Modulation by insulin and glucagon of noradrenaline-induced activation of isolated brown adipocytes from the rat

1. The effects of insulin (2 nM and 4 nM) upon oxygen consumption (VO2), lipolysis rates and indirectly derived rates of fatty acid utilization, by isolated brown adipocytes from warm-acclimated (W cells) and cold-acclimated (C cells) animals, induced by noradrenaline and glucagon separately and conjointly, are reported. 2. Changes in interrelationships (coupling) between the parameters under different treatment regimes were assessed using bivariate regression analyses. 3. Administration of glucagon with noradrenaline increased lipolysis/fatty acid utilization coupling without concomitant increase of VO2 suggesting that glucagon may increase re-esterification through glycogenolytic generation of glycerol 3-phosphate, trapping intracellular fatty acid in excess of the capacity of disposal mechanisms, thus conserving respiratory substrate. 4. W cells were unresponsive to glucagon in terms of lipolysis and VO2, C cells responded to glucagon with parallel increases in lipolysis rate and VO2. Both cell types responded to noradrenaline alone and conjointly with glucagon; C cells were more sensitive to these agonists than W cells. 5. Lipolysis/VO2 coupling was reduced in C cells suggesting that in cold acclimation, noradrenaline-induced lipolysis rates are in excess of the capacity of cellular oxidation/re-esterification mechanisms. 6. Insulin inhibited noradrenaline and glucagon-induced lipolysis, simultaneously increasing VO2, supporting the hypothesis that glucose may be a thermogenic substrate in brown adipase tissue, permitting concurrent thermogenesis and lipogenesis. C cells were more insulin-sensitive than W cells. 7. The data indicate that insulin may mediate its effects (additively with noradrenaline) by activation of pyruvate dehydrogenase, generating glycolytic flux and, in the presence of noradrenaline-inhibited lipogenesis, generate additional oxaloacetate, permitting increased beta-oxidation.     

Genistein affects lipogenesis and lipolysis in isolated rat adipocytes

Genistein is a phytoestrogen found in several plants eaten by humans and food-producing animals and exerting a wide spectrum of biological activity. In this experiment, the impact of genistein on lipogenesis and lipolysis was studied in isolated rat adipocytes. Incubation of the cells (10⁶ cells/ml in plastic tubes at 37°C with Krebs-Ringer buffer, 90 min) with genistein (0.01, 0.3, 0.6 and 1 mM) clearly restricted (1 nM) [U-¹⁴C]glucose conversion to total lipids in the absence and presence of insulin. When [¹⁴C]acetate was used as the substrate for lipogenesis, genistein (0.01, 0.1 and 1 mM) exerted a similar effect. Thus, the anti-lipogenetic action of genistein may be an effect not only of alteration in glucose transport and metabolism, but this phytoestrogen can also restrict the fatty acids synthesis and/or their estrification. Incubation of adipocytes with estradiol at the same concentrations also resulted in restriction of lipogenesis, but the effect was less marked. Genistein (0.1 and 1 mM) augmented basal lipolysis in adipocytes. This process was strongly restricted by insulin (1 μM) and H-89 (an inhibitor of protein kinase A; 50 μM) and seems to be primarily due to the inhibitory action of the phytoestrogen on cAMP phosphodiesterase in adipocytes. Genistein at the smallest concentration (0.01 mM) augmented epinephrine-stimulated (1 μM) lipolysis but failed to potentiate lipolysis induced by forskolin (1 μM) or dibutyryl-cAMP (1 mM). These results suggest genistein action on the lipolytic pathways before activation of adenylate cyclase. The restriction of lipolysis stimulated by several lipolytic agents – epinephrine, forskolin and dibutyryl-cAMP were observed when adipocytes were incubated with genistein at highest concentrations (0.1 and 1 mM). These results prove the inhibitory action of this phyestrogen on the final steps of the lipolytic cascade, i.e. on protein kinase A or hormone sensitive lipase. Estradiol, added to the incubation medium, did not affect lipolysis. It can be concluded that genistein significantly affects lipogenesis and lipolysis in isolated rat adipocytes.     

