Sequential changes in brown adipose tissue composition, cytochrome oxidase activity and GDP binding throughout pregnancy and lactation in the rat

The adaptation of maternal energy metabolism to lactation and its underlying mechanisms

Maternal energy metabolism undergoes a singular adaptation during lactation that allows for the caloric enrichment of milk. Changes in the mammary gland, changes in the white adipose tissue, brown adipose tissue, liver, skeletal muscles and endocrine pancreas are pivotal for this adaptation. The present review details the landmark studies describing the enzymatic modulation and the endocrine signals behind these metabolic changes. We will also update this perspective with data from recent studies showing transcriptional and post-transcriptional mechanisms that mediate the adaptation of the maternal metabolism to lactation. The present text will also bring experimental and observational data that describe the long-term consequences that short periods of lactation impose to maternal metabolism.

Bone Morphogenetic Protein-8B Levels at Birth and in the First Year of Life: Relation to Metabolic-Endocrine Variables and Brown Adipose Tissue Activity

OBJECTIVE

Bone morphogenetic protein-8B (BMP8B) is an adipokine produced by brown adipose tissue (BAT) contributing to thermoregulation and metabolic homeostasis in rodent models. In humans, BAT activity is particularly relevant in newborns and young infants. We assessed BMP8B levels and their relationship with BAT activity and endocrine-metabolic parameters in young infants to ascertain its potentiality as biomarker in early life.

MATERIALS AND METHODS

BMP8B concentrations were assessed longitudinally by ELISA in a cohort of 27 girls and 23 boys at birth, and at age 4 and 12 months, together with adiposity parameters (DXA), and circulating endocrine-metabolic variables. BAT activity was measured by infrared thermography. BMP8B gene expression (qRT-PCR) was determined in BAT, white fat, and liver samples from neonatal necropsies, and in placenta and cord blood.

RESULTS

BMP8B levels were high at birth, particularly in boys (P = 0.04 vs. girls), declined progressively, and remained well above those in healthy adults and pregnant women at age 1 year (P < 0.05 and P < 0.001, respectively). Neonatal BMP8B transcript levels were higher in BAT than in white fat, liver and cord blood. Circulating BMP8B levels during the first year of life marginally correlated with bone mineral density and gains in lean mass.

CONCLUSION

BMP8B levels are high at birth and decline progressively over the first year of life remaining above adult levels. Although changes in BMP8B concentrations overall reflect those in BAT activity during development, BMP8B levels are unlikely to be useful to predict individual variations in endocrine-metabolic status and BAT activity in healthy young infants.

The role of autophagy in brown and beige adipose tissue plasticity

Since the rediscovery of active brown and beige adipose tissues in humans a decade ago, great efforts have been made to identify the mechanisms underlying the activation and inactivation of these tissues, with the hope of designing potential strategies to fight against obesity and associated metabolic disorders such as type 2 diabetes. Active brown/beige fat increases the energy expenditure and is associated with reduced hyperglycemia and hyperlipidemia, whereas its atrophy and inactivation have been associated with obesity and aging. Autophagy, which is the process by which intracellular components are degraded within the lysosomes, has recently emerged as an important regulatory mechanism of brown/beige fat plasticity. Studies have shown that autophagy participates in the intracellular remodeling events that occur during brown/beige adipogenesis, thermogenic activation, and inactivation. The autophagic degradation of mitochondria appears to be important for the inactivation of brown fat and the transition from beige-to-white adipose tissue. Moreover, autophagic dysregulation in adipose tissues has been associated with obesity. Thus, understanding the regulatory mechanisms that control autophagy in the physiology and pathophysiology of adipose tissues might suggest novel treatments against obesity and its associated metabolic diseases.

Dwarfism and insulin resistance in male offspring caused by α1-adrenergic antagonism during pregnancy

OBJECTIVE

Maternal and environmental factors control the epigenetic fetal programming of the embryo, thereby defining the susceptibility for metabolic or endocrine disorders in the offspring. Pharmacological interventions required as a consequence of gestational problems, e.g. hypertension, can potentially interfere with correct fetal programming. As epigenetic alterations are usually only revealed later in life and not detected in studies focusing on early perinatal outcomes, little is known about the long-term epigenetic effects of gestational drug treatments. We sought to test the consequences of maternal α1-adrenergic antagonism during pregnancy, which can occur e.g. during hypertension treatment, for the endocrine and metabolic phenotype of the offspring.

