The blood flow and oxygen consumption of brown adipose tissue in the new-born rabbit

1. A direct method for measuring venous outflow from brown adipose tissue in anaesthetized new-born rabbits is described.

2. During noradrenaline infusion the mean blood flow through brown adipose tissue increased from 87 to 360 ml./100 g tissue (wet wt.).min, and the mean rate of oxygen consumption of brown adipose tissue rose from 9·3 to 60 ml. O₂/100 g tissue.min.

3. During cold exposure the mean blood flow through brown adipose tissue increased from 90 to 304 ml./100 g tissue.min.

4. The mean cardiac output was 266 ml./kg body weight.min; during noradrenaline infusion it was 405 ml./kg body weight.min. At rest about one tenth, and during noradrenaline infusion about one quarter of the cardiac output went to brown adipose tissue.

5. It was calculated that most of the extra oxygen consumed during the metabolic response of the anaesthetized new-born rabbit to noradrenaline infusion or cold exposure was consumed by brown adipose tissue.

6. Hypoxia (breathing 10% O₂ in N₂) greatly reduced the increase in oxygen consumption but not the increase in blood flow in brown adipose tissue caused by noradrenaline infusion.

     

Interscapular brown adipose tissue blood flow in the rat

The xenon clearance method was adapted to continuous measurement of interscapular brown adipose tissue (ISBAT) blood flow in anaesthetized rats.The ISBAT-blood partition coefficient for xenon was determined to 3.6 ml·g^–1. The blood flow values obtained by Xe clearance were compared with flow values obtained concomitantly by the microsphere technique in 17 cold acclimated rats, at ISBAT blood flows between 0.1 and 6 ml·g^–1·min^–1. Variations in blood flows were obtained by infusion of noradrenaline at different rates.The blood flow values obtained from the xenon clearance method showed a close correlation to the blood flow values determined with microspheres.Y=0.98.X+0.15 (r=0.96,P<0.001).The Xe clearance method has the advantages compared to the microsphere technique that it permits continuous monitoring of the blood flow and does not require the sacrifice of the animal.     

Potassium-induced increase in oxygen consumption of brown adipose tissue from the rat.

1. In brown adipose tissue, noradrenaline induces an increase in respiration and a depolarization of the cells. The effect of an increase in potassium concentration in a range known to depolarize the brown adipocytes was tested on the O2 consumption. 2. Isolated interscapular brown adipose tissue from the rat was incubated in chambers that allowed O2 consumption to be measured over prolonged periods. 3. 45-50 mM-KC1 were found to induce a more that fourfold increase in O2 consumption, which was stable, reversible and dependent upon the presence of calcium in the meduim. 4. When rats were pre-treated with reserpine or 6-hydroxydopamine the KC1-induced increase in O2 consumption was sharply reduced or entirely adsent. 5. The effect of KC1 was greatly inhibited by (-)-propranolol, but not by (+)-propranolol. 6. Moderate increases in O2 consumption induced by low concentrations of potassium were potentiated by desipramine, a drug which is known to block the uptake of catecholamines by adrenergic nerve endings. 7. Surgical denervation caused a decrease in the catecholamine content of the tissue, but had no effect on the KC1 response. 8. It is concluded that in brown adipose tissue, potassium stimulates O2 consumption by causing a release of noradrenaline from nerve endings. This implies that surgical denervation as it is commonly performed on this tissue does not denervate the brown adipocytes but probably only the blood vessels.     

Effects of glucagon and noradrenaline on the blood flow through brown adipose tissue in temperature-acclimated rats

After 30 min infusion of glucagon or noradrenaline, blood flow through brown adipose tissue (BAT) from various sites was investigated with the aid of 113Sn-labeled microspheres under hexobarbital anesthesia in cold-acclimated (CA), heat-acclimated (HA), and warm control (WC) rats. Glucagon increased cardiac output in both CA and HA, while noradrenaline increased it in HA but not in CA. Blood flow through BAT as well as the fractional distribution of cardiac output to BAT increased by glucagon dose-dependently and reached a maximum level in a dose of 2 micrograms/min. These glucagon-induced responses were significantly higher in CA and smaller in HA as compared with WC. Noradrenaline in a dose of 2 microgram/min caused larger responses than glucagon in all groups. Glucagon- or noradrenaline-induced blood flow per unit weight of BAT increased or tended to increase by cold acclimation. These results suggest that an in vivo enhanced glucagon-induced thermogenesis in cold-acclimated BAT is partly due to an increased blood flow through this tissue.     

