Response of white adipocyte of mouse and rabbit to catecholamines and ACTH

Summary Hormone stimulated lipolysis of mouse and rabbit adipocytes as measured by both free fatty acid and glycerol release, is proportionally elevated with increase in the adipocyte cAMP level up to 1 nmole/g. The correlation coefficients are 0.94 and 0.97 for FFA/cAMP and glycerol/cAMP respectively. Increments in cAMP greater than 1 nmole/g show no correlation with increase in lipolysis. The release of lipolytic products, glycerol and free fatty acids, from white adipocytes in response to ACTH, epinephrine or morepinephrine was measured using radiochemical assays in short term incubation systems, with cAMP levels measured at the same time and from the same cell sample. Under the conditions studied, epinephrine is a more effective lipolytic hormone than ACTH in mouse adipocyte, and ACTH is more effective than epinephrine in rabbit adipocyte. The effect of catecholamines on the rabbit adipocyte is not modified by phentolamine (10 μM), but it is potentiated by 1-methyl-3-isobutyl xanthine (0.1 mM). The results suggest that cAMP mediates the action of these lipolytic hormones in white adipocytes of mouse and rabbit.     

Some aspects of metabolic adaptations in lipid metabolism during starvation are mimicked by epinephrine in rat adipocytes

1. The effects of fasting on the neutral lipid synthesis to insulin and/or epinephrine in isolated fat cells have been examined using [1-14C]glucose. 2. The ability of adipocytes from starved rats to synthesize fatty acids from both labeled substrates was markedly diminished compared to adipocytes from control rats. 3. The response of lipogenic stimulation to insulin at all concentrations tested was greatly diminished in adipocytes from 24 hr starved rats. 4. [1-14C]glucose utilization rates in the absence or in the presence of insulin were not significantly different in adipocytes from 24 hr starved rats as compared with control adipocytes, although basal and insulin stimulated glyceride-glycerol synthesis were significantly higher in starved adipocytes. 5. Epinephrine acutely inhibited [1-14C]acetate incorporation into fatty acids for insulin-stimulated lipogenesis in control adipocytes, in contrast, this lipolytic agent strongly increased [1-14C]glucose conversion to triacylglycerols. 6. In both cases, the differences in lipid synthesis capacities found in both nutritional states were abolished by epinephrine.     

Studies on the utilization and mobilization of lipid in skeletal muscles from streptozotocin-diabetic and control rats.

Several aspects of lipid metabolism in the soleus and diaphragm muscles of streptozotocin-diabetic and control rats were investigated. The triglyceride content of both muscles was elevated in the diabetic state and the presence of increased intracellular lipid was confirmed by electron microscopy. In vitro glucose and palmitate oxidation studies showed that both types of muscle from the diabetic animals metabolized more fat than did the soleus and diaphragm from control rats. While isoproterenol alone produced a significant lipolytic response in both the soleus and diaphragm from control and diabetic animals, there was no difference in the percent increase in fatty acids released from muscles of diabetic rats compared to controls. However, the absolute difference was greater when the diaphragms were compared. Muscles from experimental and control animals showed a marked reduction in the amount of free fatty acids released in response to insulin. In addition, in the presence of the hormone, both the absolute and percent isoproterenol-stimulated increases in fatty acids were significantly greater for both diaphragm and soleus muscles from diabetic rats. The effects of insulin, isoproterenol, and the combination of these two hormones on the amount of glycerol released into the incubation medium were similar to those found on free fatty acid release. The results of these experiments show that there is an apparent increase in fat utilization in skeletal muscle of diabetic rats. Furthermore, measurements of triglyceride concentration and the enhanced response to isoproterenol stimulation in the muscles from these animals suggests that they may have an increased capacity for mobilization of intracellular lipids. Finally, in the diabetic state, both the soleus and diaphragm appear to demonstrate an increased response to the antilipolytic effect of insulin as measured by the decreased amount of fatty acid released into the incubation medium, the percent change also being significant for the soleus muscle.-Stearns, S. B., H. M. Tepperman, and J. Tepperman. Studies on the utilization and mobilization of lipid in skeletal muscles from streptozotocin-diabetic and control rats.     