METHODS

We treated C57BL/6NCrl female mice with the α1-adrenergic antagonist prazosin during pregnancy and analyzed the male and female offspring for endocrine and metabolic abnormalities.

RESULTS

Our data revealed that maternal α1-adrenergic blockade caused dwarfism, elevated body temperature, and insulin resistance in male offspring, accompanied by reduced IGF-1 serum concentrations as the result of reduced hepatic growth hormone receptor (Ghr) expression. We subsequently identified increased CpG DNA methylation at the transcriptional start site of the alternative Ghr promotor caused by the maternal treatment, which showed a strong inverse correlation to hepatic Ghr expression.

CONCLUSIONS

Our results demonstrate that maternal α1-adrenergic blockade can constitute an epigenetic cause for dwarfism and insulin resistance. The findings are of immediate clinical relevance as combined α/β-adrenergic blockers are first-line treatment of maternal hypertension.

Limits to sustained energy intake. XVI. Body temperature and physical activity of female mice during pregnancy

Lactation is the most energy-demanding phase of mammalian reproduction, and lactation performance may be affected by events during pregnancy. For example, food intake may be limited in late pregnancy by competition for space in the abdomen between the alimentary tract and fetuses. Hence, females may need to compensate their energy budgets during pregnancy by reducing activity and lowering body temperature. We explored the relationships between energy intake, body mass, body temperature and physical activity throughout pregnancy in the MF1 mouse. Food intake and body mass of 26 females were recorded daily throughout pregnancy. Body temperature and physical activity were monitored every minute for 23 h a day by implanted transmitters. Body temperature and physical activity declined as pregnancy advanced, while energy intake and body mass increased. Compared with a pre-mating baseline period, mice increased energy intake by 56% in late pregnancy. Although body temperature declined as pregnancy progressed, this served mostly to reverse an increase between baseline and early pregnancy. Reduced physical activity may compensate the energy budget of pregnant mice but body temperature changes do not. Over the last 3 days of pregnancy, food intake declined. Individual variation in energy intake in the last phase of pregnancy was positively related to litter size at birth. As there was no association between the increase in body mass and the decline in intake, we suggest the decline was not caused by competition for abdominal space. These data suggest overall reproductive performance is probably not constrained by events during pregnancy.

Limits to sustained energy intake. X. Effects of fur removal on reproductive performance in laboratory mice

The maximum rate of sustained energy intake (SusEI) may limit reproductive effort and other aspects of animal performance. We have previously suggested that lactating mice are not limited centrally by the alimentary tract or peripherally by the mammary glands, but that the limits to SusEI are imposed by the capacity of the animal to dissipate body heat generated as a by-product of processing food and producing milk. To explore the nature of the limits to SusEI, we bred MF1 laboratory mice at 21 degrees C and then dorsally shaved lactating females to reduce their external insulation and thereby elevate their capacity to dissipate body heat. These mice increased their food intake by 12.0% and assimilated on average 30.9 kJ day(-1) more energy than unshaved animals. With nearly identical mean litter sizes (11.4 pups for shaved and 11.3 pups for unshaved mice), shaved mothers exported 15.2% (22.0 kJ day(-1)) more energy as milk than control individuals. The elevated milk production of shaved mice enabled them to wean litters that were 15.4% (12.2 g) heavier than offspring produced by unshaved mice. Our results argue against central, peripheral or extrinsic limits to SusEI at peak lactation and provide strong support for the heat dissipation limit hypothesis. More generally, we see many situations where heat dissipation may be a previously unrecognised factor constraining the evolution of endothermic animals - for example, the latitudinal and altitudinal trends in clutch and litter sizes and the migration patterns of birds.

Rat brown adipose tissue thermogenic features are altered during mid-pregnancy

Brown adipose tissue (BAT) thermogenesis is inhibited during late-pregnancy and lactation in the rat. However, scarce information concerning BAT functionality during mid-pregnancy is available. The aim of this work was to investigate uncoupling proteins and leptin expression during placentation in rat BAT as well as other key parameters in the thermogenic function of the tissue. BAT mitochondrial content was found to be reduced 50% in 11 and 13 day pregnant rats as compared to nonpregnant controls, although uncoupling protein 1 (UCP1) content was not modified. Furthermore, UCP3 mRNA levels were found to be highly increased during this period. beta3-adrenergic receptor (beta3-AR) decreased expression resulted in a higher alpha2/beta3 ratio. Finally, leptin mRNA levels in BAT were found to be 3-fold up-regulated in pregnant animals. In conclusion, we show the existence of profound changes in thermogenic features in BAT during gestational days 11 and 13, pointing to the importance of this tissue during mid-pregnancy.