Pervanadate [peroxide(s) of vanadate] mimics insulin action in rat adipocytes via activation of the insulin receptor tyrosine kinase

Both vanadate and hydrogen peroxide (H2O2) are known to have insulin-mimetic effects. We previously reported that the mixture of vanadate plus H2O2 results in the generation of a peroxide(s) of vanadate, which strongly enhances IGF-II binding to rat adipocytes (Kadota et al., 1987b). We now report that pervanadate mimics insulin in isolated rat adipocytes to (1) stimulate lipogenesis, (2) inhibit epinephrine-stimulated lipolysis, and (3) stimulate protein synthesis. The efficacy of pervanadate is comparable to that of insulin. However, it is 10(2)-10(3) times more potent than vanadate alone. Exposure of intact rat adipocytes to pervanadate was found to activate the WGA-purified insulin receptor tyrosine kinase assayed with the exogenous substrate poly(Glu80/Tyr20) in a dose-dependent manner to a maximum of 1464% of control at 10(-3) M compared with a maximum insulin effect of 1046% at 10(-6) M. In contrast, in vitro assayed autophosphorylation of the WGA-purified extract was increased 3-fold after exposure of intact cells to insulin but not significantly increased after pervanadate. Furthermore, high concentrations of pervanadate (10(-5) M) inhibited subsequent in vitro added insulin-stimulated autophosphorylation. In vitro addition of pervanadate to WGA-purified receptors could not stimulate autophosphorylation or exogenous tyrosine kinase activity and did not inhibit insulin-stimulated autophosphorylation. Labeling of intact adipocytes with [32P]orthophosphate followed by exposure to 10(-4) M pervanadate increased insulin receptor beta-subunit phosphorylation (7.9 +/- 3.0)-fold, while 10(-7) M insulin and 10(-4) vanadate increased labeling (5.3 +/- 1.8)- and (1.1 +/- 0.2)-fold, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of dietary fat and adipose tissue location on insulin action in young boar adipocytes

1. Regulation of lipogenesis and lipolysis by insulin was studied on adipocytes isolated from 100 kg Large white male pigs. Two adipose tissues were studied: subcutaneous and perirenal. Animals were fed either a control low fat diet or a diet containing 14.7% sunflower seed oil. 2. The cell diameter was higher in the group fed the sunflower diet. 3. De novo lipogenesis was decreased for each adipose tissue in the group fed the sunflower diet. The perirenal site had a higher lipogenic activity than subcutaneous site whatever the diet. 4. Insulin did not significantly stimulate lipogenesis but had an important antilipolytic effect on stimulated lipolysis by isoproterenol. 5. The antilipolytic action of insulin was higher in perirenal adipocytes with the control diet. With the sunflower diet, the decrease was about 54.4% for subcutaneous adipocytes, whereas the inhibition was decreased in perirenal adipocytes. Addition of theophylline reversed the antilipolytic action of insulin. 6. Insulin binding was not affected neither by the dietary fat nor by the adipose tissue location. 7. Absence of de novo lipogenesis stimulation by insulin was not due to an impairment in insulin binding. 8. The different effects of dietary fat and adipose tissue location on the antilipolytic action of insulin could not be explained by a modification of insulin binding but rather by a latter event, probably at a post-insulin binding stage.     

