Nancy J. Rothwell

Chronic effects of beta 2-adrenergic agonists on body composition and protein synthesis in the rat

Chronic treatment of rats with the β2-adrenergic agonists clenbuterol and fenoterol over 16–19 d raised energy intake, expenditure, and body weight gain but did not affect fat or energy deposition, and body protein gain was increased by 50 and 18%, respectively. Both drugs increased the protein content and mitochondrial GDP-binding capacity of brown adipose tissue. Clenbuterol did not affect plasma insulin, growth hormone, or triiodothyronine levels, although insulin levels were reduced by fenoterol. Both drugs caused hypertrophy of skeletal muscle (gastrocnemius), and muscle protein synthesis in vivo (fractional rate) was elevated by 34 and 26% in clenbuterol and fenoteroltreated rats, respectively.

Influence of noradrenaline on blood flow to brown adipose tissue in rats exhibiting diet-induced thermogenesis.

Abstract1.The influence of noradrenaline on regional blood flow was determined using radioactive microspheres in rats maintained on either stock diet or a palatable cafeteria diet.2.Cardiac output and blood flow to brain, lungs, liver and skeletal muscle were similar for rats on the two diets.3.Blood flow to total dissectable brown adipose tissue in control and cafeteria rats represented 1 and 2% of cardiac output respectively but these values rose to 7 and 15.5% during infusion of noradrenaline.4.Arterial oxygen content was similar for all groups but the oxygen content of venous blood draining the interscapular brown adipose tissue fell to 6 ml O2/100 ml blood in control rats and 1 ml/100 ml in cafeteria rats after noradrenaline.5.The total oxygen consumption of brown adipose tissue was calculated and found to account for 42% of the response to noradrenaline in control rats and 74% in cafeteria animals. The increments in the oxygen consumption of other tissues were almost identical in both groups and so all the diet-induced changes in thermogenic capacity can be attributed to increases in brown adipose tissue metabolism.6.These findings demonstrate the quantitative importance of brown adipose tissue in diet-induced thermogenesis and confirm the similarities between diet and non-shivering thermogenesis.

Activation of brown fat thermogenesis in response to central injection of corticotropin releasing hormone in the rat

Injection of CRF-41 (2-5 nmol) into the third ventricle, or the paraventricular nucleus of anaesthetized rats caused a marked rise in the temperature of the interscapular brown adipose tissue (BAT) depot (peak rise 0.8-1 degree C) which was inhibited by prior intravenous injection of propranolol. There was also a significant increase in the mitochondrial proton conductance pathway of brown adipose tissue, assessed from the binding of guanosine diphosphate (GDP) to mitochondria isolated from the interscapular (89% above control) and perirenal and para-aortic depots (130%). Acute surgical sympathectomy of interscapular brown adipose tissue immediately prior to injection of CRF significantly attenuated the increase in mitochondrial GDP-binding. Hypophysectomized (HYPX) rats showed a large (180%) increase in GDP-binding of brown adipose tissue 7 days after surgery and this was almost completely prevented by denervation of the interscapular depot prior to hypophysectomy. Acute injection of morphine also reduced the GDP-binding in hypophysectomized, but not in control rats. These data demonstrate that central injection of CRF stimulates thermogenesis in brown adipose tissue, probably by modifying sympathetic outflow. The activation of brown adipose tissue following hypophysectomy was also dependent on the sympathetic innervation and could be due to an increase in release of CRF.