Limits to sustained energy intake IX: a review of hypotheses

Several lines of evidence indicate that animals in the wild may be limited in their maximal rates of energy intake by their intrinsic physiology rather than food availability. Understanding the limits to sustained energy intake is important because this defines an envelope within which animals must trade-off competing activities. In the first part of this review, we consider the initial ideas that propelled this area and experimental evidence connected with them. An early conceptual advance in this field was the idea that energy intake could be centrally limited by aspects of the digestive process, or peripherally limited at the sites of energy utilisation. A model system that has been widely employed to explore these ideas is lactation in small rodents. Initial studies in the late 1980s indicated that energy intake might be centrally limited, but work by Hammond and colleagues in the 1990s suggested that it was more likely that the limits were imposed by capacity of the mammary glands, and other works tended to support this view. This consensus, however, was undermined by studies that showed milk production was higher in mice at low temperatures, suggesting that the capacity of the mammary gland is not a limiting factor. In the second part of the review we consider some additional hypotheses that might explain these conflicting data. These include the heat dissipation limits hypothesis, the seasonal investment hypothesis and the saturated neural control hypothesis. Current evidence with respect to these hypotheses is also reviewed. The limited evidence presently available does not unambiguously support any one of them.

Brown adipose tissue. More than an effector of thermogenesis?

Brown adipose tissue (BAT) produces heat by oxidation of fatty acids. This takes place when the tissue is stimulated by norepinephrine; the molecular background for the ability of BAT to produce heat is the tissue-specific mitochondrial protein UCP1. In the classic view of BAT with respect to fever, BAT is an effector organ, producing heat especially during the onset phase of the fever. There is good evidence that BAT thermogenesis is stimulated via a lipopolysaccharide (LPS), interleukin (IL)-1 beta, IL-6, prostaglandin E cascade. Under physiologic conditions of constantly stimulated activity, BAT is expected to be recruited, but in fevers this is only evident in thyroxine fever. However, BAT may be more than merely an effector. There are indications of a correlation between the amount of BAT and the intensity of fevers, and brown adipocytes can indeed produce IL-1 alpha and IL-6. Furthermore, brown adipocytes are directly sensitive to LPS; this LPS sensitivity is augmented in brown adipocytes from IL-1 beta-deficient mice. Thus, BAT may also have a controlling role in thermoregulation. The existence of transgenic mice with ablations of proteins central in fever and in BAT thermogenesis opens up possibilities for identification and elucidation of this putative new role for brown adipose tissue as an endocrine organ involved in the control of fever.

AVP mediates the attenuated febrile response to administration of PGE1 in rats near term of pregnancy

Rats have an attenuated febrile response to intracerebroventricular injection of PGE1 near the term of pregnancy, the mechanism of which is unknown. The present experiments were carried out to test the hypothesis that arginine vasopressin (AVP), functioning as an endogenous antipyretic substance in the central nervous system, mediates this attenuated febrile response. The febrile response to intracerebroventricular injection of 0.2 microg PGE1 was determined in pregnant and nonpregnant rats after an intracerebroventricular injection of either vehicle or a vasopressin V1-receptor antagonist. After intracerebroventricular administration of vehicle, intracerebroventricular administration of 0.2 microg PGE1 produced significant increases in core temperature in both nonpregnant and pregnant animals. The increase in core temperature, however, was attenuated both in magnitude and duration in pregnant compared with nonpregnant animals. After intracerebroventricular administration of a vasopressin V1-receptor antagonist, intracerebroventricular administration of 0.2 microg PGE1 produced significant increases in core temperature that were similar in nonpregnant and pregnant animals. Our data support the hypothesis that a pregnancy-related activation of AVP as an endogenous antipyretic substance in the central nervous system attenuates the febrile response to intracerebroventricular administration of PGE1 near term of pregnancy in rats.