The short-term regulation of fatty acid synthesis in the rat epididymal adipocytes

Lipogenesis and fatty acid synthetase (FAS) activity of isolated rat adipocytes that were treated with insulin or epinephrine were studied. Insulin stimulated incorporation of radioactivity from D-[U-14C]glucose into CO2, saponifiable and non-saponifiable fractions, whereas epinephrine promoted lipolysis and oxidation of glucose into CO2. Whereas insulin stimulated fatty acid synthesis, epinephrine had no effect. Changes in FAS specific activity of insulin- or epinephrine-treated adipocytes were insignificant and could not account for insulin-stimulated lipogenesis. Rat adipocyte FAS, unlike hepatic FAS, was not subject to short-term regulation by insulin, although fatty acid synthesis showed such a response.     

Changes in adipocyte cell size, gene expression of lipid metabolism markers, and lipolytic responses induced by dietary fish oil replacement in gilthead sea bream (Sparus aurata L.)

The effects of fish oil (FO) substitution by 66% vegetable oils in a diet with already 75% vegetable protein (66VO) on adipose tissue lipid metabolism of gilthead sea bream were analysed after a 14-month feeding trial. In the last 3 months of the experiment, a FO diet was administrated to a 66VO group (group 66VO/FO) as a finishing diet. Hormone-sensitive lipase (HSL) activity was measured in adipose tissue and adipocyte size, and HSL, lipoprotein lipase and liver X receptor gene expression in isolated adipocytes, on which lipolysis and glucose uptake experiments were also performed. Lipolysis was measured after incubation with tumour necrosis factor-α (TNFα), linoleic acid, and two conjugated linoleic acid isomers. Glucose uptake was analysed after TNFα or insulin administration. Our results show that FO replacement increased lipolytic activity and adipocyte cell size. The higher proportion of large cells observed in the 66VO group could be involved in their observed lower response to fatty acid treatments and lower insulin sensitivity. The 66VO/FO group showed a moderate return to the FO conditions. Therefore, FO replacement can affect the morphology and metabolism of gilthead sea bream adipocytes which could potentially affect other organs such as the liver.     

Regulation of macrophage migration inhibitory factor (MIF) expression by glucose and insulin in adipocytes in vitro.

BACKGROUND: It has been reported that macrophage migration inhibitory factor (MIF) stimulated insulin secretion from pancreatic islet beta-cells in an autocrine manner, which suggests its pivotal role in the glucose metabolism. According to this finding, we evaluated MIF expression in cultured adipocytes and epididymal fat pads of obese and diabetic rats to investigate its role in adipose tissue. MATERIALS AND METHODS: The murine adipocyte cell line 3T3-L1 was used to examine MIF mRNA expression and production of MIF protein in response to various concentrations of glucose and insulin. Epididymal fat pads of Otsuka Long-Evans Tokushima fatty (OLETF) and Wistar fatty rats, animal models of obesity and diabetes, were subjected to Northern blot analysis to determine MIF mRNA levels. RESULTS: MIF mRNA of 3T3-L1 adipocytes was up-regulated by costimulation with glucose and insulin. Intracellular MIF content was significantly increased by stimulation, whereas its content in the culture medium was decreased. When the cells were treated with cytochalasin B, MIF secretion in the medium was increased. Pioglitazone significantly increased MIF content in the culture medium of 3T3-L1 cells. However, MIF mRNA expression of both epididymal fat pads of OLETF and Wistar fatty rats was down-regulated despite a high plasma glucose level. The plasma MIF level of Wistar fatty rats was significantly increased by treatment with pioglitazone. CONCLUSION: We show here that the intracellular glucose level is critical to determining the MIF mRNA level as well as its protein content in adipose tissue. MIF is known to play an important role in glucose metabolism as a positive regulator of insulin secretion. In this context, it is conceivable that MIF may affect the pathophysiology of obesity and diabetes.     

Direct and indirect effects of leptin on adipocyte metabolism

Leptin is hypothesized to function as a negative feedback signal in the regulation of energy balance. It is produced primarily by adipose tissue and circulating concentrations correlate with the size of body fat stores. Administration of exogenous leptin to normal weight, leptin responsive animals inhibits food intake and reduces the size of body fat stores whereas mice that are deficient in either leptin or functional leptin receptors are hyperphagic and obese, consistent with a role for leptin in the control of body weight. This review discusses the effect of leptin on adipocyte metabolism. Because adipocytes express leptin receptors there is the potential for leptin to influence adipocyte metabolism directly. Adipocytes also are insulin responsive and receive sympathetic innervation, therefore leptin can also modify adipocyte metabolism indirectly. Studies published to date suggest that direct activation of adipocyte leptin receptors has little effect on cell metabolism in vivo, but that leptin modifies adipocyte sensitivity to insulin to inhibit lipid accumulation. In vivo administration of leptin leads to a suppression of lipogenesis, an increase in triglyceride hydrolysis and an increase in fatty acid and glucose oxidation. Activation of central leptin receptors also contributes to the development of a catabolic state in adipocytes, but this may vary between different fat depots. Leptin reduces the size of white fat depots by inhibiting cell proliferation both through induction of inhibitory circulating factors and by contributing to sympathetic tone which suppresses adipocyte proliferation. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.     

