Metabolism of Different Adipose Tissues in Vivo in the Rat

Circulating Dopamine Is Regulated by Dietary Glucose and Controls Glucagon-like 1 Peptide Action in White Adipose Tissue

Dopamine directly acts in the liver and white adipose tissue (WAT) to regulate insulin signaling, glucose uptake, and catabolic activity. Given that dopamine is secreted by the gut and regulates insulin secretion in the pancreas, we aimed to determine its regulation by nutritional cues and its role in regulating glucagon-like peptide 1 (GLP-1) action in WAT. Solutions with different nutrients were administered to Wistar rats and postprandial dopamine levels showed elevations following a mixed meal and glucose intake. In high-fat diet-fed diabetic Goto-Kakizaki rats, sleeve gastrectomy upregulated dopaminergic machinery, showing the role of the gut in dopamine signaling in WAT. Bromocriptine treatment in the same model increased GLP-1R in WAT, showing the role of dopamine in regulating GLP-1R. By contrast, treatment with the GLP-1 receptor agonist Liraglutide had no impact on dopamine receptors. GLP-1 and dopamine crosstalk was shown in rat WAT explants, since dopamine upregulated GLP-1-induced AMPK activity in mesenteric WAT in the presence of the D2R and D3R inhibitor Domperidone. In human WAT, dopamine receptor 1 (D1DR) and GLP-1R expression were correlated. Our results point out a dietary and gut regulation of plasma dopamine, acting in the WAT to regulate GLP-1 action. Together with the known dopamine action in the pancreas, such results may identify new therapeutic opportunities to improve metabolic control in metabolic disorders.

Reduction of litter size during lactation in rats greatly influences fatty acid profiles in dams

This study aimed to determine in lactating rats how fatty acid profiles are affected by litter size. On day 2 after parturition, litters of lactating rats were adjusted to a normal litter size of 9 pups/dam (NL) or to a small litter of 4 pups/dam (SL), and dams were studied at day 21 of lactation. Plasma glucose, insulin, and docosahexaenoic acid (DHA) concentrations were higher in SL than in NL dams, whereas the concentrations of most other fatty acids, triacylglycerols (TAG), and non-esterified fatty acids were lower in the SL dams. In the liver, the concentration of TAG was lower in SL than in NL dams as was the concentration of most fatty acids, with the exception of stearic acid (STA), arachidonic acid (ARA), and DHA concentrations that were higher in SL. Both plasma and liver Δ⁹ desaturase indices were lower in SL than in NL dams, whereas both Δ⁵ and Δ⁶ desaturase indices were higher in SL dams. In the liver, the expression of acetyl CoA carboxylase was lower in SL than in NL dams, and among the different adipose tissue depots, only mesenteric adipose tissue showed a higher concentration of most fatty acids in SL than in NL dams. It is proposed that reduction of litter size during lactation decreases liver lipogenesis de novo, although the synthesis of long-chain polyunsaturated fatty acids from their corresponding precursors increases, and lipolytic activity in mesenteric adipose tissue decreases probably as result of increased insulin responsiveness.

Epigenetic Regulation of Adipogenesis in Development of Metabolic Syndrome

Obesity is one of the biggest public health concerns identified by an increase in adipose tissue mass as a result of adipocyte hypertrophy and hyperplasia. Pertaining to the importance of adipose tissue in various biological processes, any alteration in its function results in impaired metabolic health. In this review, we discuss how adipose tissue maintains the metabolic health through secretion of various adipokines and inflammatory mediators and how its dysfunction leads to the development of severe metabolic disorders and influences cancer progression. Impairment in the adipocyte function occurs due to individuals' genetics and/or environmental factor(s) that largely affect the epigenetic profile leading to altered gene expression and onset of obesity in adults. Moreover, several crucial aspects of adipose biology, including the regulation of different transcription factors, are controlled by epigenetic events. Therefore, understanding the intricacies of adipogenesis is crucial for recognizing its relevance in underlying disease conditions and identifying the therapeutic interventions for obesity and metabolic syndrome.