Regulation of mitochondrial biogenesis in brown adipose tissue: nuclear respiratory factor-2/GA-binding protein is responsible for the transcriptional regulation of the gene for the mitochondrial ATP synthase beta subunit

The regulation of transcription of the gene for the beta subunit of the FoF1 ATP synthase (ATPsynbeta) in brown adipose tissue has been studied as a model to determine the molecular mechanisms for mitochondrial biogenesis associated with brown adipocyte differentiation. The expression of the ATPsynbeta mRNA is induced during the brown adipocyte differentiation that occurs during murine prenatal development or when brown adipocytes differentiate in culture. This induction occurs in parallel with enhanced gene expression for other nuclear and mitochondrially-encoded components of the respiratory chain/oxidative phosphorylation system (OXPHOS). Transient transfection assays indicated that the expression of the ATPsynbeta gene promoter is higher in differentiated HIB-1B brown adipocytes than in non-differentiated HIB-1B cells. A major transcriptional regulatory site was identified between nt -306 and -266 in the ATPsynbeta promoter. This element has a higher enhancer capacity in differentiated brown adipocyte HIB-1B cells than in non-differentiated cells. Electrophoretic shift analysis indicated that Sp1and nuclear respiratory factor-2/GA-binding protein (NRF2/GABP) were the main nuclear proteins present in brown adipose tissue that bind this site. Double-point mutant analysis indicated a major role for the NRF2/GABP site in the enhancer capacity of this element in brown fat cells. It is proposed that NRF2/GABP plays a pivotal role in the co-ordinated enhancement of OXPHOS gene expression associated with mitochondrial biogenesis in brown adipocyte differentiation.

Influence of nicotine on the core temperature response to a novel environment in pregnant rats

Exposure of a male or nonpregnant female rat to a novel environment, such as a simulated open field, induces a transient increase in core temperature, which is often called stress-induced hyperthermia. Pregnancy alters this response such that the core temperature index increases significantly during exposure to a simulated open field on day 10 but not on days 15 and 20 of gestation in rats. The present experiments were carried to investigate the effect of chronic administration of nicotine (0, 1, 2, 4, or 8 mg.kg-1.24 h-1 for 13-15 days) on the core temperature response to a simulated open field in chronically instrumented pregnant (day 20 or 21 of gestation) and nonpregnant Sprague-Dawley rats. In nonpregnant rats, the core temperature index increased more during exposure to a simulated open field after chronic administration of nicotine at all doses than after chronic administration of vehicle; the core temperature response was not dependent on the dose of nicotine. In pregnant rats, significant increases in core temperature as well as in the core temperature index occurred only during exposure to a simulated open field after chronic administration of nicotine in doses of 2, 4, or 8 mg.kg-1.24 h-1; the core temperature response was dependent on the dose of nicotine. Our data provide evidence that chronic exposure to nicotine enhances the core temperature response to a simulated open field in nonpregnant rats and unmasks a maternal thermogenic response that is not seen to the same stimulus near term of pregnancy. The possible physiological consequences for the fetus are presently unknown and require investigation.

Thermoregulatory control during pregnancy and lactation in rats

Although the mechanisms remain unknown, maternal core temperature (Tc) decreases near term of pregnancy and is increased throughout lactation in rats. The purpose of our present experiments was to determine whether pregnancy and lactation shift the thermoneutral zone of rats and to investigate whether the changes in maternal Tc during pregnancy and lactation result from "forced" or "regulated" thermoregulatory responses. Conscious, chronically instrumented nonpregnant and pregnant and lactating rats were studied both in a thermocline (a chamber with a linear temperature gradient from 12 to 36 degrees C) and in a metabolic chamber to determine the influence of pregnancy and lactation on selected ambient temperature as well as the thermoregulatory response to changes in ambient temperature. We found that selected ambient temperature, oxygen consumption, and thermal conductance did not change in rats studied in a thermocline as Tc decreased near term of pregnancy. There was, however, a downward shift in the thermoneutral zone of rats studied in a metabolic chamber near term of pregnancy. During lactation, selected ambient temperature decreased in rats studied in a thermocline as oxygen consumption and Tc increased. The thermoneutral zone of lactating rats was not different from that of nonpregnant animals. Thus our data provide evidence that the decrease in Tc near term of pregnancy in rats results from a regulated thermoregulatory response, whereas the increase in Tc during lactation results from a forced thermoregulatory response.