Long-term alcohol consumption and brown adipose tissue in man

Summary The purpose of the present work was to study whether long-term alcohol consumption in man affects the develeopment of brown adipose tissue. The adipose tissue around the thoracic aorta and common carotid arteries was collected at medicolegal autopsies on adults with a positive record of heavy alcohol consumption. Adults without any evident history of alcohol consumption served as controls. Histochemical reactions of the oxidative mitochondrial enzymes, cytochrome oxidase and succinate dehydrogenase were studied in samples of this adipose tissue and the activities of the enzymes were measured biochemically.There was histological evidence of some multilocular adipose tissue around the thoracic aorta and common carotid arteries of the alcohol consumers, whereas the adipose tissue from the non-drinkers was mostly unilocular resembling white adipose tissue. Histochemical evidence of brown adipose tissue was found in all alcohol consumers, but also in some of the controls. Biochemical cytochrome oxidase (CYO) and succinate dehydrogenase measurements in isolated mitochondria showed activity in 70% of the cases of drinkers and in one of the eight controls. Activity of CYO was measurable in the mitochondria from two other controls. The protein content of the samples from the alcoholics was twice that of the controls. The results suggest that chronic alcohol intake may induce a change in the white adipose tissue around the thoracic aorta and common carotid arteries of human adults into brown fat.     

Exercise and brain catecholamine relationships with brown adipose tissue and whole-body oxygen consumption in rats

Brown Adipose Tissue (BAT) is thought to maintain caloric homeostasis by increasing its metabolic rate during periods of energy surfeit or low environmental temperatures and decreasing it during periods of deprivation or high environmental temperatures. The present study investigated the relationship of BAT and whole-body metabolic rate with negative energy balance induced by a minimally stressful exercise regimen. Twenty-four rats exercised in a running wheel for 2 hours per day for 8 weeks for food reinforcement either under a variable ratio or variable time schedule. Whole-body oxygen consumption and the thermic effect of a test meal were then measured, the rats sacrificed, BAT weight and oxygen consumption measured, and the brains assayed for catecholamines. Food intake was found to be positively correlated with expenditure either in the form of exercise, overnight weight loss, or BAT metabolic rate. Brain dopamine concentrations were found to be related to BAT metabolic rate which was in turn related to the amount of weight lost overnight and the amount of food intake. Running was positively correlated with BAT metabolic rate, suggesting that brown adipose tissue is not simply an organ of caloric homeostasis or thermoregulation. BAT, then, appears to be a multi-purpose tool used by a number of systems which regulate body temperature and energy metabolism.     

Effects of insulin on adipose tissue blood flow in man

Adipose tissue blood flow (ATBF) rises after nutrient ingestion. It is not clear whether this is due to insulin. The aim of this study was to investigate the role of insulin in the regulation of subcutaneous ATBF. We have investigated the role of insulin in the regulation of ATBF in normal, healthy subjects in a three-step procedure to determine the functional level at which insulin may potentially exert its effect. Fifteen subjects were studied on two occasions. On the first visit, 75 g oral glucose was given. In the second, similar plasma concentrations of insulin and glucose were achieved by dynamic intravenous infusions of insulin and glucose. The increase in ATBF after oral glucose (4.2 ± 1.4 ml min⁻¹ (100 g tissue)⁻¹,   P = 0.01  ) was significantly greater (   P < 0.05  ) than that after intravenous infusions (1.5 ± 0.6 ml min⁻¹ (100 g tissue)⁻¹   P < 0.05  ). For the local delivery of potentially vasoactive substances and simultaneous measurement of ATBF, we describe a novel combination of methods, which we have called 'microinfusion'. We have used this technique to show that locally infused insulin, even at pharmacological concentrations, had no demonstrable effect on ATBF in nine subjects. We conclude that whilst insulin does not have a direct effect on ATBF, it is likely to be an important mediator, possibly acting via sympathetic activation. In the postprandial state, other candidate peptides and hormones are also likely to play important roles.     

Effect of ventromedial hypothalamic stimulation on blood flow of brown adipose tissue in rats

The changes in regional blood flows to the rat's interscapular brown adipose tissue and several other tissues during electrical stimulation of the ventromedial hypothalamus (VMH) were studied using radioactively labelled microspheres. Measurement of blood flow was carried out, along with monitoring heart rate, under anesthesia and at thermoneutrality. During VMH stimulation the heart rate was clearly augmented and cardiac output increased about 45%. Regional blood flows were significantly increased in response to VMH stimulation in interscapular brown adipose tissue, adrenal glands, diaphragm and gastrocnemius muscles. The response of interscapular brown adipose tissue was the most prominent (approx. fiftyfold increase). Blood flows tended to decrease in spleen, lungs and kidneys during VMH stimulation, but did not change in liver or in other visceral organs. These observations suggest that the VMH is concerned with the regulation of regional blood flow to brown adipose tissue and contributes to thermogenesis in this tissue.     

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

1. 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.

     

Noradrenaline-induced changes in rat brown adipose tissue glucagon

Brown adipose tissue (BAT) glucagon level was higher in cold-acclimated rats (CA) than in warm controls (WC). Noradrenaline (NA) injection increased BAT glucagon levels in both WC and CA with increases in plasma glucagon levels. The magnitude of increase was significantly greater in CA for plasma glucagon, while it did not differ for BAT between groups. However, BAT glucagon was kept at a higher level in CA after NA injection than in WC.     