Effects of epinephrine on glucose transport and metabolism in adipose tissue of normal and hypothyroid rats

Epinephrine increases the oxidation of glucose in adipose tissue even when its lipolytic effects are markedly reduced or abolished by propranolol, nicotinic acid, ouabain, or thyroidectomy. In order to locate the site(s) at which epinephrine stimulates glucose utilization, we studied the effects of epinephrine on the oxidation of various metabolites of glucose. Epinephrine neither increased the production of (14)CO(2) from 1- or 3-(14)C-pyruvate nor affected pyruvate conversion to glyceride-glycerol. To assess the possibility that epinephrine might accelerate the entry of glucose into adipocytes, we studied the accumulation of the nonmetabolized sugar l-arabinose in the intracellular water of adipose tissue. Epinephrine increased arabinose penetration into adipocytes to a degree comparable with that caused by 0.1 mU/ml of insulin. Virtually identical results were obtained in tissues from thyroidectomized rats in which the lipolytic effects of epinephrine were significantly reduced. It is concluded that epinephrine increases glucose oxidation by promoting its entry into adipose tissue and that the effect is independent of lipolysis.     

Fat accumulation in the rat during early pregnancy is modulated by enhanced insulin responsiveness

Insulin sensitivity has been implicated in the variation of fat accumulation in early gestation by as-yet-unknown mechanisms. In the present study, we analyzed the insulin sensitivity of lipolysis and lipogenesis in lumbar adipocytes from rats at 0, 7, 14, and 20 days of gestation. In adipocytes of 7-day pregnant rats, we found a twofold decrease in both beta-agonist (isoproterenol and BRL-37344)-stimulated lipolysis and beta3-adrenoceptor protein but not in lipolysis initiated by forskolin or isobutylmethylxanthine, suggesting a modification of the lipolytic pathway at the receptor level. Whereas adipocytes from 7-day pregnant rats showed a twofold increase in fatty acid synthesis from glucose, those from 20-day pregnant animals displayed a decreased lipogenic activity. Insulin responsiveness of the lipolytic and lipogenic pathways was analyzed by dose-response experiments, giving evidence for the involvement of improved insulin responsiveness in the enhanced lipogenic and reduced lipolytic activities of adipocytes in early pregnancy. In contrast, insulin resistance is responsible for lower antilipolytic and lipogenic actions of insulin in late pregnant animals. In conclusion, the present study shows that enhanced adipose tissue insulin responsiveness during early pregnancy contributes to maternal fat accumulation, whereas decreased insulin responsiveness during late gestation modulates fat breakdown.     

Studies on hormonal regulation of lipolysis and lipogenesis in fat cells of various mammalian species

1. Corticotropin-stimulated lipolysis in adipocytes of rats, mice, hamsters, guinea pigs and rabbits. Melanotropins elicited high lipolytic activity only in guinea pig and rabbit adipocytes. Opiate peptides were active only in rabbit adipocytes. Pituitary and chorionic gonadotropins and somatotropin were lipolytic in guinea pig adipocytes. Other hormones tested including prolactin, somatostatin, substance P, neurotensin, angiotensin II, thyrotropin releasing hormone and pancreatic polypeptide were devoid of lipolytic activity in all of the adipocytes studied. 2. In the rabbit adipocytes gamma-melanotropin was lipolytic only at high doses. At these doses the peptide inhibited the lipolytic response to a high dose of corticotropin. 3. Lipolysis stimulated by vasoactive intestinal peptide and epinephrine in rat adipocytes was antagonized by insulin. The lipolytic hormones corticotropin, epinephrine, vasoactive intestinal peptide and secretin suppressed basal and insulin-stimulated lipogenesis.     