Non-invasive Assessments of Adipose Tissue Metabolism In Vitro

Adipose tissue engineering is a diverse area of research where the developed tissues can be used to study normal adipose tissue functions, create disease models in vitro, and replace soft tissue defects in vivo. Increasing attention has been focused on the highly specialized metabolic pathways that regulate energy storage and release in adipose tissues which affect local and systemic outcomes. Non-invasive, dynamic measurement systems are useful to track these metabolic pathways in the same tissue model over time to evaluate long term cell growth, differentiation, and development within tissue engineering constructs. This approach reduces costs and time in comparison to more traditional destructive methods such as biochemical and immunochemistry assays and proteomics assessments. Towards this goal, this review will focus on important metabolic functions of adipose tissues and strategies to evaluate them with non-invasive in vitro methods. Current non-invasive methods, such as measuring key metabolic markers and endogenous contrast imaging will be explored.

Tail skin temperatures reflect coping styles in rats

This study was carried out to elucidate the predictive value of tail skin temperatures (TSTs) assessed in naïve rats as a non-invasive pre-experimental method of classification of coping style. Male Lewis rats were classified according to tail skin temperatures (TST), and relative size within cage-groups. TSTs were monitored over two-hour periods following exposure to physical and emotional stressors. Bodyweight-shifts associated to the experiments were analysed. Six organs of neuroendocrine relevance to allostasis were weighed. Challenge-specific TST-profiles were size-related and consistent with proactive or reactive coping. Pro-active (A) rats showed a more pronounced TST-response to unknown conspecifics, but reactive (B) rats to environmental novelties. B-rats showed challenge-specific weight-losses while A-rats gained more after experiments. Second size males showed rapidly decreased TSTs (vasoconstriction) after nociceptive stimulation. Males that showed the highest basal TSTs and weight-loss in emotionality tests had lost a first rank position during a pre-experimental period, suggesting long-lasting effects of social defeat. Pre-experimental growth correlated positively to adiposisity post-experimentally, but negatively to testes relative weight in B-rats. Scaling effects explained heart-size in B-rats and pituitary-size in A-rats. The overall patterns that emerged, in factor analyses including organ sizes, were consistent with pro-active coping in A-rats and reactive in B-rats. Our results, controlling for rank-effects, suggest that non-invasively assessed TSTs may predict individual stress-coping phenotypes pre-experimentally in rats housed in groups.

Hepatic branch vagotomy, like insulin replacement, promotes voluntary lard intake in streptozotocin-diabetic rats

Although high insulin concentrations reduce food intake, low insulin concentrations promote lard intake over chow, possibly via an insulin-derived, liver-mediated signal. To investigate the role of the hepatic vagus in voluntary lard intake, streptozotocin-diabetic rats with insulin or vehicle replaced into either the superior mesenteric or jugular veins received a hepatic branch vagotomy (HV) or a sham operation. All rats received a pellet of corticosterone that clamped the circulating steroid at moderately high concentrations to enhance lard intake. After 5 d of recovery, rats were offered the choice of lard and chow for 5 d. In streptozotocin-diabetic rats, HV, like insulin replacement, restored lard intake to nondiabetic levels. Consequently, this reduced chow intake without affecting total caloric intake, and insulin site-specifically increased white adipose tissue weight. HV also ablated the effects of insulin on reducing circulating glucose levels and attenuated the streptozotocin-induced weight loss in most groups. Collectively, these data suggest that the hepatic vagus normally inhibits lard intake and can influence glucose homeostasis and the pattern of white adipose tissue deposition. These actions may be modulated by insulin acting both centrally and peripherally.

Depot-specific modulation of rat intraabdominal adipose tissue lipid metabolism by pharmacological inhibition of 11beta-hydroxysteroid dehydrogenase type 1

