Brown Adipose Tissue Norepinephrine Research Articles

Supplementary prenatal copper increases plasma triiodothyronine and brown adipose tissue uncoupling protein-1 gene expression but depresses thermogenesis in newborn lambs.

ObjectiveWe tested the hypothesis that increasing dietary copper (Cu) to gravid ewes would enhance brown adipose tissue (BAT) thermogenesis in their offspring.MethodsTwin-bearing ewes were assigned on d 70 of gestation to diets containing 3, 10, or 20 ppm dietary Cu (n = 8 per group). Twin lambs were assigned at birth to a cold (6°C) or warm (28°C) environmental chamber for 48 h. Blood was collected from ewes and from lambs and perirenal BAT was collected after 48 h in the environmental chambers.ResultsPrenatal Cu exposure increased ewe plasma triiodothyronine (T3) and thyroxine concentration (T4) (p<0.01) but prenatal Cu exposure had no effect on lamb plasma concentrations of T3, T4, glucose, or nonesterified fatty acid concentration (p≥0.08). The high level of prenatal Cu exposure depressed 48-h rectal temperature (p = 0.03). Cold exposure decreased BAT norepinephrine (NE) and increased BAT dopamine (p≤0.01), but prenatal Cu exposure had no effect on BAT cytochrome C oxidase activity or BAT NE or dopamine (p≥0.07). However, BAT of lambs from high-Cu ewes maintained higher uncoupling protein-1 (UCP1) gene expression than BAT of lambs from low- and medium-Cu ewes following warm or cold exposure in environmental chambers (p = 0.02). Cold exposure caused near depletion of BAT lipid by 48 h (p<0.001), increased BAT cytochrome c oxidase activity (p< 0.01), and depressed plasma fatty acid concentrations (p<0.001).ConclusionAlthough prenatal Cu exposure increased BAT UCP1 expression during warm and cold exposure, prenatal cold Cu exposure depressed 48-h rectal temperature. Cold exposure decreased BAT lipid content by over 80% and decreased lamb plasma fatty acid concentration by over 40%, indicating that fuel reserves for thermogenesis were nearly depleted by 48 h of cold exposure.

Perilipin regulates the thermogenic actions of norepinephrine in brown adipose tissue

In response to cold, norepinephrine (NE)-induced triacylglycerol hydrolysis (lipolysis) in adipocytes of brown adipose tissue (BAT) provides fatty acid substrates to mitochondria for heat generation (adaptive thermogenesis). NE-induced lipolysis is mediated by protein kinase A (PKA)-dependent phosphorylation of perilipin, a lipid droplet-associated protein that is the major regulator of lipolysis. We investigated the role of perilipin PKA phosphorylation in BAT NE-stimulated thermogenesis using a novel mouse model in which a mutant form of perilipin, lacking all six PKA phosphorylation sites, is expressed in adipocytes of perilipin knockout (Peri KO) mice. Here, we show that despite a normal mitochondrial respiratory capacity, NE-induced lipolysis is abrogated in the interscapular brown adipose tissue (IBAT) of these mice. This lipolytic constraint is accompanied by a dramatic blunting ( approximately 70%) of the in vivo thermal response to NE. Thus, in the presence of perilipin, PKA-mediated perilipin phosphorylation is essential for NE-dependent lipolysis and full adaptive thermogenesis in BAT. In IBAT of Peri KO mice, increased basal lipolysis attributable to the absence of perilipin is sufficient to support a rapid NE-stimulated temperature increase ( approximately 3.0 degrees C) comparable to that in wild-type mice. This observation suggests that one or more NE-dependent mechanism downstream of perilipin phosphorylation is required to initiate and/or sustain the IBAT thermal response.

Norepinephrine release in brown adipose tissue remains robust in cold-exposed senescent Fischer 344 rats.