Sex- and depot-specific lipolysis regulation in human adipocytes: interplay between adrenergic stimulation and glucocorticoids

The purpose of this investigation was to explore interactions between adrenergic stimulation, glucocorticoids, and insulin on the lipolytic rate in isolated human adipocytes from subcutaneous and omental fat depots, and to address possible sex differences. Fat biopsies were obtained from 48 nondiabetic subjects undergoing elective abdominal surgery. Lipolysis rate was measured as glycerol release from isolated cells and proteins involved in lipolysis regulation were assessed by immunoblots. Fasting blood samples were obtained and metabolic and inflammatory variables were analyzed. In women, the rate of 8-bromo-cAMP- and isoprenaline-stimulated lipolysis was approximately 2- and 1.5-fold higher, respectively, in subcutaneous compared to omental adipocytes, whereas there was no difference between the two depots in men. Dexamethasone treatment increased the ability of 8-bromo-cAMP to stimulate lipolysis in the subcutaneous depot in women, but had no consistent effects in fat cells from men. Protein kinase A, Perilipin A, and hormone sensitive lipase content in adipocytes was not affected by adipose depot, sex, or glucocorticoid treatment. In conclusion, catecholamine and glucocorticoid regulation of lipolysis in isolated human adipocytes differs between adipose tissue depots and also between sexes. These findings may be of relevance for the interaction between endogenous stress hormones and adipose tissue function in visceral adiposity and the metabolic syndrome.     

Impaired insulin-mediated inhibition of lipolysis and glucose transport with aging

Regulation of hormone action with aging has been extensively studied; adipocytes provide an interesting model for some of these questions. We have compared the ability of insulin to stimulate glucose uptake and suppress lipolysis in adipocytes isolated from two month and twelve month-old rats. The ability of insulin to stimulate maximal glucose transport was decreased in adipocytes from the older rats (P less than 0.001); as well, insulin's EC50 was also higher (P less than 0.01) in these cells. Furthermore, these defects were present when insulin-stimulated glucose transport was measured in the presence or absence of adenosine deaminase which metabolizes endogenously released adenosine. Endogenously released adenosine is a stimulator of glucose transport and an inhibitor of lipolysis. Maximal suppression of isoproterenol-induced lipolysis by insulin was similar when adipocytes isolated from the two age groups were incubated in the absence of adenosine deaminase. However, maximal insulin-mediated suppression of lipolysis was found to be significantly decreased (P less than 0.001) in adipocytes isolated from older rats when the experiments were done in the presence of adenosine deaminase; also, insulin's EC50 was increased in these cells under these conditions (P less than 0.001). These results emphasize the importance of the adenosine receptor in modulating the response of isolated adipocytes to insulin, particularly for lipolysis, and document the presence of age-associated defects in insulin regulation of both glucose transport and lipolysis.     

Mannose, glucosamine and inositol monophosphate inhibit the effects of insulin on lipogenesis. Further evidence for a role for inositol phosphate-oligosaccharides in insulin action.

The mechanism of insulin signalling is not yet understood in detail. Recently, a role for inositol phosphate (IP)-oligosaccharides as second messengers transmitting the insulin signal at the post-kinase level was proposed. To evaluate this hypothesis further, we studied whether IP-oligosaccharides isolated from 'haemodialysate' have insulin-like activity. We found that these compounds mimic, in a dose-dependent fashion, the following effects of insulin in adipocytes. (1) Lipogenesis. Incorporation of [3H]glucose into lipids (expressed in nmol/min per 10(6) cells): basal, 0.74 +/- 0.05; insulin (1 mu unit/ml), 4.43 +/- 0.21; IP-oligosaccharide (2 micrograms/ml), 4.07 +/- 0.19. (2) Inhibition of isoprenaline (isoproterenol) (1 microM)-stimulated cyclic AMP levels and lipolysis. Cyclic AMP (pmol/10(5) cells): basal 0.84 +/- 0.05; isoprenaline, 4.03 +/- 0.19; isoprenaline + insulin (200 mu units/ml), 2.06 +/- 0.7; isoprenaline + IP-oligosaccharides (2 micrograms/ml), 2.4 +/- 0.29. Inhibition of lipolysis (mumol of glycerol/mg of protein): isoprenaline (1 microM), 166 +/- 11; isoprenaline+insulin (150 mu units/ml), 53 +/- 3.5; isoprenaline+IP-oligosaccharides (2 micrograms/ml), 58 +/- 5. (3) Stimulation of 3-O-methylglucose transport; basal, 9 +/- 3%; insulin (1 mu unit/ml), 67 +/- 4%, IP-oligosaccharides (2 micrograms/ml), 54 +/- 2%. To identify the active molecules of the IP-oligosaccharide fraction, competition experiments were performed. IP-oligosaccharide effects on lipogenesis were blocked by inositol monophosphate, glucosamine and mannose. In contrast, these compounds did not inhibit IP-oligosaccharide effects on membrane-mediated functions (3-O-methylglucose transport, cyclic AMP levels, lipolysis). We also found that the effect of insulin on lipogenesis was blocked by mannose, glucosamine and inositol monophosphate, whereas the insulin effects on 3-O-methylglucose, cyclic AMP and lipolysis were unaffected. The following conclusions were reached. (1) IP-oligosaccharides mimic the major metabolic effects of insulin in adipocytes. This is consistent with the proposed role of IP-oligosaccharides as second messengers of certain insulin effects. (2) Mannose and glucosamine are functionally important sugar residues for the effect of IP-oligosaccharide on lipogenesis. (3) The observation that mannose, inositol monophosphate and glucosamine block the action of insulin of on lipogenesis supports a role of mannose- and glucosamine-containing IP-oligosaccharides as second messengers for this insulin effect.     