Mechanisms of thermogenesis induced by low protein diets

Weanling (22-day-old) rats fed a low protein (8% casein) diet consumed the same amount of energy as controls (22% casein diet), but intake corrected for body size (kJ/kg0.75) was increased in the former group. Weight gain and the efficiency of gain (g gain/MJ) were markedly reduced in low protein fed rats. Resting oxygen consumption (VO2) was elevated by 15% in the low protein group but this difference was completely abolished by beta-adrenergic blockade with propranolol. Interscapular brown adipose tissue (BAT) mass, protein content, mitochondrial yield and GDP binding were increased in low protein fed rats but mitochondrial alpha-glycophosphate shuttle activity of BAT was unaltered, although shuttle activity was elevated in liver mitochondria. Plasma triiodothyronine levels were increased by 64% in the low protein group, whereas insulin levels were markedly reduced in spite of normal blood glucose levels. Resting VO2 and BAT mass were also increased in older (55-day-old) rats fed the low protein diet, but the changes were smaller than in weanling rats. These data suggest that the decreased metabolic efficiency seen in rats fed protein deficient diets involves sympathetic activation of BAT, and is therefore similar to the thermogenic responses seen in cold adapted and cafeteria-fed animals.

Effect of chronic food restriction on energy balance, thermogenic capacity, and brown-adipose-tissue activity in the rat

Young male rats were fed a pelleted stock diet either ad libitum (control) or in restricted amounts (65% of control intake) for 17 d. Body energy gain and energy expenditure, determined from energy balance measurements, were reduced by 73 and 27% respectively compared to controls. Resting oxygen consumption was similar for both groups during the light phase but was significantly depressed in energy-restricted rats at night, and the thermogenic response to noradrenaline was also reduced in these animals. Brown-adipose-tissue mass, protein content, and mitochondrial protein were all decreased in the food-deprived rats, and specific and total depot mitochondrial GDP-binding capacity were 29 and 53% lower than controls. The reduced energy expenditure which occurs during food restriction may be due partly to a lower activity of brown adipose tissue which is associated with a decrease in thermogenic capacity.

Changes in tissue blood flow and β‐receptor density of skeletal muscle in rats treated with the β2‐adrenoceptor agonist clenbuterol

1 Rats injected with the β2‐adrenoceptor agonist clenbuterol (2 mg kg−1per day) for 18 days gained significantly more weight than controls. 2 Tissue blood flow assessed 24 h after the last injection from the distribution of radiolabelled microspheres was increased in white (5 fold) and brown (3 fold) adipose tissue of clenbuterol‐treated rats but was unaffected in kidney, brain and diaphragm, and was reduced by about 80% in skeletal muscle. 3 Acute injection of clenbuterol one hour before measuring blood flow, increased blood flow to brown fat (20 fold) in both treated and control groups. Blood flow to skeletal muscle increased more in the rats treated chronically with clenbuterol (6 fold increase) than in control rats (2 fold increase), but absolute flow rates were still significantly lower in the rats treated chronically with clenbuterol. 4 Skeletal muscle β‐adrenoceptor density and subtype were assessed from ligand binding and displacement studies using [3H]‐dihydroalprenolol. Rats treated with clenbuterol for 18 days showed a 50% reduction in β‐receptor density, but the ratio of β1/β2‐receptors was unaffected (15% β1/85% β2). 5 The results indicate that, although clenbuterol produces acute increases in muscle blood flow, chronic treatment results in lower flow rates immediately (1 h) and 24 h after the previous injection. The attenuated response following chronic treatment is associated with a marked reduction in skeletal muscle β‐adrenoceptor density. 6 The data suggest that any anabolic effects of clenbuterol on muscle which may require, or may be mediated by increases in blood supply, cannot be sustained by chronic treatment. Conversely, blood flow to white and brown adipose tissue would appear to be potentiated by chronic treatment, possibly reflecting increases in lipolytic and/or thermogenic activity.