Near the end of life, old F344 rats undergo a transition, marked by spontaneous and rapidly declining function. Food intake and body weight decrease, and these rats, which we call senescent, develop severe hypothermia in the cold due in part to blunted brown fat [brown adipose tissue (BAT)] thermogenesis. We tested the hypothesis that this attenuation may involve diminished sympathetic signaling by measuring cold-induced BAT norepinephrine release in freely moving rats using linear microdialysis probes surgically implanted into interscapular BAT 24 and 48 h previously. In response to 2 h at 15 degrees C, senescent rats increased BAT norepinephrine release 6- to 10-fold but did not maintain homeothermy. This increase was comparable to that of old presenescent (weight stable) rats that did maintain homeothermy during even greater cold exposure (2 h at 15 degrees C followed by 1.5 h at 8 degrees C). Tail temperatures, an index of vasoconstrictor responsiveness to cold, exhibited similar cooling curves in presenescent and senescent rats. Thus cold-induced sympathetic signaling to BAT and tail vasoconstrictor responsiveness remain robust in senescent rats and cannot explain their cold-induced hypothermia.

Sympathetic activation by fenfluramine depletes brown adipose tissue norepinephrine content in rats

The antiobesity agent fenfluramine (FEN) has been reported to produce an activation of sympathetic neurons to brown adipose tissue (BAT) resulting in thermogenesis. The present study was conducted to determine if FEN-induced activation of BAT is affected by the ambient temperature at which animals are maintained. Body temperature was determined in conscious male Sprague–Dawley rats using implanted temperature transmitters, and sympathetic activation of BAT was determined by measuring norepinephrine (NE) content. Animals maintained at 22 and 4 °C incurred a significant decline in core body temperature following FEN administration. FEN produced a significant depletion of BAT NE content, and the magnitude of BAT NE depletion was related to the dose of FEN (−57.4% at 3 mg/kg dose and −75.9% at 10 mg/kg dose). However, the extent of BAT NE depletion was equal at 22 and 4 °C. BAT NE depletion by FEN appears to depend on activation of sympathetic neurons because it was prevented by pretreatment with the ganglionic blocker pentolinium (10 mg/kg ip). Furthermore, NE depletion by FEN could be blocked with fluoxetine (10 mg/kg ip) pretreatment, indicating that the action of FEN is mediated through the release of serotonin.

Sympathetic activity in brown adipose tissue from rats adapted to a high protein, carbohydrate-free diet

Previous studies have shown that adaptation of rats to a high protein, carbohydrate-free (HP) diet results in a marked reduction in brown adipose tissue (BAT) thermogenic capacity. The present experiments were carried out to assess BAT sympathetic activity in HP diet-adapted rats. It was found that interscapular brown adipose tissue (IBAT) norepinephrine (NE) content, fractional turnover rate and calculated rate of turnover were markedly reduced in HP-fed rats. Replacement of the HP diet by a control, balanced diet for 24 h did not affect BAT NE content significantly, but restored fractional turnover rate and turnover rate values to those of control animals. Exposure to cold (4°) for a short period (8 h) induced a marked increase in IBAT NE fractional turnover rate and calculated turnover rate in both HP and control rats. The cold-induced rise of turnover rate over values at ambient temperature (25°C) was higher in HP rats than in controls. Rectal temperature after 8 h of cold exposure did not differ in HP and control rats. The data suggest that the decreased thermogenic capacity of rats adapted to a high protein, carbohydrate-free diet is due to a reduced sympathetic outflow to BAT, which can be rapidly reactivated by cold stimulation.

Defective stimulation of thyroxine 5'-deiodinase activity by cold exposure and norepinephrine in brown adipose tissue of monosodium glutamate-obese mice.

In order to examine the possible change in the thyroid hormone metabolism in the monosodium glutamate (MSG)-obese mice, we determined the iodothyronine deiodinase activity of brown adipose tissue (BAT), liver and kidney of male and female mice. There was no significant difference in the type II thyroxine 5'-deiodinase (T4 5'DII) activity in BAT between MSG-obese and control mice when they were kept at the ambient temperature of 22 degrees C. T4 5'DII activity in BAT of control mice increased markedly after exposure to cold (4 degrees C) for 4 h; however, the extent of cold-induced increase in T4 5'DII activity in BAT of MSG-obese mice was greatly reduced. Injection of norepinephrine (NE) (0.8 mg/kg, s.c.) 4 h previously increased T4 5'DII activity in BAT of control mice, but NE-induced increase in T4 5'DII activity was also markedly reduced in BAT of MSG-obese mice. Both cold- and NE-induced increase in T4 5'DII activity was greater in female, although similar tendency was obtained in male mice. Type I 3,3',5'-triiodothyronine deiodinase (rT3 5'DI) activity of liver and kidney, and serum thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels in MSG-obese mice were essentially the same as those of the control male and female mice irrespective of cold exposure. These results suggest that defective stimulation of T4 5'DII activity of BAT by cold in the MSG-obese mice is due to deficient sympathetic input to BAT and/or to diminished response of BAT to NE, and may contribute to a possible cause of inability of MSG-obese mice to maintain body temperature under cold exposure.