Insulin-mediated antilipolysis in permeabilized rat adipocytes

Elucidating the mechanism by which insulin inhibits lipolysis has been hampered by the unavailability of a broken cell preparation in which the intact cell responses to the hormone could be duplicated. Here we report, using digitonin-permeabilized rat adipocytes, that physiological concentrations of insulin inhibit cyclic AMP-activated lipolysis despite the absence of cytosolic and plasma membrane integrity. Cyclic AMP (1.0 mM) maximally activates lipolysis in permeabilized adipocytes greater than 10-fold. Insulin inhibits this activation in a biphasic manner with maximum inhibition of 59 +/- 8% (N = 7) at 10(-9) M. At the submaximal concentrations of cyclic AMP (1.0 to 10 microM), insulin (10(-9) M) inhibits lipolysis 80 to 90%. Additionally, the antilipolytic effect of insulin is rapid (3 min) and it is specific, with the relatively inactive desoctapeptide analogue of insulin being three orders of magnitude less inhibitory than native insulin. In contrast to permeabilized cells, intact cells demonstrate only a small lipolytic response to cyclic AMP which is insensitive to insulin. These findings suggest the following about insulin's antilipolytic effects: 1) an intact cell is not required; 2) the intracellular mechanism of action does not require physiological concentrations of the freely diffusible cytosolic components; and 3) a site of insulin action independent of adenylate cyclase may play a major role.     

Epidermal and nerve growth factors manifest antilipolytic and lipogenic activities in isolated rat adipocytes

The effects of epidermal growth factor (EGF) and nerve growth factor (NGF) from mouse submaxillary glands on lipolysis and lipogenesis in isolated rat adipocytes were studied. EGF and NGF at nanomolar concentrations augmented basal lipogenesis. The lipogenic responses to EGF and NGF were additive with a submaximal response induced by insulin but not with that of a maximal response to insulin, indicating a similarity in the mechanisms of action of EGF, NGF and insulin. EGF and NGF also inhibited epinephrine-induced lipolysis. The antilipolytic and lipogenic activities of EGF and NGF were considerably less potent by concentration than those of insulin.     