Influence of pregnancy on the febrile response to ICV administration of PGE1 in rats studied in a thermocline

Rats near term of pregnancy have an attenuated febrile response to intracerebroventricular (ICV) injection of prostaglandin E1 (PGE1) when they are studied at an ambient temperature below their thermoneutral zone. Given that nonshivering thermogenesis in brown adipose tissue is impaired in rodents near term of pregnancy, it is possible that the attenuated febrile response is forced by impairment of this component of the autonomic thermoregulatory response. If this were the case, then near-term pregnant rats should develop a "normal" fever after PGE1 administration if they were studied in a thermocline where they could utilize behavioral as well as autonomic thermoregulatory effectors to increase their body core temperature (Tbc). Experiments were, therefore, carried out on 13 nonpregnant and 14 pregnant chronically instrumented rats in a thermocline (temperature gradient 10-40 degrees C) to investigate their Tbc responses to ICV injection of PGE1. ICV injection of 0.2 microgram PGE1 produced significant increases in Tbc and fever index in both nonpregnant and pregnant animals (day 19 of gestation); the increases, however, were significantly attenuated in the pregnant compared with the nonpregnant rats. Behavioral (e.g., selected ambient temperature) and autonomic (e.g., oxygen consumption) thermoregulatory effectors were activated to increase Tbc after ICV PGE1 in both groups of animals, but the duration of activation was shortened in pregnant compared with nonpregnant rats. The abbreviated thermoregulatory effector responses and the resulting attenuated febrile response to PGE1 in the pregnant rats may have resulted from a pregnancy-related activation of an endogenous antipyretic system.

Pregnancy alters body-core temperature response to a simulated open field in rats

Exposure of a rat to a novel environment (e.g., a simulated open field) induces a transient increase in body-core temperature, which is often called stress-induced hyperthermia. Although pregnancy is known to influence thermoregulatory control, its effect on stress-induced hyperthermia is unknown. Therefore, 24 Sprague-Dawley rats (8 nonpregnant and 16 pregnant) were studied to test the hypothesis that pregnancy would alter the development of stress-induced hyperthermia after exposure to a simulated open field. Body-core temperature index increased significantly after exposure to a simulated open field in nonpregnant and gestation day-10 rats but not in gestation day-15 and day-20 rats. Thus our data provide evidence that pregnancy influences the body-core temperature response of rats exposed to a simulated open field in a gestation-dependent fashion. The functional consequences as well as the mechanisms involved remain to be determined.

Co-ordinate decrease in the expression of the mitochondrial genome and nuclear genes for mitochondrial proteins in the lactation-induced mitochondrial hypotrophy of rat brown fat

The relative abundance of the mitochondrial-encoded mRNAs for cytochrome c oxidase subunit II and NADH dehydrogenase subunit I was lower in brown adipose tissue (BAT) from lactating rats than in virgin controls. This decrease was in parallel with a significant decrease in mitochondrial 16 S rRNA levels and in the relative content of mitochondrial DNA in the tissue. BAT from lactating rats showed lowered mRNA expression of the nuclear-encoded genes for the mitochondrial uncoupling protein, subunit IV of cytochrome c oxidase and the adenine nucleotide translocase isoforms ANT1 and ANT2, whereas mRNA levels for the ATP synthase beta-subunit were unchanged. However, the relative content of this last protein was lower in BAT mitochondria from lactating rats than in virgin controls. It is concluded that lactation-induced mitochondrial hypotrophy in BAT is associated with a co-ordinate decrease in the expression of the mitochondrial genome and nuclear genes for mitochondrial proteins. This decrease is caused by regulatory events acting at different levels, including pre- and post-transcriptional regulation. BAT appears to be a useful model with which to investigate the molecular mechanisms involved in the co-ordination of the expression of the mitochondrial and nuclear genomes during mitochondrial biogenesis.

Body temperature regulation in rats near term of pregnancy

Experiments were carried out to define the effects of pregnancy on body temperature (Tb) regulation in rats. Tb was measured by biotelemetry in six animals from day 10 of pregnancy (term day 21) to postpartum day 10. Average 24-h Tb decreased from day 15 of gestation to the time of parturition. Furthermore, there was a loss of the normal circadian variation of Tb late in gestation, which was again present by postpartum day 2. The decrease in 24-h Tb on day 15 of gestation resulted from this loss of circadian variation, as Tb did not increase during the dark period. The further decrease in Tb on day 20 of gestation resulted from an overall decrease in Tb during the light and dark periods as well as from a loss of the circadian variation in Tb. Tb increased dramatically within 4 h of birth of the first pup, which always occurred on day 21 during the light period. The mechanisms responsible for these dramatic changes in thermoregulation during late gestation and around the time of parturition are presently unknown.