The metabolic consequences of visceral obesity have been associated with amplification of glucocorticoid action by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) in adipose tissue. This study aimed to assess in a rat model of diet-induced obesity the effects of pharmacological 11beta-HSD1 inhibition on the morphology and expression of key genes of lipid metabolism in intraabdominal adipose depots. Rats fed a high-sucrose, high-fat diet were treated or not with a specific 11beta-HSD1 inhibitor (compound A, 3 mg/kg.d) for 3 wk. Compound A did not alter food intake or body weight gain but specifically reduced mesenteric adipose weight (-18%) and adipocyte size, without significantly affecting those of epididymal or retroperitoneal depots. In mesenteric fat, the inhibitor decreased (to 25-50% of control) mRNA levels of genes involved in lipid synthesis (FAS, SCD1, DGAT1) and fatty acid cycling (lipolysis/reesterification, ATGL and PEPCK) and increased (30%) the activity of the fatty acid oxidation-promoting enzyme carnitine palmitoyltransferase 1. In striking contrast, in the epididymal depot, 11beta-HSD1 inhibition increased (1.5-5-fold) mRNA levels of those genes related to lipid synthesis/cycling and slightly decreased carnitine palmitoyltransferase 1 activity, whereas gene expression remained unaffected in the retroperitoneal depot. Compound A robustly reduced liver triacylglycerol content and plasma lipids. The study demonstrates that pharmacological inhibition of 11beta-HSD1, at a dose that does not alter food intake, reduces fat accretion specifically in the mesenterical adipose depot, exerts divergent intraabdominal depot-specific effects on genes of lipid metabolism, and reduces steatosis and lipemia.

Visceral obesity: a "civilization syndrome"

The controversial question of the relationship between obesity and disease has been considerably clearer after the demonstration in several prospective, epidemiological studies that the subgroup of central, visceral obesity is particularly prone to develop cardiovascular disease, stroke, and non-insulin dependent diabetes mellitus. Visceral obesity is associated with multiple central endocrine aberrations. The hypothalamo-adrenal axis is apparently sensitive to stimuli, sex steroid hormone secretion blunted, and hyperandrogenicity is found in women. In addition, there seem to be signs of central dysfunctions in the regulation of hemodynamic factors after stress, and growth hormone secretion appears to be particularly blunted. Several of these endocrine abnormalities are associated with insulin resistance, particularly glycogen synthesis in muscle. Fiber composition with low type I/type II ratio might be secondary to the prevailing hyperinsulinemia, but low capillary density in muscle may well be of importance. In combination with elevated turn-over of free fatty acids (FFA) this will probably provide powerful mechanisms whereby insulin resistance is created. Portal FFA, from the highly lipolytic visceral depots may, in addition, affect hepatic metabolism to induce increased gluconeogenesis, production of very low density lipoproteins as well as to perhaps inhibit clearance of insulin. By these mechanisms a Metabolic Syndrome Visceral adipocytes seem to have a high density of several steroid hormone receptors, directing steroid hormone effects particularly to these depots. The net effect of cortisol is apparently a stimulation of lipid storage, with opposing effects of sex steroid hormones which also facilitate lipid mobilization, regulations most often found at the gene transcription level. Growth hormone inhibits cortisol effects on lipid accumulation, and amplifies the lipid mobilizing effects of steroid hormones. The combined perturbations of hormonal secretions will therefore probably direct triglycerides toward visceral depots. Circulatory and nervous regulatory mechanisms require, however, more attention. The multiple central endocrine and nervous aberrations of visceral obesity suggest neuroendocrine dysregulations, and have features characteristic of the hypothalamic arousal seen after certain types of stress, alcohol intake, and smoking. Such factors can be traced to subjects with visceral fat accumulation. Standardized stress, eliciting a "defeat reaction" in primates is followed by an apparently identical syndrome. This integrated picture of the multiple symptoms of visceral obesity is based on epidemiological, clinical, experimental, cellular, and molecular evidence. The ingredients of positive energy balance, including physical inactivity, stress, smoking, and alcohol consumption are frequent features of modern, urbanized society. Visceral obesity may therefore be an expression of a "Civilization Syndrome."

