had lower rectal temperatures and higher plasma NE and dopamine levels at 21--22 degrees C ambient temperature, a relative failure to increase plasma NE and E levels after 1 hr of immobilization, but normal or supranormal plasma catecholamine levels after decapitation compared to the lean rats. These results suggest that the obese Zucker rat has abnormalities of both peripheral sympatho-adrenal function and thermoregulation, which may play roles in the development and/or maintenance of many of the physiological and metabolic defects in this animal model of genetic obesity.

Adipocyte size distribution in fa/fa rats during development

Epididymal fat pad adipocyte frequency distributions of obese (fa/fa) and nonobese (non-fa/fa) Zucker rats were determined from 2 to 20 wk of age. The fa/fa rats consistently displayed a higher proportion of larger adipocytes than the non-fa/fa rats of all ages studied. Increased adipocyte size preceded an increase in the number of adipocytes during the early phases of obese development. Among the fa/fa rats, the frequency of small cells continued to increase, which resulted in a bimodal distribution by 20 wk of age. Discriminant analysis suggests that the differences in adipocyte size frequency distributions at 2 wk of age may be another useful means to identify fa/fa and non-fa/fa pups prior to expression of overt obesity. Increased fat storage may reflect the lower energy expended by the very young fa/fa rats.

Consumption of O2 and early detection of fa/fa genotype in rats

Oxygen consumption was determined in 10--12-wk-old fa/fa and non-fa/fa rats at ambient temperatures of 10 degrees--40 degrees C. The fa/fa rats exhibited a lower oxygen consumption than non-fa/fa rats from 10 degrees--30 degrees C, but not at 35 degrees C and 40 degrees C. A lower oxygen consumption was also observed among fa/fa rats as early as 18 days of age, prior to the phenotypic expression of apparent obesity. Fourteen hundred microliter O2 consumed/hr/g body weight at STP was used as a value, below which future obese rats could be identified among 18-day-old pups from fa/+ X fa/+ crosses. Only a 10% error was found in the use of this value for the early identification of the fa/fa genotype.