Physiological modulation of the uptake and fate of glucose in brown adipose tissue

Glucose utilization indices (GUI values) and rates of fatty acid synthesis in interscapular brown adipose tissue (IBAT) varied during the diurnal cycle in virgin and late-pregnant rats permitted unrestricted access to food. In virgin rats, peak GUI values and lipogenic rates were observed at the end of the dark (feeding) phase, but were not sustained during the light phase. Whereas peak GUI values were comparable with those observed during re-feeding after 24 h starvation, maximum rates of IBAT fatty acid synthesis in virgin rats during the diurnal cycle were only approx. 25% of those measured during re-feeding after 24 h starvation. Despite hyperphagia, GUI values during the diurnal cycle in late-pregnant rats fed ad libitum were generally lower than those of age-matched virgin controls. The percentage of pyruvate dehydrogenase complex present in the active form (PDHa) was also significantly decreased. Suppression of GUI and PDHa was not parallelled by suppression of fatty acid synthesis. IBAT GUI values in late-pregnant rats during chow re-feeding ad libitum after 24 h starvation were only 25% of those of corresponding virgin controls, and stimulation of fatty acid synthesis was also dramatically attenuated. The suppression of IBAT GUI values after re-feeding in pregnancy was not due to depletion of GLUT 4 protein. The results are discussed in relation to the importance of glucose as a precursor for fatty acid synthesis in IBAT.

Metabolic fuels and reproduction in female mammals

A complete reproductive cycle of ovulation, conception, pregnancy, and lactation is one of the most energetically expensive activities that a female mammal can undertake. A reproductive attempt at a time when calories are not sufficiently available can result in a reduced return on the maternal energetic investment or even in the death of the mother and her offspring. Numerous physiological and behavioral mechanisms link reproduction and energy metabolism. Reproductive attempts may be interrupted or deferred when food is scarce or when other physiological processes, such as thermoregulation or fattening, make extraordinary energetic demands. Food deprivation suppresses both ovulation and estrous behavior. The neural mechanisms controlling pulsatile release of gonadotropin-releasing hormone (GnRH) and, consequently, luteinizing hormone secretion and ovarian function appear to respond to minute-to-minute changes in the availability of metabolic fuels. It is not clear whether GnRH-secreting neurons are able to detect the availability of metabolic fuels directly or whether this information is relayed from detectors elsewhere in the brain. Although pregnancy is less affected by fuel availability, both lactational performance and maternal behaviors are highly responsive to the energy supply. When a reproductive attempt is made, changes in hormone secretion have dramatic effects on the partitioning and utilization of metabolic fuels. During ovulatory cycles and pregnancy, the ovarian steroids, estradiol and progesterone, induce coordinated changes in the procurement, ingestion, metabolism, storage, and expenditure of metabolic fuels. Estradiol can act in the brain to alter regulatory behaviors, such as food intake and voluntary exercise, as well as adenohypophyseal and autonomic outputs. At the same time, ovarian hormones act on peripheral tissues such as adipose tissue, muscle, and liver to influence the metabolism, partitioning and storage of metabolic fuels. During lactation, the peptide hormones, prolactin and growth hormone, rather than estradiol and progesterone, are the principal hormones controlling partitioning and utilization of metabolic fuels. The interactions between metabolic fuels and reproduction are reciprocal, redundant, and ubiquitous; both behaviors and physiological processes play vital roles. Although there are species differences in the particular physiological and behavioral mechanisms mediating nutrition-reproduction interactions, two findings are consistent across species: 1) Reproductive physiology and behaviors are sensitive to the availability of oxidizable metabolic fuels. 2) When reproductive attempts are made, ovarian hormones play a major role in the changes in ingestion, partitioning, and utilization of metabolic fuels.

The effects of insulin and the pituitary peptide beta-cell tropin on the incorporation of D-3-3H-glucose into lipid in brown adipocytes from lactating and non-lactating rats

Lactating and non-lactating rat brown adipocytes were used to study the dose-dependent stimulation of lipogenesis by Beta-cell tropin (BCT) and insulin. In non-lactating animals BCT increased lipogenesis approximately 2-fold compared to a 3-fold stimulation with insulin; however BCT was effective at a substantially lower molar concentration than insulin. In lactating animals resistance was observed to both BCT and insulin action.

Inactivation of brown adipose tissue thermogenesis by oestradiol treatment in cold-acclimated rats

Both cold-acclimated female rats and rats at thermoneutrality received 0.15-0.20 mg daily of 17 beta-oestradiol over 15 days via a Silastic capsule implanted subcutaneously. Controls received empty implants. Comparison between the oestradiol-treated animals and the untreated controls revealed that at thermoneutrality, oestradiol treatment decreased food intake and body weight gain, but did not affect brown adipose tissue (BAT) thermogenesis and composition. By contrast, in cold-acclimated rats, oestradiol treatment did not modify food intake or body weight gain, but it decreased BAT thermogenesis. It is concluded that the effects of oestradiol treatment on BAT depend on the activity of the tissue, i.e. it has no effect on BAT when the tissue is thermogenically inactive, but it decreases cold-induced BAT thermogenesis. It is suggested that oestradiol could be the hormonal factor responsible for the previously observed inactivation of BAT thermogenesis during pregnancy in cold-acclimated rats.