Subcutaneous lipectomy causes a metabolic syndrome in hamsters

The insulin resistance syndrome X is related to excess intra-abdominal adipose tissue. With lipectomy of >50% of subcutaneous adipose tissue (SQAT) in nonhibernating, adult female Syrian hamsters on high-fat (HF; 50 calorie%) diet and measurements of oral glucose tolerance, oral [(14)C]oleic acid disposal, serum triglycerides, serum leptin, liver fat, perirenal (PR) adipose tissue cellularity, and body composition, we studied the role of SQAT. Sham-operated (S) animals on HF or low-fat (LF; 12.5 calorie%) diets served as controls. After 3 mo there was no visible regrowth of SQAT but HF diet led to similar levels of body weight and body fat in lipectomized and sham-operated animals. Lipectomized (L) animals had more intra-abdominal fat as a percentage of total body fat, higher insulinemic index, a strong trend toward increased liver fat content, and markedly elevated serum triglycerides compared with S-HF and S-LF. Liver and PR adipose tissue uptake of fatty acid were similar in L-HF and S-HF but reduced vs. S-LF, and were inversely correlated with liver fat content and insulin sums during the oral glucose tolerance test. In summary, lipectomy of SQAT led to compensatory fat accumulation implying regulation of total body fat mass. In conjunction with HF diet these lipectomized hamsters developed a metabolic syndrome with significant hypertriglyceridemia, relative increase in intra-abdominal fat, and insulin resistance. We propose that SQAT, via disposal and storage of excess ingested energy, acts as a metabolic sink and protects against the metabolic syndrome of obesity.

Growth hormone treatment of hypophysectomized rats increases catecholamine-induced lipolysis and the number of beta-adrenergic receptors in adipocytes: no differences in the effects of growth hormone on different fat depots

Growth hormone (GH) has a lipolytic effect in adipose tissue but this effect may differ in adipose tissue from various fat depots. This latter possibility was investigated in the present study, in which the effects of GH in vivo on catecholamine-induced lipolysis and the number of beta-adrenergic receptors in isolated adipocytes from different fat depots of hypophysectomized rats were investigated. Female and male Sprague-Dawley rats were hypophysectomized or sham-operated at 45 days of age. One week after the operation, hormonal replacement therapy with L-thyroxine and hydrocortisone acetate was given. In addition, groups of rats were treated with GH (1.33 mg/kg per day, given as two daily subcutaneous injections). After 1 week of hormonal treatment, adipocytes were isolated from the parametrial, epididymal and inguinal fat pads, and glycerol release after catecholamine-stimulation and 125I-cyanopindolol binding were measured. Hypophysectomy resulted in a marked decrease in the lipolytic response to catecholamines. GH treatment significantly increased catecholamine-induced lipolysis with similar effects in adipocytes from parametrial or epididymal and inguinal fat depots in both female and male rats. There were no differences between norepinephrine compared with isoproterenol-induced responses. 125I-cyanopindolol binding was reduced after hypophysectomy and normalized by GH treatment, without differences between parametrial and inguinal adipose tissue regions. We conclude that the lipolytic effects of GH in the rat may partly be mediated by a stimulatory effect on beta-adrenergic receptors in adipocytes. In addition, GH exerted similar effect on catecholamine induced lipolysis and beta-adrenergic receptors in adipocytes from parametrial, epididymal and inguinal fat depots.

Beta-adrenergic responsiveness of adenylate cyclase in human adipocyte plasma membranes in obesity and after massive weight reduction

The aim of this study was to find out how beta-adrenergic responsiveness of adipocytes is altered in obesity and by weight loss and to investigate what mechanisms lead to potential alterations in responsiveness. Crude plasma membranes were prepared from adipocytes of massively obese and normal-weight individuals, as well as previously obese patients that had lost an average of 38% of their initial weight after bariatric surgery. Stimulation of adenylate cyclase by isoproterenol varied considerably in fat cell plasma membranes from different individuals. Crude fat cell plasma membranes from obese patients were less responsive to isoproterenol than those from normal-weight subjects, whereas those from postgastroplasty patients were hyperresponsive. The response was correlated negatively with cell size and positively with beta-adrenergic receptor density and with the ratio of beta-receptors and stimulatory G-proteins (Gs). There was no correlation with Gs content. However, differences in receptor density between small and large cells or normal-weight, obese, and post-bypass patients could not explain the observed differences in responsiveness to isoproterenol between the different groups.