Effects of Arachis hypogaea nutshell extract on lipid metabolic enzymes and obesity parameters

The aim of the present study was to assess the effects of peanut (Arachis hypogaea L.) shell extracts (PSE) on lipases and to evaluate its potential development for the treatment of obesity. The peanut shells were extracted in 95% ethanol, and the extracts were screened for inhibitory effects on pancreatic lipase (PL) and lipoprotein lipase (LPL) activities as well as on lipolysis of 3T3-L1 adipocytes. We also examined in vivo whether PSE could prevent the body weight gain induced by feeding a high-fat diet to male Wistar rats for 12 weeks. PSE inhibits a number of lipases, including PL, LPL and, possibly, hormone sensitive lipase (HSL). PSE-treated Wistar rats showed increased fecal lipid excretion respect to the control group. Body weight and body weight gain, and liver size, were significantly lower in rats fed the high-fat diet with 1% of PSE (w:w diet) than in those fed the high-fat diet alone. The rats treated with PSE showed reduced triacylglycerol content in the liver, as well as the serum glucose and insulin. The inhibitory activity of PSE on the lipid metabolic enzymes and the increase in fecal fat excretion suggests that PSE might be useful as a treatment to reduce the dietary fat absorption. The observed reduction in intracellular lipolytic activity of cultured 3T3-L1 adipocytes may reduce the levels of circulating free fatty acids. The observed effects are likely induced by more than one bioactive component of PSE. The PSE actions may, at least in part, be attributed to the inhibition of fat absorption in the digestive tract and the reduction of the adipocyte lipolysis.     

Effect of aging and cellularity on lipolysis in isolated mouse fat cells

The effects of age and cellularity on lipolysis have been investigated in isolated epididymal fat cells from both Swiss albino mice and Sprague-Dawley rats. No significant lipolytic response to glucagon could be demonstrated with adipocytes from either young or old mice, while glycerol output was increased by this hormone with fat cells from young rats. Larger adipocytes from older mice showed significantly greater isoproterenol-stimulated lipolysis than those from younger animals if the glycerol output was expressed on a per cell basis. However, the lipolytic response per cell appeared to be equivalent in young and old rat adipocytes with either isoproterenol or ACTH-(1-24). In a complete aging study, relationships between body weight, epididymal fat pad weight and cellularity were examined covering the life span of the mouse. ACTH-(1-24)- and dibutyryl cyclic AMP-stimulated lipolysis increased with age and cell size but fell at senescence when adipocyte size diminished. Although an effect of aging per se cannot be ruled out with the experimental techniques used in the present study, a dominant influence of adipocyte size on the lipolytic process was demonstrated.     

Suppressed gene expression of adipocyte resistin in an insulin-resistant rat model probably by elevated free fatty acids.

Resistin, the peptide specifically secreted from adipocytes, is a hormone antagonistic to insulin action and, thus, may serve as a link between human obesity due to adiposity and insulin resistance associated with type 2 diabetes. To test this hypothesis, we studied the gene expression of resistin in adipocytes isolated from rats fed with a fructose diet which induced insulin resistance. Compared to the control rats (C) on a normal chow diet, the fructose-fed rats (F) developed hyperinsulinemia, glucose intolerance, hypertriglyceridemia and hypertension, a profile reminiscent of the syndrome X of patients with non-insulin-dependent diabetes mellitus (NIDDM). The F rats had significantly elevated plasma free fatty acids (FFA), enlarged epididymal fat pads, and increased adipocyte size compared with the C rats. We examined the glucose transport and the relative quantity of resistin mRNA produced in the adipocytes of these two groups of rats. Compared to the C rats, the F rats had a clearly reduced insulin-stimulated glucose transport. The gene expression of resistin and other adipocyte peptides was measured on the mRNA by semiquantitative RT-PCR; the validity of this technique was established in advance with a rat-fasting and then refeeding experiment. The F rats showed a decreased expression of the resistin gene, whereas gene expression of leptin and angiotensinogen in contrast increased. Free fatty acids were found to suppress the expression of resistin gene in normal rat adipocytes. These results demonstrate that an insulin-resistant instance in the fructose diet rat model exists with the decreased gene expression of resistin.     