Effects of cold and immobilization stress on noradrenaline turnover in brown adipose tissue of rat

Noradrenaline (NA) turnover of the interscapular brown adipose tissue (BAT) was determined in order to evaluate a role of sympathetic NA of this tissue in an enhanced nonshivering thermogenesis which had been previously evidenced in the repetitively stressed rats by immobilization (daily 3-h immobilization for 4 weeks) and the cold-acclimated ones (5 degrees C, 4 weeks). The disappearance rate of NA from the BAT following blockade of NA synthesis with alpha-methyl-p-tyrosine was adopted for estimation of NA turnover of the tissue. Cold acclimation increased both fractional turnover rate (%/h) (k) and turnover rate (ng/(g BAT.h)). Repetitive immobilization stress also elevated turnover rate, but not k. In the warm non-stressed controls acute cold exposure to -5 degrees C and acute immobilization stress elevated the turnover rate. The effect of cold exposure was significantly greater than that of immobilization stress for both indices of NA turnover. In the cold-acclimated rats acute cold exposure increased k as well as turnover rate, but not acute immobilization stress. In the repetitively immobilized rats both acute cold exposure and acute immobilization stress elevated k and turnover rate. These results indicate that immobilization enhances sympathetic activity of thermogenic tissue, BAT. The results also suggest that the extent of sympathetic participation is not necessarily the same between the cold-acclimated and the stressed rats.     

Cerebral blood flow and oxygen consumption in the rat in hypoxic hypoxia.

In order to measure cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRo(2)) at pronounced degrees of hypoxic hypoxia the Pao(2) of artificially ventilated and normocapnic rats was reduced to between 47 and 22 mm Hg for 15-25 min with subsequent measurements of CBF, using a -133Xenon modification of the Kety and Schmidt technique, and of the arteriovenous difference in oxygen content, the venous blood being obtained from the superior sagittal sinus. When the Pao(2) was reduced to minimal values of 22 mm Hg CBF increased 4- to 6-fold, the increase in CBF being unrelated to changes in blood pressure or Paco(2). The CMRo(2) remained unchanged at all levels of hypoxia. It is concluded that the maintenance of a normal, or near-normal, cerebral energy state even at extreme degrees of hypoxic hypoxia depends solely on a homeostatic increase in CBF.     

Effects of 4-hydroxyamphetamine on in vivo brown adipose tissue thermogenesis and feeding behavior in the rat

The influence of 4-hydroxyamphetamine (4-OHAM) on food and water intake and in vivo brown adipose thermogenesis was examined in two experiments. In Experiment 1, female rats were treated with 0.00, 0.25, 0.50, 1.00, or 2.00 mg/kg 4-OHAM (ip) prior to assessment of interscapular brown adipose tissue (IBAT) thermogenesis. The 4-OHAM treatment induced dose-dependent activation of IBAT thermogenesis consistent with the enhanced serum levels of norepinephrine and epinephrine observed in 4-OHAM-treated rats immediately after temperature measurement. In Experiment 2, the influence of 4-OHAM on food and water intake was assessed during 120-min test intervals in female rats fed food and water ad lib. Although there was a trend for 4-OHAM to increase water intake, there was no significant effect of 4-OHAM (0.40, 0.80, 1.00, 2.00, and 4.00 mg/kg) on either food or water intake. These data suggest that brown adipose thermogenesis does not play a role in the anorexia induced by amphetamine or in the regulation of feeding.     

Influence of intravenously administered catecholamines on cerebral oxygen consumption and blood flow in the rat

In order to study effects of catecholamines on cerebral oxygen consumption (CMRo₂) and blood flow (CBF), rats maintained on 75 % N₂O and 25 % O₂, were infused i.v. with noradrenaline (2, 5, or 8 μpg. kg^-1. min^-1) or adrenaline (2 or 8, μg. kg^-1.min^-1) for 10 min before CBF and CMR_oz were measured. In about 50% of animals infused with 2–8, μg. kg^-1 min^-1 of noradrenaline, CMRo_z (and CBF) rose. However, there was no dose-dependent response, and CMRo₂, did not exceed 150% of control. The effects of noradrenaline in a dose of 5 μg. kg^-l. min^-1 on CMRo₂, and CBF were blocked by propranolol (2.5μg.kg^-1). In animals infused with adrenaline (8 μg.kg^-1.min^-1) CMRo₂, was doubled and, in many, CBF rose 4- to 6-fold. It is concluded that, when given in sufficient amounts, catecholamines have pronounced effects on cerebral metabolism and blood flow, the effects of adrenaline on CMRo₂, and CBF resembling those observed in status epilepticus.