Triglyceride uptake in muscles in rats

Exogenous lipid is assimilated with different priorities in adipose tissue regions and varies in the fasting and fed conditions. The quantitative role of uptake of lipid in muscle has not been evaluated. In order to examine the uptake in other than adipose tissues, U14C-oleic acid in sesame oil was administered orally to conscious rats, and lipid label measured after different times in serum, heart, liver, mesenteric, retroperitoneal, inguinal and epididymal fat pads, as well as in red and white parts of gastrocnemius, extensor digitorum longus and soleus muscles. Lipid uptake in total adipose tissue was calculated from dissected adipose tissues plus lipids extracted from the eviscerated, skinned carcass. Lipid uptake in total muscle tissue was estimated from label in dissected muscles plus that in the carcass, assuming similar intracellular lipid contents and radioactivity as that averaged from dissected muscles. Lipid uptake in the liver was calculated from directly extracted lipid. Four hours after lipid administration to fed rats lipid radioactivity in heart and serum was minimal and had essentially disappeared at 8 hours. Liver label declined rapidly from peak values at or before 4 hours. Adipose tissue radioactivity increased gradually up to 16 hours and then decreased. Label in muscles was highest at 4 hours in the red gastrocnemius, and then decreased, while the other muscles showed a constant radioactivity over the observation period (24 hours). Radioactivity expressed per unit muscle mass seemed to be proportional to the oxidative capacity of muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Androgen hormone binding to adipose tissue in rats

Nuclear binding of androgen was examined, using R 1881, a synthetic androgen. The amount of androgen-receptor complexes bound to isolated nuclei was determined in isolated adipocytes from the epididymal (Epi), retroperitoneal (Ret), inguinal (Ing) and mesenteric (Mes) adipose tissues from intact and castrated rats. The binding was specific and saturable with a Kd in the nanomolar range. Binding was examined after 2 days and after 1 and 2 weeks after castration, showing a higher binding in the Mes tissue in comparison with Ing at all time-points (P < 0.05). Mes adipocytes showed a trend (0.05 < P < 0.1) to up-regulate their binding capacity 2 days after castration, and a significant (P < 0.05) downregulation 2 weeks after castration. Two days after castration, R 1881 binding, expressed per mg triacylglycerol (TG), was generally higher in the Mes region (P < 0.05). This was not fully significant in comparison with Epi tissue in intact rats. When expressed per cell the differences were somewhat diminished, due to differences in cell sizes. Androgen binding showed a negative correlation with TG-uptake in vivo (r = 0.85, P < 0.01), suggesting that a higher density of androgen receptors leads to a more inhibited lipid uptake. In conclusion, a specific androgen receptor was demonstrated in adipose tissue in rat, showing regional differences and a negative correlation with the lipid accumulation of the tissue.

Effects of testosterone on triglyceride uptake and mobilization in different adipose tissues in male rats in vivo

The effects of testosterone (T) on uptake and mobilization of orally administered triglyceride were examined in male rats. In order to attempt to explain regional differences, adipose tissue metabolism was studied in vivo. (U-14 C) oleic acid in sesame oil was given by gastric gavage to male, sham operated, castrated and castrated + T substituted rats, and accumulation and half-life of radioactivity measured. In castrated rats in comparisons with sham-operated and castrated + T rats, serum T was absent, and body weight lower (p < 0.05 or 0.01), but adipocytes in retroperitoneal and mesenteric tissues became significantly heavier. Radioactivity (dpm/mg triglyceride) was higher, in retroperitoneal tissue at 4 hours, 7, 30 days, and in mesenteric tissue at 4 hours, and at 30 and 60 days after oral label administration (0.1 > p > 0.05 or p < 0.05), no differences were seen in epididymal or inguinal depots at 4 hours. When radioactivity was expressed per adipocyte, the castrated group showed significantly higher radioactivity when compared to sham and castrated + T groups at 7 and 30 days in retroperitoneal and at 60 days in mesenteric adipocytes (p < 0.05 or 0.01). Half life (T 1/2) of radioactivity was longer in mesenteric tissue in the castrated rats than the other two groups (sham group, 33 days +/- 2; castrated group, 58 days +/- 6; and castrated + T group, 39 days +/- 3, p < 0.05), but there were no differences between groups in retroperitoneal adipose tissue.(ABSTRACT TRUNCATED AT 250 WORDS)