Genipin ameliorates diet-induced obesity via promoting lipid mobilization and browning of white adipose tissue in rats.

Genipin is the major active component of Gardeniae fructus and has been shown to ameliorate diabetes and insulin resistance in rat models. In this study, we first investigated the effect of genipin on obesity and the related lipid metabolism mechanisms in diet-induced obese rats. Our results showed that genipin reduced body weight, food intake, and visceral fat mass; ameliorated dyslipidemia, glucose intolerance, insulin intolerance, adipocyte hypertrophy, and hepatic steatosis; and reduced serum tumor necrosis factor-α level in diet-induced obese rats. Quantitative real-time reverse-transcription polymerase chain reaction results further illustrated that genipin promoted lipolysis and ß-oxidation of fatty acid by upregulating gene expressions of hormone-sensitive lipase and adipose triglyceride lipase in white adipose tissue (WAT) and peroxisome proliferator-activated receptor-α and carnitine palmitoyltransferase 1α in hepatic tissue. Moreover, genipin promoted browning of WAT by upregulating the mRNA and protein levels of uncoupling protein 1 and PRD1-BF1-RIZ1 homologous domain containing 16 in WAT. Additionally, genipin inhibited gene expressions of activin receptor-like kinase 7, tumor necrosis factor-α, and interlukin-6 in WAT. These results indicated that genipin had a potential therapeutic role in obesity, in which regulation of lipid mobilization and browning of WAT were involved.

Short-term fasts increase levels of halogenated flame retardants in tissues of a wild incubating bird.

Many species are adapted for fasting during parts of their life cycle. For species undergoing extreme fasts, lipid stores are mobilized and accumulated contaminants can be released to exert toxicological effects. However, it is unknown if short-term fasting events may have a similar effect. The objective of this study was to determine if short successive fasts are related to contaminant levels in liver and plasma of birds. In ring-billed gulls (Larus delawarensis), both members of the pair alternate between incubating the nest for several hours (during which they fast) and foraging, making them a useful model for examining this question. Birds were equipped with miniature data loggers recording time and GPS position for two days to determine the proportion and duration of time birds spent in these two activities. Liver and plasma samples were collected, and halogenated flame retardants (HFRs) (PBDEs and dechlorane plus) and organochlorines (OCs) (PCBs, DDTs, and chlordane-related compounds) were determined. Most birds (79%) exhibited plasma lipid content below 1%, indicating a likely fasted state, and plasma lipid percent declined with the number of hours spent at the nest site. The more time birds spent at their nest site, the higher were their plasma and liver concentrations of HFRs. However, body condition indices were unrelated to either the amount of time birds fasted at the nest site or contaminant levels, suggesting that lipid mobilization might not have been severe enough to affect overall body condition of birds and to explain the relationship between fasting and HFR concentrations. A similar relationship between fasting and OC levels was not observed, suggesting that different factors are affecting short-term temporal variations in concentrations of these two classes of contaminants. This study demonstrates that short fasts can be related to increased internal contaminant exposure in birds and that this may be a confounding factor in research and monitoring involving tissue concentrations of HFRs in wild birds.

Panax red ginseng extract regulates energy expenditures by modulating PKA dependent lipid mobilization in adipose tissue.

Regulation of balance between lipid accumulation and energy consumption is a critical step for the maintenance of energy homeostasis. Here, we show that Panax red ginseng extract treatments increased energy expenditures and prevented mice from diet induced obesity. Panax red ginseng extracts strongly activated Hormone Specific Lipase (HSL) via Protein Kinase A (PKA). Since activation of HSL induces lipolysis in WAT and fatty acid oxidation in brown adipose tissue (BAT), these results suggest that Panax red ginseng extracts reduce HFD induced obesity by regulating lipid mobilization.

Lipolysis: cellular mechanisms for lipid mobilization from fat stores.

The perception that intracellular lipolysis is a straightforward process that releases fatty acids from fat stores in adipose tissue to generate energy has experienced major revisions over the last two decades. The discovery of new lipolytic enzymes and coregulators, the demonstration that lipophagy and lysosomal lipolysis contribute to the degradation of cellular lipid stores and the characterization of numerous factors and signalling pathways that regulate lipid hydrolysis on transcriptional and post-transcriptional levels have revolutionized our understanding of lipolysis. In this review, we focus on the mechanisms that facilitate intracellular fatty-acid mobilization, drawing on canonical and noncanonical enzymatic pathways. We summarize how intracellular lipolysis affects lipid-mediated signalling, metabolic regulation and energy homeostasis in multiple organs. Finally, we examine how these processes affect pathogenesis and how lipolysis may be targeted to potentially prevent or treat various diseases.