Metabolic patterns and insulin responsiveness of enlarging fat cells

The rate and pattern of glucose metabolism, basal lipolysis, and intracellular concentration of free fatty acids were determined in isolated epididymal fat cell preparations (mean volume 30-800 pl) from rats on the basis of fat cell number and in relation to the cell volume. The effects of increasing glucose concentrations in the medium and of insulin on the cellular metabolic activities were compared. Expanding fat cell volume correlated positively and significantly (P < 0.001) with the synthesis of glyceride glycerol from glucose (correlation coefficient, r = 0.919), with rates of basal lipolysis (r = 0.663), and with intracellular free fatty acid accumulation (r = 0.796); it correlated negatively and significantly with glucose conversion to glyceride fatty acids (r = -0.814, P < 0.01). The differences in patterns of glucose metabolism and basal lipolysis between small (<100 pl) and large (>400 pl) fat cells were not modified by insulin or by increments in glucose concentration. The results indicate that the reduced capacity of the large fat cells to respond to insulin cannot be attributed solely to a limited capacity of the cells to take up and metabolize increasing amounts of glucose. The acquired unresponsiveness of the large cells to insulin may result from an alteration in the mechanism of action of insulin and may be related to an intracellular metabolic derangement with increased basal lipolysis, free fatty acid accumulation, and accelerated glyceride synthesis resulting from the accumulation of triglyceride.     

Lipoprotn lipase content and triglyceride fatty acid uptake in adipose tissue of rats of differing body weights

Increasing body weight appears to alter lipid metabolism in adipose tissue. We have measured the content of lipoprotein lipase and the uptake of chylomicron triglyceride fatty acids in epididymal fat pads of rats of different weights. In order that the results might be expressed in terms of cell numbers, the relationship between the weights of fat pads and the numbers and volumes of fat cells isolated from them was determined. Highly significant correlations were found between fat pad weight and both the number and the volume of the individual adipocytes. In rats weighing from 140 to 350 g, the increase in the size of fat pads was attributable almost equally to increases in cell size and in cell number. Lipoprotein lipase activity was measured in acetone powders of whole fat pads and of isolated fat cell preparations. With both, lipoprotein lipase activity per cell diminished significantly as the weight of fat tissue increased, i.e., larger fat cells contained less enzyme per cell than smaller cells. The uptake of triglyceride fatty acid radioactivity was measured after incubation of fat pads with radiolabeled rat lymph chylomicrons in flasks containing either buffer alone or with added glucose or glucose plus insulin. The addition of glucose and insulin led to a mean increase of 70% in the uptake of radioactivity, but larger adipocytes were stimulated less than smaller cells. This resulted in a significant negative correlation between the weights of fat pads and the uptake of radioactivity. Enlargement of fat cells also led to a diminution in their capacity to esterify fatty acids.     

Effect of insulin on lipolysis in adipose tissue of rats of different ages

Several authors have not been able to find any antilipolytic effect of insulin in adipose tissue "in vitro". We investigated the possible role of cell size and/or age of donors on this phenomenon. The lipolytic rates (glycerol release per cell) were lower in the small cells of the 4-6 weeks old rats than in the larger cells of the 25-30 weeks old animals; however, the difference disappeared when the data were expressed per unit of cell surface area. Insulin (0.5-50 ng/ml) failed to inhibit both maximally and submaximally noradrenaline stimulated lipolysis in the adipocytes of the young rats, but its antilipolytic action was fully restored by using glucose-free medium. Therefore, at our experimental conditions, a glucose dependent factor, possibly involving the preferential hydrolysis of newly synthetized triglycerides, seems to blunt or to mask the insulin induced inhibition of glycerol release. Relatively higher rates of glucose metabolism and a lower lipolysis in small fat cells might explain the difference in the action of insulin on glycerol release in the adipose tissue of young rats as compared to the older ones.     

Inhibition of adipose S-100 protein release by insulin

The release of S-100 protein brought about in rat epididymal fat pads by 10 microM epinephrine was inhibited by about 50% in the presence of more than 8 nM insulin. The inhibitory effect of insulin was also observed in the release of S-100 protein induced by isoproterenol or adrenocorticotropin (ACTH), but not in the release induced by a high concentration (5 mM) of dibutyryl cyclic AMP. Since insulin suppressed (to about 50%) the increase in cyclic AMP content induced by epinephrine under the same conditions, it is suggested that the inhibitory mechanism is mediated by the cyclic AMP levels in adipocytes. The S-100 protein release induced by catecholamine was significantly decreased (to about 50%) in the fat pads obtained from insulin-injected rats. In contrast, in the fat pads obtained from diabetic or long-term starved rats, the S-100 protein release was greatly enhanced, showing several-fold higher levels of basal release in the absence of hormones, and S-100 protein contents in the epididymal adipose tissues of these rats were significantly lower than those of the control rats. These results suggest that the S-100 protein content in adipocytes is regulated by insulin as well as the lipolytic hormones.     

