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Single injections of recombinant human interleukin 1 beta (IL-1 beta) caused large (up to 2 degrees C) and sustained (3 h) increases in body temperature in conscious rats. Intracerebroventricular injections (10-100 ng) were much more effective and elicited greater responses than intravenous injections (0.1-1 microgram). IL-1 beta increased resting oxygen consumption by 25-49% in a dose-dependent manner. The activity of the thermogenic proton conductance pathway in brown adipose tissue (BAT) mitochondria was assessed from purine nucleotide (GDP) binding and was elevated by 40 and 86% 1 h after intravenous (1 microgram) or intracerebroventricular (100 ng) injection of IL-1 beta, respectively. Regional tissue blood flow was determined in anesthetized rats from the distribution of radiolabeled microspheres. Blood flow to liver (hepatic arterial), testes, skin, and white adipose tissue was unaffected by IL-1 beta injection. Blood flow to brain and kidney was increased (142 and 50%) but reduced (58%) to skeletal muscle after intravenous but not intracerebroventricular injection of interleukin. In contrast, blood flow to BAT was markedly elevated after intravenous (288%) or intracerebroventricular (382%) injection of IL-1 beta. Severing the sympathetic nerves supplying the interscapular BAT depot prevented the increase in blood flow. These data indicate that the potent pyrogenic effects of IL-1 beta in the rat are due largely to a central action. Fever is associated with increases in metabolic rate and BAT activity, and these results provide support for the involvement of brown fat in thermogenesis associated with fever.

Adult male rats were made obese either by tube feeding varying fractions (34%, 47%, 68% or 75%) of their normal food intake or by offering them a varied and palatable diet (cafeteria diet). After 17--30 days of these regimens, the treatments were withdrawn, and the animals were allowed free access to the normal stock diet. Tube-fed animals precisely adjusted voluntary food intake to compensate for the energy delivered by tube but nevertheless became obese as a result of an increased metabolic efficiency. Cafeteria-fed rats were hyperphagic and became obese without any apparent change in metabolic efficiency. Recovery from obesity was more rapid in the cafeteria animals and was due to a pronounced increase in heat production as well as concomitant hypophagia. Animals previously made obese by tube feeding exhibited hypophagia and returned to normal weight without any change in heat production. The relevance of these results to the concept of lipostasis and the relative roles of energy intake and expenditure in the regulation of energy balance are discussed.

Voluntary intake of protein, fat and carbohydrate (CHO) was modified by feeding young rats either a control purified diet [% metabolizable energy (ME): protein 21, fat 7, CHO 72], a control diet plus sucrose solution (20%) to drink (final intakes 17, 6 and 77% ME as protein, fat and CHO, respectively) or a low protein diet substituted with either CHO (8, 7 and 85% ME as protein, fat and CHO, respectively) or fat (8, 20 and 72% ME as protein, fat and CHO, respectively). Total ME intakes corrected for body size were similar for all rats, but body weight, energy gain and net energetic efficiency were lower in both low protein-fed groups than in the control group. The acute thermogenic response (% rise in oxygen consumption) to a standard balanced-nutrient meal was higher (12%) in sucrose-supplemented and in low protein groups (15-16%) than in control rats (8%). Brown adipose tissue protein content and thermogenic capacity (assessed from purine nucleotide binding to isolated mitochondria) were greater than control values in sucrose-fed and protein-deficient animals, and the greatest levels of activity were seen in low protein-fed rats with a high fat intake. The results demonstrate that the changes in energy balance, thermogenesis and brown adipose tissue activity that result from protein deficiency cannot be ascribed to changes in the level of energy intake or to a specific increase in the amount or proportion of either CHO or fat. They suggest that the protein-to-energy ratio must be the primary influence on thermogenesis and brown fat activity in these animals.

The effects of several bacterial endotoxins on body temperature and resting oxygen consumption (VO2) were compared in normal rats. Low doses (0.05 mg/kg, i.m.) of 0127:B8 phenol-extracted endotoxin caused significant increases in both parameters. Maximal febrile responses (+1.6 degrees C) occurred at a dose of 0.05 mg/kg, but higher doses produced smaller effects. The maximal increase in VO2 (17%) occurred at doses of 0.5-1.0 mg/kg. A TCA extract of the same strain of endotoxin elicited a similar pattern of responses but was less potent than the phenol extract, whereas another endotoxin 026:B6 (TCA extract) was much less potent. The data illustrate the importance of constructing dose-response curves when comparing different endotoxins and indicate that in the rat, oxygen consumption provides a useful index of the response to pyrogens.