Effects of feeding a palatable ‘cafeteria’ diet on energy balance in young and adult lean (+/?) Zucker rats

1. The effects of feeding a palatable and varied cafeteria diet on energy balance were studied in young (5.5 week) and adult (5.5 month) lean male Zucker (+/?) rats. 2. Estimates of metabolizable energy (ME) intake derived from food composition tables were almost identical to values obtained from bomb calorimetry of foods, urine and faeces, and ME intake was elevated by approximately 73% in all cafeteria animals compared to stock-fed controls. 3. Cafeteria feeding had no effect on the body-weight of young rats but induced excess weight gains in the older animals and resulted in increased deposition of fat and energy in both groups. Energy expenditure, calculated from ME intake and body-energy gain, was elevated by 77 and 57% in young and adult-cafeteria rats respectively. The energy cost of fat deposition could account for only a small proportion of this increased expenditure. 4. The present results confirm previous findings in another strain of rat and show that the increased energy expenditure (i.e. diet-induced thermogenesis, DIT) which occurs in response to hyperphagia is not restricted to young animals but is also seen in older rats. Measurements of resting oxygen consumption after injections of noradrenaline or a beta-adrenergic antagonist (propranolol), and changes in brown adipose tissue mass are consistent with the suggestion that the DIT of cafeteria-fed rats results from sympathetic activation of brown fat.

Central effects of TNF alpha on thermogenesis and fever in the rat.

Intracerebroventricular (icv) injection of purified recombinant human tumour necrosis factor α (TNF α, 4–8μg) in conscious rats, produced increases in colonic temperature (1.0°C) and resting oxygen consumption (VO2, 14%) which were maximal after 80–90 minutes. Pretreatment with propranolol (10mg/kg s.c) significantly inhibited the rise in VO2, and prevented the increase in body temperature. Icv injection of an antagonist to corticotropin releasing factor (α-helical CRF 9-41, 25 μg), which prevents the pyrogenic and thermogenic actions of interleukin-1β, did not influence the effects of TNFα on temperature or VO2. Injection of a fragment of TNFα (113–130 amino acid sequence) did not affect body temperature or VO2. TNFα injection (icv) significantly increased brown adipose tissue (BAT)in vitro mitochondrial GDP binding, and this effect was slightly inhibited, but not prevented, by surgical denervation of the tissue, and was unaffected by pretreatment with α-helical CRF 9-41. These data indicate that TNFα can stimulate thermogenesis by a direct central action. The effects are largely, but not totally, dependent on the sympathetic nervous system but, unlike the thermogenic actions of interleukin they do not require release of CRF.

Sympathetic activation of brown-adipose-tissue thermogenesis in cachexia

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 8-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 brownadipose-tissue mitochondria (an index of thermogenic capacity) was significantly increased in turnout-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 tumour-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.

Acute effects of food, 2-deoxy-D-glucose and noradrenaline on metabolic rate and brown adipose tissue in normal and atropinised lean and obese (fa/fa) Zucker rats.

Abstract1.Intragastric feeding (40 kJ) produced a 17% rise in metabolic rate in lean Zucker rats but only an 8% increase in obese (fa/fa) rats, and both of these responses were significantly reduced by β-adrenergic blockade with propranolol (10 mg/kg, s.c.).2.Parasympathetic blockade with atropine (0.5 mg/kg, s.c.) caused a doubling of the response to food in lean rats and a threefold increase in the obese mutants, such that all atropinised animals showed the same increase in metabolic rate after food.3.Feeding also caused a significant rise in interscapular brown adipose tissue temperature, which was greatest in the lean animals and was enhanced by atropine in both groups.4.Injection of noradrenaline (250 μg/kg, s.c.) caused a similar (40%) rise in metabolic rate in lean and obese animals but this response was unaffected by atropine.5.2-Deoxy-d-glucose injection (360 mg/kg, s.c.) depressed oxygen consumption by 25 and 8% in lean and obese rats respectively and this effect was totally abolished by atropine.6.These results suggest that the rise in metabolic rate after a meal is partly due to sympathetic activation of brown adipose tissue. The reduced thermic response in obese Zucker rats is not due to insensitivity to noradrenaline, but may be partly due to parasympathetic inhibition of thermogenesis and partly to insensitivity to glucose availability.