Resveratrol,a naturally occurring diphenolic compound,affects lipogenesis,lipolysis and the antilipolytic action of insulin in isolated rat adipocytes

Resveratrol is a naturally occurring diphenolic compound exerting numerous beneficial effects in the organism. The present study demonstrated its short-term, direct influence on lipogenesis, lipolysis and the antilipolytic action of insulin in freshly isolated rat adipocytes. In fat cells incubated for 90 min with 125 and 250 μM resveratrol (but not with 62.5 μM resveratrol), basal and insulin-induced lipogenesis from glucose was significantly reduced. The antilipogenic effect was accompanied by a significant diminution of CO₂ release and enhanced production of lactate. The inhibition of glucose conversion to lipids found in the presence of resveratrol was not attenuated by activator of protein kinase C. However, acetate conversion to lipids appeared to be insensitive to resveratrol.In adipocytes incubated for 90 min with epinephrine, 10 and 100 μM resveratrol significantly enhanced lipolysis, especially at lower concentrations of the hormone. However, the lipolytic response to dibutyryl-cAMP, a direct activator of protein kinase A, was unchanged. Further studies demonstrated that, in cells stimulated with epinephrine, 1, 10 and 100 μM resveratrol significantly enhanced glycerol release despite the presence of insulin or H-89, an inhibitor of protein kinase A. The influence of resveratrol on epinephrine-induced lipolysis and on the antilipolytic action of insulin was not abated by the blocking of estrogen receptor and was accompanied by a significant (with the exception of 1 μM resveratrol in experiment with insulin) increase in cAMP in adipocytes. It was also revealed that resveratrol did not change the proportion between glycerol and fatty acids released from adipocytes exposed to epinephrine.Results of the present study revealed that resveratrol reduced glucose conversion to lipids in adipocytes, probably due to disturbed mitochondrial metabolism of the sugar. Moreover, resveratrol increased epinephrine-induced lipolysis. This effect was found also in the presence of insulin and resulted from the synergistic action of resveratrol and epinephrine. The obtained results provided evidence that resveratrol affects lipogenesis and lipolysis in adipocytes contributing to reduced lipid accumulation in these cells.     

Characterization of lipolytic responses of isolated white adipocytes from hamsters.

1. Lipolysis by isolated white adipocytes from hamsters, as measured by glycerol production, was stimulated by corticotropin, isopropylnorepinephrine (INE), norepinephrine, or epinephrine (EPI), in a dose-dependent fashion. 2. Lipolysis was stimulated by five inhibitors of cyclic 3',5'-adenosine monophosphate phosphodiesterase: caffeine, theophylline, 1-methyl-3-isobutyl xanthine, 1-ethyl-4-(isopropylidenehydrazine)-1H-pyrazolo-(3,4,-b)-pyridine-5-carboxylic acid ethyl ester (SQ 20009), and 4-(3,4-dimethoxybenzyl)-2-imidazolidinone (Ro 7-2956). Caffeine-stimulated lipolysis consistently attained higher rates than did hormone-stimulated lipolysis. However, when cells were stimulated by both caffeine and a hormone, lipolytic rates were consistently lower than those attained under the influence of caffeine alone. 3. Isolated white adipocytes from hamsters were sensitive to both alpha- and beta-adrenergic antagonists. The beta-adrenergic antagonist propranolol could completely inhibit norepinephrine-stimulated glycerol production. The alpha-adrenergic antagonist phentolamine, on the other hand, had a biphasic effect on the cells. At 5-10(-7) M or 5-10(-6) M, phentolamine enhanced norepinephrine-stimulated lipolysis, while concentrations higher than 5-10(-5) M caused inhibition. 4. The effects of two different concentrations of six antilipolytic agents, prostaglandin E1, nicotinic acid, phenylisopropyladenosine, 5-methylpyrazole-3-carboxylic acid, adenosine and insulin, were measured. With the exception of insulin, all of these agents showed much more potent inhibition of caffeine-stimulated lipolysis than of hormone-stimulated lipolysis. Insulin, in contrast, showed only modest inhibition of hormone-stimulated lipolysis and virtually no inhibition of caffeine-stimulated lipolysis.     