Lipoprotein lipase mRNA expression in brown adipose tissue: translational and/or posttranslational events are involved in the modulation of enzyme activity

Lipoprotein lipase mRNA abundance in rat brown adipose tissue increases during the first 24 h of cold exposure. Lipoprotein lipase mRNA levels do not change in brown fat throughout pregnancy and lactation, whereas enzyme activity is significantly lowered. After 5 h of acute cold or noradrenaline administration there is a 2-fold increase in lipoprotein lipase mRNA abundance, whereas lipoprotein lipase activity is stimulated to more than 6-fold the basal values. It is concluded that translational and/or posttranslational mechanisms are involved in the noradrenergic modulation of lipoprotein lipase activity in brown fat.

Insulin resistance of glucose metabolism in isolated brown adipocytes of lactating rats. Evidence for a post-receptor defect in insulin action

The mechanism responsible for the insulin resistance described in vivo in brown adipose tissue (BAT) of lactating rats was investigated. The effect of insulin on glucose metabolism was studied on isolated brown adipocytes of non-lactating and lactating rats. Insulin stimulation of total glucose metabolism is 50% less in brown adipocytes from lactating than from non-lactating rats. This reflects a decreased effect of insulin on glucose oxidation and lipogenesis. However, the effect of noradrenaline (8 microM) on glucose metabolism was preserved in brown adipocytes from lactating rats as compared with non-lactating rats. The number of insulin receptors is similar in BAT of lactating and non-lactating rats. The insulin-receptor tyrosine kinase activity is not altered during lactation, for receptor autophosphorylation as well as tyrosine kinase activity towards the synthetic peptide poly(Glu4-Tyr1). The defect in the action of insulin is thus localized at a post-receptor level. The insulin stimulation of pyruvate dehydrogenase activity during euglycaemic/hyperinsulinaemic clamps is 2-fold lower in BAT from lactating than from non-lactating rats. However, the percentage of active form of pyruvate dehydrogenase is similar in non-lactating and lactating rats (8.6% versus 8.9% in the basal state, and 37.0% versus 32.3% during the clamp). A decrease in the amount of pyruvate dehydrogenase is likely to be involved in the insulin resistance described in BAT during lactation.

Adaptative decrease in expression of the mRNA for uncoupling protein and subunit II of cytochrome c oxidase in rat brown adipose tissue during pregnancy and lactation

Uncoupling-protein (UCP) mRNA expression is decreased to 15% of virgin control levels between days 10 and 15 of pregnancy, and remains at these low values during late pregnancy and lactation. Abrupt weaning of mid-lactating rats causes a slight but significant increase in UCP mRNA. Expression of mRNA for subunit II of cytochrome c oxidase (COII) decreased to half that of virgin control in late pregnancy and during lactation. Whereas COII mRNA expression is in step with the known modifications of brown-fat mitochondria content during the breeding cycle of the rat, UCP mRNA expression appears to be diminished much earlier than the mitochondrial proton-conductance-pathway activity. On the other hand, the reactivity of brown fat to increase expression of UCP and COII mRNAs in response to acute cold or noradrenaline treatment is not impaired during lactation.

Effects of hypothyroidism and hyperthyroidism on GDP binding to brown-adipocyte mitochondria from rats

1. Rats were made hypothyroid by giving them a low-iodine diet with propylthiouracil for 4 weeks, or were made hyperthyroid by injection with tri-iodothyronine (T3) over a 3-day period. 2. Brown adipocytes were isolated from the interscapular depots of these animals or from their euthyroid controls, followed by isolation of mitochondria from the cells. 3. Relative to cell DNA content, hypothyroidism decreased the maximum binding (Bmax.) of [3H]GDP to mitochondria by 50%. T3 treatment increased binding by 37%. 4. These findings, which are discussed in relation to previously observed changes in brown adipose tissue after alteration of thyroid status, suggest that mitochondrial uncoupling for thermogenesis is less or more effective in hypothyroidism or hyperthyroidism respectively.