Cellular adaptations in fat tissue of exercise-trained miniature swine: role of excess energy intake.

This study examined the influence of energy expenditure and energy intake on cellular mechanisms regulating adipose tissue metabolism. Twenty-four swine were assigned to restricted-fed sedentary, restricted-fed exercise-trained, full-fed sedentary, or full-fed exercise-trained groups. After 3 mo of treatment, adipocytes were isolated and adipocyte size, adenosine A(1) receptor characteristics, and lipolytic sensitivity were measured. Swine were infused with epinephrine during which adipose tissue extracellular adenosine, plasma fatty acids, and plasma glycerol were measured. Results revealed that adipocytes isolated from restricted-fed exercised swine had a smaller diameter, a lower number of A(1) receptors, and a greater sensitivity to lipolytic stimulation, compared with adipocytes from full-fed exercised swine. Extracellular adenosine levels were transiently increased on infusion of epinephrine in adipose tissue of restricted-fed exercised but not full-fed exercised swine. These results suggest a role for adenosine in explaining the discrepancy between in vitro and in vivo lipolysis findings and underscore the notion that excess energy intake dampens the lipolytic sensitivity of adipocytes to beta-agonists and adenosine, even if accompanied by exercise training.     

Cholesterol, a cell size-dependent signal that regulates glucose metabolism and gene expression in adipocytes

Enlarged fat cells exhibit modified metabolic capacities, which could be involved in the metabolic complications of obesity at the whole body level. We show here that sterol regulatory element-binding protein 2 (SREBP-2) and its target genes are induced in the adipose tissue of several models of rodent obesity, suggesting cholesterol imbalance in enlarged adipocytes. Within a particular fat pad, larger adipocytes have reduced membrane cholesterol concentrations compared with smaller fat cells, demonstrating that altered cholesterol distribution is characteristic of adipocyte hypertrophy per se. We show that treatment with methyl-beta-cyclodextrin, which mimics the membrane cholesterol reduction of hypertrophied adipocytes, induces insulin resistance. We also produced cholesterol depletion by mevastatin treatment, which activates SREBP-2 and its target genes. The analysis of 40 adipocyte genes showed that the response to cholesterol depletion implicated genes involved in cholesterol traffic (caveolin 2, scavenger receptor BI, and ATP binding cassette 1 genes) but also adipocyte-derived secretion products (tumor necrosis factor alpha, angiotensinogen, and interleukin-6) and proteins involved in energy metabolism (fatty acid synthase, GLUT 4, and UCP3). These data demonstrate that altering cholesterol balance profoundly modifies adipocyte metabolism in a way resembling that seen in hypertrophied fat cells from obese rodents or humans. This is the first evidence that intracellular cholesterol might serve as a link between fat cell size and adipocyte metabolic activity.     

Regulation of lipolysis in isolated adipocytes of rainbow trout (Oncorhynchus mykiss): the role of insulin and glucagon

In the present study, we have examined the effects of insulin and glucagon on the lipolysis of rainbow trout (Oncorhynchus mykiss). To this end, adipocytes were isolated from mesenteric fat and incubated in the absence (basal lipolysis) or presence of different concentrations of insulin and glucagon. In addition, to further elucidate the effects of these hormones in vivo on adipocyte lipolysis, both fasting and intraperitoneal glucagon injection experiments were performed. Basal lipolysis, measured as the glycerol released in the adipocyte medium, increased proportionally with cell concentration and incubation time. Cell viability was verified by measuring the release of lactate dehydrogenase (LDH) activity in the medium. Insulin (at doses of 35 and 350 nM) decreased lipolysis in isolated adipocytes of rainbow trout in vitro, while glucagon was clearly lipolytic at concentrations of 10 and 100 nM. Furthermore, hypoinsulinemia induced by fasting, as well as glucagon injection, significantly increased lipolysis in isolated adipocytes approximately 1.5- and 1.4-fold, respectively, when compared with adipocytes from control fish. Our data demonstrate that lipolysis, as measured in isolated adipocytes of rainbow trout, can be regulated by both insulin and glucagon. These results not only indicate that insulin is an important hormone in lipid deposition via its anti-lipolytic effects on rainbow trout adipocytes, but also reveal glucagon as a lipolytic hormone, as shown by both in vitro and in vivo experiments.