Effects of insulin and norepinephrine on glucose transport and metabolism in rat brown adipocytes. Potentiation by insulin of norepinephrine-induced glucose oxidation

Glucose is an important fuel for rat brown adipose tissue in vivo and its utilization is highly sensitive to insulin. In this study, the different glucose metabolic pathways and their regulation by insulin and norepinephrine were examined in isolated rat brown adipocytes, using [6-14C]glucose as a tracer. Glucose utilization was stimulated for insulin concentrations in the range of 40-1000 microU/ml. Furthermore, the addition of adenosine deaminase (200 mU/ml) or adenosine (10 microM) did not alter insulin sensitivity of glucose metabolism. The major effect of insulin (1 mU/ml) was a respective 7-fold and 5-fold stimulation of lipogenesis and lactate synthesis, whereas glucose oxidation remained very low. The 5-fold stimulation of total glucose metabolism by 1 mU/ml of insulin was accompanied by an 8-fold increase in glucose transport. In the presence of norepinephrine (8 microM), total glucose metabolism was increased 2-fold. This was linked to a 7-fold increase of glucose oxidation, whereas lipogenesis was greatly inhibited (by 72%). In addition, norepinephrine alone did not modify glucose transport. The addition of insulin to adipocytes incubated with norepinephrine, induced a potentiation of glucose oxidation, while lipogenesis remained very low. In conclusion, in the presence of insulin and norepinephrine glucose is a oxidative substrate for brown adipose tissue. However the quantitative importance of glucose as oxidative fuel remains to be determined.     

Effects of insulin on lipolysis and lipogenesis in adipocytes from genetically obese (ob/ob) mice.

A method for the preparation of isolated adipocytes from obese mice is described. Similar yields of adipocytes (50--60%), as judged by several criteria, are obtained from obese mice and lean controls. Few fat-globules and no free nuclei were observed in cell preparations, which are metabolically active, respond to hormonal control and appear to be representative of intact adipose tissue. Noradrenaline-stimulated lipolysis was inhibited by insulin, equally in adipocytes from lean and obese mice. Inhibition in obese cells required exogenous glucose, and the insulin dose--response curve was shifted to the right. Basal lipogenesis from glucose was higher in adipocytes from obese mice, and the stimulatory effect of insulin was greater in cells from obese mice compared with lean controls. A rightward shift in the insulin dose--response curve was again observed with cells from obese animals. This suggests that adipose tissue from obese mice is insulin-sensitive at the high blood insulin concentrations found in vivo. The resistance of obese mice to the hypoglycaemic effect of exogenous insulin and their impaired tolerance to glucose loading appear to be associated with an impaired insulin response by muscle rather than by adipose tissue.     

In vitro effects of leptin on human adipocyte metabolism

OBJECTIVE: Leptin receptors are expressed in adipocytes, suggesting potential autocrine/paracrine effects. Studies on the direct effects of leptin on adipose tissue metabolism in different species have yielded controversial data. To assess the in vitro effects of leptin on human adipocyte metabolism: lipolysis, the insulin-induced inhibition of lipolysis and lipogenesis were studied in adipocytes obtained from infants and adults. METHODS: Lipolysis was studied by incubating adipocytes with increasing concentrations of leptin or isoprenaline. Glycerol in the incubation medium was measured as an indicator of lipolysis. For the lipogenesis and insulin-induced inhibition of lipolysis experiments, the cells were preincubated with 0, 25, or 250 ng/ml of leptin for 2 h. RESULTS: Leptin did not stimulate lipolysis in human adipocytes, either in children or adults. Preincubation with leptin did not affect the insulin-induced inhibition of lipolysis, but decreased the insulin-induced lipogenesis (p < 0.05). CONCLUSIONS: This study shows that leptin has no direct lipolytic effect in human adipocytes. The lack of effect on the insulin-induced inhibition of lipolysis and the negative effect on lipogenesis indicates that the effect of leptin is not at the proximal insulin-signalling pathway but further downstream.     