The cafeteria diet involves feeding experimental animals a choice of palatable human food items to stimulate energy intake, and has been used extensively to study diet-induced thermogenesis. In a recent commentary Moore has argued that this feeding regime is inappropriate for such studies because the nutrient composition cannot be controlled, many of the effects seen are due to protein or nutrient deficiency and accurate measurements of energy intake are difficult to achieve. We argue that all of these criticisms can be overcome by careful use of the feeding regime and well-controlled experiments. Gross nutrient composition of cafeteria diets can be modified over a wide range, and such studies demonstrate that the effects of protein deficiency can be clearly dissociated from those of hyperphagia. There is no experimental evidence for nutritional deficiency in cafeteria-fed animals even over very long periods of time. Furthermore, the alternatives suggested by Moore, i.e., presenting sucrose solutions to drink or high fat diets, suffer the same drawbacks of altered and often uncontrolled nutrient intake and yet produce little or no increase in energy intake. Criticism of the cafeteria diet is not justified simply because of its misuse by nutritionally naive experimenters. The value and validity of this feeding regime is further supported by the enormous impact it has made on our understanding of energy balance regulation and thermogenesis.

Scatchard analysis of specific guanosine-diphosphate-([3H]GDP-) binding to rat brown-adipose-tissue mitochondria revealed two distinct binding sites with apparent dissociation constants (Kd) of approximately 0.05 and 2.0 microM. Binding to both sites was insensitive to atractyloside. Reducing the pH of the binding medium from 7.1 to 6.6 caused marked reductions in the Kd of both sites, but at pH 7.6, the dissociation constants were increased about 3-fold. Acute treatment of rats with noradrenaline, 1 h before sacrifice, increased the maximum number of binding sites (Bmax, pmol/mg mitochondrial protein) of both sites and also increased the dissociation constants. The Bmax of the lower-affinity site was elevated in rats exposed to 5 degrees C or fed a palatable cafeteria diet for 10 d, compared to control animals, with the greater changes occurring in the cold-adapted group. The high-affinity site was unaltered by cold adaptation or cafeteria feeding. These results indicate the presence of two distinct nucleotide-binding sites in brown-fat mitochondria, both of which may be involved in thermogenesis.

Acute injection (sc) of the antiglucocorticoid RU-486 (5-10 mg/kg) stimulated oxygen consumption (VO2) and brown adipose tissue (BAT) activity (mitochondrial GDP binding) in the rat. A peak effect was seen 60-80 min after injection. The rise in VO2 was prevented by prior injection of the beta-adrenergic antagonist propranolol, and the effect on BAT was abolished by surgical denervation of the sympathetic supply to the tissue. Central injection (cerebroventricular) of a much lower dose (3-8 micrograms/kg) of RU-486 also stimulated VO2, and this effect was inhibited by a CRF receptor antagonist [alpha-helical CRF-(9-41)]. Peripheral injection of RU-486 also elicited acute thermogenic responses in older (greater than 12 months) rats and in genetically obese (Zucker) rats. In lean animals, daily injection of RU-486 inhibited weight gain and stimulated BAT without affecting food intake. The thermogenic effects of RU-486 appear to be due to central stimulation of sympathetic outflow and may involve CRF release. The data support previous studies on the effects of adrenalectomy and CRF on thermogenesis in the rat.

Tumour-bearing mice spontaneously lose weight 8-9 weeks after implantation of a human hypernephroma, in spite of a normal food intake. Resting oxygen consumption was up to 40% higher in these animals than in sham-operated controls, but was significantly reduced by beta-adrenergic blockade with propranolol in the former group. The injection of noradrenaline caused a marked stimulation of the metabolic rate in all the animals, but the greatest response was seen in the cachectic mice. The brown-adipose-tissue mass was similar for both groups, but guanosine diphosphate binding to brown-adipose-tissue mitochondria (an index of thermogenic capacity) was significantly increased in tumor-bearing mice, and the injection of noradrenaline 1 h prior to sacrifice caused the greatest stimulation of binding in the cachectic group. These data suggest that the rapid weight loss of tumor-bearing animals may be due to a high metabolic rate which results from sympathetic stimulation of brown-adipose-tissue metabolism. The relevance of these results to cancer-induced cachexia in man is discussed.