Dissociation of thermogenic and trophic actions of norepinephrine in brown adipocytes of Richardson ground squirrels.

Richardson ground squirrels are hibernators with seasonal changes in the thermogenic capacity of brown adipose tissue (BAT). The objective of this work was to evaluate whether norepinephrine (NE) acts both as a thermogenic and a growth factor in BAT. Brown adipocytes were isolated by collagenase digestion of axillary BAT and kept in a tissue culture incubator in methionine-free Dulbecco's modified Eagle's medium supplemented with albumin, antibiotics, and calf serum with or without NE for up to 24 h. For short-term incubations (<6 h), calf serum was omitted. Freshly isolated brown adipocytes responded to NE with 10- to 20-fold increases in rates of oxygen uptake. This ability to respond calorigenically to NE was maintained over a 24-h period. However, no significant increase in [35S]methionine incorporation into cellular proteins was observed when brown adipocytes were incubated for 1-6 or 24 h with NE. When labeled cell proteins were separated by SDS-PAGE and visualized by autoradiography, no selective change in protein labeling, particularly in the uncoupling protein (32 kDa) region was observed. Likewise, [35S]methionine incorporation into immunoprecipitable uncoupling protein was not affected by the presence of NE. These results suggest a dissociation between the thermogenic and growth effects of NE in BAT of Richardson ground squirrels.

Energy balance and lipid metabolism in transgenic mice bearing an antisense GCR gene construct.

Energy balance and lipid metabolism were investigated in transgenic mice bearing an antisense glucocorticoid receptor (GCR) gene construct that impairs the normal expression of the GCR gene. Food intake was recorded during the 15 days preceding decapitation of adult normal and transgenic mice, and feces were collected to derive the digestible energy intake. Body composition measurements consisted of the determination of energy, protein, and fat content of the carcass. Carcass energy was determined by bomb calorimetry, whereas carcass protein was measured by the Kjeldahl procedure. Energy expenditure was estimated from the continuous oxygen consumption (VO2) monitoring over a 24-h period. Lipoprotein lipase (LPL) activity was quantified in epididymal white adipose tissue (WAT), heart, and vastus lateralis muscle (VLM) by measuring the in vitro hydrolysis of labeled triolein in the presence of tissue homogenates. Norepinephrine (NE) content of both interscapular brown adipose tissue (BAT) and heart were determined by high-performance liquid chromatography (HPLC). Energy intake and expenditure were significantly lower in transgenic mice than in controls. Concurrently, both fat content and total energy of the carcasses were significantly higher in the transgenic animals. In comparison with normal mice, heart and VLM LPL activity was significantly reduced in transgenic mutants. There was no difference between groups in LPL activity in WAT. Finally, heart and BAT NE contents were lower in transgenic animals than in control mice. These results suggest that a defective GCR system may affect energy balance through increasing energetic efficiency, and they emphasize the modulatory effects of hypothalamic-pituitary-adrenal axis changes on muscle LPL activity.

Neural influences on trophic changes in brown adipose tissue during cold acclimation.

We studied the role of the sympathetic innervation in development and maintenance of increased levels of uncoupling protein (UCP) and of thyroxine 5'-deiodinase (TD) during cold-induced growth of brown adipose tissue (BAT). Interscapular BAT was unilaterally (and in some experiments, bilaterally) denervated either before acclimation to cold (4 degrees C) for 12 days or after 14 days of a total 28-day period of acclimation to cold. BAT norepinephrine was reduced to 3-7% of the normal level in denervated BAT for up to 26 days. Denervation slowed, but did not prevent, cold-induced increases in total protein, in mitochondrial GDP binding, and in mitochondrial UCP concentration, which all reached 50% or more of the elevated level in intact tissue. In contrast, TD activity did not exceed 10% of the elevated level in intact tissue at any time. Denervation after cold acclimation resulted in a very rapid loss of TD activity, a slower and selective loss (after a lag of 1 day) of UCP, and a much slower loss of tissue protein. We conclude that the sympathetic innervation is required for an optimal trophic response of BAT to cold acclimation and for maintenance in the hypertrophied state but that other factors are also involved. Induction and maintenance of TD in BAT does need the sympathetic innervation.