Regulation of lipogenesis by glucocorticoids and insulin in human adipose tissue

Patients with glucocorticoid (GC) excess, Cushing's syndrome, develop a classic phenotype characterized by central obesity and insulin resistance. GCs are known to increase the release of fatty acids from adipose, by stimulating lipolysis, however, the impact of GCs on the processes that regulate lipid accumulation has not been explored. Intracellular levels of active GC are dependent upon the activity of 11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1) and we have hypothesized that 11β-HSD1 activity can regulate lipid homeostasis in human adipose tissue (Chub-S7 cell line and primary cultures of human subcutaneous (sc) and omental (om) adipocytes. Across adipocyte differentiation, lipogenesis increased whilst β-oxidation decreased. GC treatment decreased lipogenesis but did not alter rates of β-oxidation in Chub-S7 cells, whilst insulin increased lipogenesis in all adipocyte cell models. Low dose Dexamethasone pre-treatment (5 nM) of Chub-S7 cells augmented the ability of insulin to stimulate lipogenesis and there was no evidence of adipose tissue insulin resistance in primary sc cells. Both cortisol and cortisone decreased lipogenesis; selective 11β-HSD1 inhibition completely abolished cortisone-mediated repression of lipogenesis. GCs have potent actions upon lipid homeostasis and these effects are dependent upon interactions with insulin. These in vitro data suggest that manipulation of GC availability through selective 11β-HSD1 inhibition modifies lipid homeostasis in human adipocytes.     

In vitro insulin-like actions of the growth factor from the tapeworm, Spirometra mansonoides

In vitro actions of purified plerocercoid growth factor (PGF) were compared with those of insulin and human growth hormone (hGH) in adipose tissue from normal male rats. Insulin-like effects were measured by the ability of PGF, insulin, or hGH to stimulate oxidation of [U-14C]glucose to 14CO2, to stimulate lipogenesis, and to inhibit epinephrine-induced lipolysis. PGF and insulin stimulated significant increases in glucose oxidation and lipogenesis in adipose tissue that had not been preincubated as well as in tissue that had been preincubated. hGH stimulated insulin-like effects only in tissue that had been preincubated for 3 hr. Insulin, hGH, and PGF inhibited epinephrine-induced lipolysis of preincubated (3 hr) adipose tissue. hGH produced a dramatic lipolytic response in tissue freshly removed from normal rats but no dose of PGF was lipolytic. PGF did not displace 125I-insulin from its receptors on adipocytes but did competitively inhibit 125I-hGH binding to adipocytes. These results suggest that PGF has direct insulin-like actions which are initiated by binding a GH receptor, but PGF had no anti-insulin action and the insulin-like activity of PGF was unaffected by refractoriness of adipose tissue to GH.     

Interactions of noradrenalin and tumour necrosis factor alpha, interleukin 4 and interleukin 6 in the control of lipolysis from adipocytes around lymph nodes.

The contributions of inflammatory and immunosuppressive cytokines and noradrenalin to the control of lipolysis in adipocytes surrounding and remote from lymph nodes were investigated in healthy adult guinea-pigs. A few hours after excision from fasting animals, spontaneous lipolysis in adipocytes from around the popliteal and mesenteric lymph nodes and omental "milky spots" was significantly lower than in those from elsewhere in the same depots, and much lower than in perirenal, epididymal or parametrial adipocytes. The perinodal adipocytes were consistently more sensitive to noradrenalin at 10(-8), 10(-7)and 10(-5) M, and their maximum rate of lipolysis was higher. They also responded more strongly to pre-incubation for 24 h with tumour necrosis factor alpha interleukin 6 and interleukin 4 than those elsewhere in the same depots. Tumour necrosis factor alpha and interleukin 6 applied alone stimulated lipolysis, but combined with interleukin 4, they suppressed glycerol release, especially in perinodal adipocytes, thereby creating large within-depot differences. These cytokines had minimal effects on lipolysis in perirenal or gonadal adipocytes.The authors conclude that adipocytes surrounding lymph nodes contribute little to whole-body energy supply during fasting, but are more sensitive than all others to cytokines and to noradrenalin, having higher maximum but lower minimum rates of lipolysis. These properties equip perinodal adipocytes for local interactions with lymphoid tissue.     