Iodothyronine 5'-deiodinase activity and thyroid hormone content in brown adipose tissue during the breeding cycle of the rat

Brown adipose tissue iodothyronine 5'-deiodinase activity is significantly lower in 17-day pregnant rats compared with virgin controls and remains low during late pregnancy and lactation. It fully recovers with abrupt weaning, but only partially with spontaneous weaning. Even though this profile of changes is remarkably in step with the known pattern of modifications in brown fat thermogenesis during the breeding cycle, the lowered iodothyronine 5'-deiodinase activity appearing between days 15 and 17 of pregnancy occurs earlier than the reduction in brown adipose tissue thermogenesis. Brown fat 3,3',5-tri-iodothyronine content is also reduced in late pregnant, early and mid-lactating rats, most probably as a consequence of the lowered 5'-deiodination of thyroxine in situ. Acute insulin treatment increases brown fat iodothyronine 5'-deiodinase activity in virgin animals as well as in late-pregnant and lactating rats, despite the lowered basal enzyme activity levels in the latter groups. Thus an impaired response to insulin in brown fat does not appear to be a factor leading to the lowered iodothyronine 5'-deiodinase activity during late pregnancy and lactation.

Functional atrophy of brown adipose tissue during lactation in mice. Effects of lactation and weaning on mitochondrial GDP binding and uncoupling protein

The thermogenic activity and capacity of brown adipose tissue were determined in mice during lactation and after weaning. There was a marked fall during lactation in the mitochondrial content of the tissue, and in GDP binding and the specific mitochondrial concentration of uncoupling protein. The lactation-induced functional atrophy of brown adipose tissue was fully reversible after weaning; mitochondrial content and the mitochondrial concentration of uncoupling protein were both restored, although GDP binding was not normalized.

Direct assessment of brown adipose tissue as a site of systemic tri-iodothyronine production in the rat

Tri-iodothyronine (T3)production by interscapular brown fat was studied by measurements of arterio-venous differences and blood flow across the tissue in rats exposed to the following situations: controls, acute cold, chronic cold and starvation. Results demonstrate that brown adipose tissue is a source of systemic T3 in the rat and that the T3 release is modulated according to the physiological situation of the animal: increased in cold exposure and inhibited in starvation.

Iodothyronine 5'-deiodinase activity in rat brown adipose tissue during development

Iodothyronine 5'-deiodinase activity in rat brown adipose tissue has a characteristic pattern of developmental changes that is completely different from that of the liver. Fetal brown fat exhibits an extremely high iodothyronine 5'-deiodinase activity that is approx. 10-fold that in adult rats. Even though brown fat iodothyronine 5'-deiodinase activity falls suddenly at birth, there is a new peak in the activity around days 5-7 of life, whereas it remains very low afterwards. Just after birth, brown adipose tissue iodothyronine 5'-deiodinase activity is already capable of stimulation by noradrenaline. The postnatal peak in brown fat iodothyronine 5'-deiodinase correlates with the known increase in the thermogenic activity of the tissue in the neonatal rat, thus reinforcing the suggestion that local 3',3,5-triiodothyronine generation could be an important event related to thermogenesis in brown adipose tissue. However, the high fetal activity was only slightly related to the thermogenic activity of brown fat. Moreover, the increased iodothyronine 5'-deiodinase activity of brown adipose tissue during fetal and neonatal life suggests a substantial contribution by brown fat in the overall extrathyroidal 3',3,5-triiodothyronine production in these physiological periods.

Reduced noradrenaline turnover in brown adipose tissue of lactating rats

Brown adipose tissue properties as well as noradrenaline turnover in the tissue were determined in 15-day lactating rats and virgin controls. Brown adipose tissue thermogenic activity was reduced in lactating rats as shown by a decrease in weight, cytochrome oxidase activity and mitochondrial GDP-binding. The noradrenaline turnover rate was lower in brown adipose tissue from lactating rats. It is suggested that diminished sympathetic activity in brown adipose tissue may be a major cause of the reduced tissue thermogenic activity during lactation.

Brown adipose tissue activity in hypocaloric-diet fed lactating rats

Hypocaloric diet feeding reduced the mitochondrial protein content and whole tissue GDP-binding in interscapular brown adipose tissue from both virgin and lactating rats. A reduction in brown fat lipoprotein lipase activity was also detected in underfed virgin and lactating animals. These results indicate that lactation in the rat, even though it produces a reduction in brown fat activity, does not impair the capacity of the tissue to respond to a diminished caloric intake by lowering its activity further.