Norepinephrine turnover in iron deficiency at three environmental temperatures.

Iron-deficient anemic rats had a significant elevation in urinary norepinephrine (NE) after 7 days at 30 and 24 degrees C, but not at 10 degrees C, compared with control animals. NE turnover studies were performed to examine sympathetic nervous system activity in a tissue-specific fashion. NE content in myocardium decreased by nearly 50% in hypertrophied iron-deficient hearts at all three temperatures, whereas fractional turnover rates were compensatorily increased. In contrast, interscapular brown adipose tissue NE turnover was significantly reduced to 30% of normal in iron-deficient animals at both 30 and 10 degrees C. Serum triiodothyronine concentrations were similar to controls at 30 degrees C but were decreased at lower temperatures. Serum tetraiodothyronine concentrations were lower in iron-deficient animals at all three environmental temperatures. We conclude that increased sympathetic nervous system activity compensatory to temperatures below thermoneutrality is less well controlled in iron-deficient animals than in controls, and at a low environmental temperature this may possibly explain the poor thermoregulatory capacity of iron-deficient animals. A generalized hypernoradrenergic state is not supported by our NE turnover study and does not explain the elevated urine NE concentrations.

Thyroid hormone-catecholamine interrelationship during cold acclimation in rats. Compensatory role of catecholamine for altered thyroid states.

Effects of hyper- and hypothyroidism on catecholamine (CA) metabolism in the brain, adrenal glands, liver, and brown adipose tissue (BAT) were studied in adult rats during cold acclimation. Hypothyroidism was induced by the administration of propylthiouracil (PTU) and hyperthyroidism by the injection of thyroxine (T4). After 2 weeks of treatment, they were exposed to cold (5 degrees C) and sacrificed after 1 or 4 weeks. Although the body weight gain of PTU-treated rats were markedly impaired, the body temperature was maintained within normal range. They had increased cerebral dopamine, adrenal CA and BAT norepinephrine (NE) contents, enhanced cerebral tyrosine hydroxylase and adrenal dopamine beta-hydroxylase (DBH) activities and elevated [3H]dihydroalprenolol (DHA) binding to liver plasma membranes (P less than 0.01 vs controls). T4-treated rats showed an increased brain and adrenal CA only after cold exposure. The BAT NE content, DHA binding to liver plasma membranes, and [3H]guanosine diphosphate binding to BAT mitochondria were reduced by 30 to 50% from control values after 4 weeks of cold exposure. These results indicate that during cold acclimation, thyroid hormone deficiency is associated with an accelerated CA synthesis and release, which results in an enhanced BAT thermogenesis, and the hyperthyroid state suppresses CA release, hepatic DHA binding, and BAT heat production. Thus, there is a close metabolic interrelationship between thyroid hormone and CA during exposure to cold. CA appears to ameliorate thyroid hormone excess or deficiency.

Dietary obesity: brown fat denervation fails to alter development or recovery.

We assessed the effect of scapular brown adipose tissue (BAT) denervation on dietary obesity in adult female Sprague-Dawley rats. In rats maintained for 32 days on an obesity-inducing regimen, BAT denervation produced significant reductions in food intake (83.4 vs. 94.4 kcal/day), weight gain (66.0 vs. 103.5 g), and carcass fat (19.3 vs. 27.6%). A subgroup of denervates consuming as many calories as controls (94.1 kcal/day) failed to gain more weight (92.2 g) or accumulate more fat (25.4%) than controls. In rats developing obesity for 32 days followed by 8 days on laboratory chow, weight change was unaffected by denervation during either weight gain (89.4 vs. 87.1 g for controls) or weight loss (21.2 vs. 22.1 g) phases, as was carcass fat (21.0 vs. 20.4%). BAT norepinephrine utilization was unchanged in nondenervated obese rats and those recovering from obesity. We did not, therefore, find evidence that under these experimental conditions sympathetic activation of BAT countered obesity during overconsumption or contributed to recovery from obesity.

Defective trophic response of brown adipose tissue of myopathic hamsters.