Adipocyte responses to adrenaline and insulin in active and former sportsmen

The rates of lipolysis and lipogenesis in adipocytes, isolated from biopsy samples of subcutaneous fat, was assessed by estimation of glycerol release during a 30-min incubation, and of the incorporation of 14C-glucose into lipids during a 1-h incubation at 37 degrees C, respectively. The subjects were six highly-qualified, active endurance sportsmen, eight former endurance sportsmen of international class, and six untrained young men. In the active sportsmen the basal rate of lipolysis was about half of that in the previously-active sportsmen and the untrained subjects, but after the addition of adrenaline (10(-4) or 5 x 10(-4) mol.l-1) the lipolysis rate was the highest. No differences were observed in the lipolytic rates in the former sportsmen compared to the untrained subjects. Gases of a comparatively high level of lipogenesis were found in the trained subjects. The addition of insulin (9 microU.ml-1) to isolated adipocytes caused a significant augmentation of individual rates of lipogenesis in the active sportsmen and the untrained persons but not in the previously-active sportsmen. In comparison with the active sportsmen, the previously active sportsmen revealed an increased basal rate of lipolysis and a reduced sensitivity to the lipogenic action of insulin. These findings suggest that these changes may have had significance in avoiding an increase of adipose tissue after a decrease in energy expenditure due to a change in physical activity.     

Phosphorylation of zymogen granule membrane proteins in intact rat pancreatic acinar cells

The comparative effects of insulin and ethanolamine on 14CO2 production and lipid synthesis from [U-14C]-D-glucose in isolated rat adipocytes were studied. Ethanolamine (10 mM) increased 14CO2 production (glucose oxidation) about 5-fold and lipogenesis about 3-fold as compared to the control. Ethanolamine was more efficient than 25 microU/ml insulin regarding both parameters, but it was less efficient than 200 microU/ml insulin in glucose oxidation, and equally potent in lipogenesis. The combination of ethanolamine and insulin was more active than insulin alone. The mechanisms of ethanolamine action include facilitation of glucose transport and increase of pyruvate dehydrogenase activity.     

Dimethylaminopurine inhibits metabolic effects of insulin in primary adipocytes

Dimethylaminopurine (DMAP) has previously been used as an inhibitor of phosphorylation in studies of meiotic events, and more recently to investigate TNFalpha signaling, because of its potential to inhibit activation of c-jun N-terminal kinase (JNK). Here we have addressed the effects of DMAP on metabolic insulin responses in adipocytes and on intracellular insulin signaling molecules. At 100 micromol/L, DMAP completely inhibited the ability of insulin to counteract lipolysis in isolated adipocytes. Insulin-induced lipogenesis and glucose uptake was inhibited to a lesser degree in a concentration-dependent manner starting at 10 micromol/L DMAP. Insulin-induced tyrosine phosphorylation of the insulin receptor was not affected by DMAP. Insulin-induced activation of protein kinase B, a known mediator of insulin action, was not inhibited by 100 micromol/L, but to a low extent by 1 mmol/L DMAP in intact cells. This inhibition was not sufficient to affect activation of the downstream protein kinase B substrate phosphodiesterase 3B. The inhibition of activation of JNK as a possible mechanism whereby DMAP affects insulin-induced antilipolysis, lipogenesis, and glucose uptake, was investigated using the JNK inhibitor SP600125. At 100 micromol/L, SP600125 completely reversed the antilipolytic effect of insulin, as well as partially inhibited insulin-induced lipogenesis and glucose-uptake, indicating that JNK may be involved in mediating these actions of insulin. Inhibition of JNK by DMAP may therefore partly explain the negative impact of DMAP on insulin action in adipocytes.