Gastric intubation with 40 kJ of a carbohydrate slurry (4 mL) produced increases in resting oxygen consumption (VO2) of 15% to 22% in control, cold-adapted, and hyperthyroid rats, but the absolute rise in metabolic rate after food was greater in the latter group. Tube-feeding methyl cellulose (4 mL, 7% wt/vol) evoked similar increases in VO2 to carbohydrate (15% to 23%), but all of these responses were inhibited by beta-adrenergic blockade with propranolol. In cold-adapted animals, fat (40 kJ, 1.2 mL) produced a greater thermic effect than carbohydrate or methyl cellulose, and water (4 mL) also induced a small (8%), significant increase in VO2. Treatment with diazoxide shortly before the meal, to inhibit insulin release, almost completely inhibited the thermic responses to carbohydrate and methyl cellulose in all groups, but did not alter the effects of fat or water. Ingestion of a nonmetabolizable substance (methyl cellulose), would appear to stimulate metabolic rate to a similar extent to carbohydrate, possibly by causing gastric distention. Thermic effects of both these substances appear to involve insulin release and activation of the sympathetic nervous system. The thermic response to fat can also be inhibited by beta-adrenergic blockade, but apparently does not involve insulin release.

Binding of (-)-[3H]dihydroalprenolol [( 3H]DHA) to isolated brown adipose tissue (BAT) microsomal membranes was used to estimate beta-adrenoreceptor density, and this was found to be saturable, reversible, and stereospecific. Scatchard analysis indicated a single class of binding sites of equilibrium dissociation constant 2.2 nM, with a maximum number of binding sites of 160-170 fmol/mg protein. These values were unaffected by the age or sex of the animals, and similar Kd values were obtained for binding to heart and lung membranes. The kinetically derived Kd from forward and reverse reactions was 1.4 nM for BAT. Hofstee analysis of displacement of [3H]DHA produced linear plots for propranolol but curvilinear plots for atenolol and ICI 118,551. Atenolol showed 50-fold selectivity for beta 1-receptors and ICI 118,551 50-fold selectivity for beta 2-receptors in heart, lung, and BAT membranes. These plots indicated beta 1:beta 2-receptor ratios of 59:41 for BAT and heart and 25:75 for lung. It is possible that both beta-adrenoreceptor subtypes in BAT may be involved in the activation of thermogenesis in the tissue.

The rise in metabolic rate after intragastric feeding with fat and carbohydrate was enhanced in cold-acclimated (5 degrees C) rats and diminished in warm-acclimated (30 degrees C) rats compared to controls (24 degrees C), but the response was largest in cold-acclimated animals intubated with fat. These acute effects of nutrients were almost completely abolished by beta-adrenergic blockade with propranolol in all groups, while the parasympathetic antagonist atropine sulphate enhanced the responses in control rats, but had little effect in cold-acclimated animals. Feeding carbohydrate produced similar increases in interscapular brown adipose tissue (BAT) temperature in control and cold-acclimated rats, but fat caused a much greater rise in the latter group. The thermic effects of both nutrients were lower in genetically obese Zucker rats than in their lean littermates. Atropine slightly increased the thermic responses to fat and carbohydrate in the lean Zucker rats and caused marked potentiation in obese rats intubated with fat, but did not alter the effect of carbohydrate in the obese animals These data suggest that the size of the acute rise in metabolic rate after fat and carbohydrate is dependent on the thermogenic capacity of the animal. The response to fat was particularly large in cold-acclimated rats, where BAT activity is high, possibly due to a direct action of fat on the tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Gastric intubation of young male rats with 100 kJ/d of medium chain (C8 and C10) triglyceride (MCT) reduced their voluntary intake of stock diet such that total metabolizable energy intake was similar to that of rats intubated with water, and 41% of their energy intake was derived from MCT. Body weight, energy gain, and energetic efficiency were all markedly suppressed in MCT-fed rats, but energy expenditure over the 14-day experiment was significantly increased. Resting oxygen consumption, measured at thermoneutrality, was also enhanced in MCT-fed rats, but this difference was abolished by injection of the animals with the beta-adrenergic antagonist, propranolol. Brown adipose tissue mass was similar for both groups, but the activity of the mitochondrial proton conductance pathway, assessed from the binding of purine nucleotides, was increased by over 70%. These data indicate that the reduced weight gains of animals fed MCT are due to elevated rates of energy expenditure, possibly resulting from sympathetic activation of brown fat thermogenesis.

The injection of (+/-)-baclofen intravenously or directly into the ventromedial hypothalamus of urethane-anaesthetized rats, produced an activation of brown fat metabolism. This was seen as an increase of brown fat and rectal temperatures, and an increase of GDP binding in brown fat mitochondria. The activation was mediated by the sympathetic supply.