Hamsters with muscular dystrophy (BIO 14.6) have a smaller than normal amount of brown adipose tissue. Two stimuli that promote growth of brown adipose tissue in normal hamsters, short photoperiod and eating a high-fat diet, are here shown to be without effect on brown adipose tissue of myopathic hamsters. Cold-induced growth of brown adipose tissue occurs normally [Am. J. Physiol. 239 (Cell Physiol. 8): C18-C22, 1980]. There is a normal rate of turnover of norepinephrine in brown adipose tissue of the myopathic hamster but a failure of the tissue to hypertrophy in response to norepinephrine is unlikely since norepinephrine does not appear to mediate the trophic response [Am. J. Physiol. 247 (Endocrinol. Metab. 10): E793-E799, 1984]. Denervation results in a marked reduction in size (protein content) of brown adipose tissue of normal hamsters but has very little effect on the size of brown adipose tissue of myopathic hamsters. A central, possibly hypothalamic, defect in the myopathic hamster is postulated to underlie its abnormal control of brown adipose tissue hypertrophy.

ABSENCE OF TRANSSYNAPTIC BETA ADRENERGIC RECEPTOR REGULATION IN OVINE FETAL LUNG AND HEART

We examined the effect of altered transsynaptic neural activity on lung and heart BAR development in the ovine fetus following chemical sympathectomy (CS) with guanethidine sulfate (GS). CS was induced by chronic administration of GS using subcutaneously implanted miniosmotic infusion pumps programmed to deliver 17 mg/kg/day for 14 days (4 animals). Control fetuses were infused with vehicle alone (4 animals). Animals were delivered by cesarean section. The extent of sympathetic denervation was assessed by measurement of tissue norepinephrine (NE) content using a sensitive, specific radioenzymatic assay. BAR concentration and affinity were measured using the tritiated radioligand dihydroalprenolol (DHA). Lung NE was unchanged following CS while cardiac atrial NE was 10% of control (p<.001) and ventricular NE was 19% of control values (p<.001). Lung and ventricular BAR were 112 and 100% of control values, respectively (p>0.05). Brown adipose tissue NE and adrenal catecholamine (CAT) concentrations were unchanged by CS. Conclusions: 1) GS at the dosage utilized causes extensive atrial and ventricular sympathetic denervation but does not affect lung, BAT or adrenal CAT contents. 2) Sympathetic denervation of the developing ventricle is not associated with up-regulation of BAR. Speculation: Myocardial BAR development in fetal life is not subject to the “normal” postpartum transsynaptic regulatory influences.

Effects of sucrose feeding and denervation on lipogenesis in brown adipose tissue

Rats were fed standard laboratory chow or sucrose + chow for 2–3 weeks. Sucrose feeding significantly increased the size and norepinephrine content of interscapular brown adipose tissue (BAT). Sucrose feeding stimulated in vivo lipogenesis in BAT, white adipose tissue, and liver. Unilateral denervation reduced BAT norepinephrine content by 90%, and significantly reduced BAT protein and DNA content. Prior surgical denervation of BAT had no effect on lipogenesis in chow-fed rats, but reduced the sucrose-induced lipogenesis in BAT by 75% and increased lipogenesis in retroperitoneal white adipose tissue of sucrose-fed rats. These data demonstrate that the optimal stimulation of lipogenesis in BAT by sucrose feeding requires innervation of this tissue.

The effect of cortisone on brown adipose tissue of young rats.

Rats aged 7 to 9 days were injected for 1 to 3 days with 5 mg/100 g body weight cortisone acetate per day. This led to an increase in the weight of brown adipose tissue, an increase in its fat content, and a decrease in its protein content. The content of norepinephrine in brown adipose tissue was decreased by cortisone treatment. The in vitro increase in O2 consumption following the addition of 5 μg nore-pinephrine/ml in vitro to brown adipose tissue from control animals was abolished by cortisone administration. Lipolysis due to norepinephrine was, however, not affected and was even slightly increased in cortisone-treated rats. Mitochondrial succinate–tetrazolium reductase activity was decreased after treatment with cortisone. It is concluded that cortisone alters the mitochondria of brown adipose tissue but has little effect on lipolysis as such. The content of norepinephrine in brown adipose tissue was found to decrease immediately after birth and then to return rapidly to higher levels.