Co-Administration of Soluble Fibres and Lactobacillus casei NCDC19 Fermented Milk Prevents Adiposity and Insulin Resistance Via Modulation of Lipid Mobilization Genes in Diet-Induced Obese Mice.

BACKGROUND: Numerous reports explaining the beneficial health effects of soluble fibres and probiotics on lifestyle disorders have been published. However, a little information is available on coadministration of soluble fibres such as gum acacia & inulin and probiotic lactobacilli. Therefore, in the present study, we have evaluated the synergistic effects of soluble fibres and probiotic fermented milk on adiposity, insulin resistance and dyslipidemia in C57BL/6 mice fed high-fat and sucrose diet for 18 weeks. OBJECTIVE: To explore the synergistic effect of soluble fibres (gum acacia/inulin) and Lactobacillus casei NCDC19 fermented milk on adiposity, insulin resistance and lipid mobilization genes in dietinduced obese mice. METHODS: C57BL/6 mice were divided randomly into three groups (n = 9/group) according to their body weights. The HFS group was fed high-fat and sucrose diet, the HFS-GFM group was fed HFS diet incorporated with gum acacia (7%, w/w) along with L. casei NCDC19 fermented milk and HFSIFM group was fed HFS diet incorporated with inulin (7%, w/w) along with L. casei NCDC19 fermented milk. RESULTS: At the end of the experiment, final body weight, epididymal fat (E.fat) weight, and adipocyte size were found to be lower in groups received either gum acacia or inulin in combination with L. casei NCDC19 fermented milk (HFS-GFM or HFS-IFM). Also, fasting blood glucose, serum insulin, triglycerides, and VLDL-cholesterol levels were decreased significantly in both HFS-GFM and HFSIFM fed groups. Furthermore, relative mRNA expression of genes (cpt1, foxa2, pgc1ß, and pparα) related to fatty acid oxidation enhanced significantly in the liver. In E.fat pad, expression of adiponectin was upregulated, whereas, leptin expression was reduced considerably. Also, expression of fasting-induced adipose factor enhanced significantly in the distal ileum of mice in HFS-GFM and HFS-IFM groups. CONCLUSION: Overall, we demonstrate that co-administration of soluble fibres viz. gum acacia, inulin and L. casei NCDC19 fermented milk exhibited the anti-adiposity effects, improved insulin sensitivity and dyslipidemia in mice via modulation of lipid mobilization genes.

Inability of oxytocin to activate pyruvate dehydrogenase in the Brattleboro rat.

Oxytocin has insulin-like activity in that it stimulates lipogenesis and increases pyruvate dehydrogenase activity. However, in adipocytes from homozygous Brattleboro rats oxytocin is incapable of stimulating lipogenesis or pyruvate dehydrogenase activity, although insulin stimulation of both processes is normal and the antilipolytic activity of oxytocin is normal. Thus, the Brattleboro rat provides a new genetic model for the study of oxytocin action, wherein recognition of the chemical mediator is partially defective.

Divergent biological effects of adenosine and dibutyryl adenosine 3',5'-monophosphate on the isolated fat cell.

Adenosine 3',5'-monophosphate stimulated production of carbon dioxide and lipid from glucose, whereas its dibutyryl derivative inhibited this conversion. Addition of the dibutyryl derivative to the isolated fat cell further stimulated lipolysis induced by adrenocorticotropic hormone, whereas addition of adenosine 3',5'-monophosphate inhibited this lipolysis. Hence, measured by these two parameters, the biologic properties of adenosine 3',5'-monophosphate and its dibutyryl derivative are distinctly different.

Site-related white adipose tissue lipid-handling response to oleoyl-estrone treatment in overweight male rats.

BACKGROUND: Oleoyl-estrone (OE) decreases energy intake while maintaining glucose homeostasis, and energy expenditure at the expense of body fat. White adipose tissue (WAT) depots behave differently under starvation, postprandial state and pharmacologically induced lipolysis. AIM OF THE STUDY: To understand the mechanism of massive lipid loss from WAT elicited by OE treatment. METHODS: We used overweight male rats. Rats receiving OE (10 nmol/g) gavages were compared with controls and a pair-fed group. Whole fat pads from the mesenteric, retroperitoneal, epididymal and inguinal subcutaneous sites were excised and analyzed for lipid, DNA, mRNA and the expression of lipogenic, fatty acid transporters and lipase genes. RESULTS: In OE and pair-fed rats, WAT weights decreased, with the limited loss of cells. Patterns of gene expression in most WAT sites were similar for OE and PF, suggesting a shared mechanism of fat mobilization, but in mesenteric WAT, PF increased lipogenic and fatty acid transporter gene expressions. However, OE inhibited lipogenic expressions more deeply than PF. CONCLUSIONS: White adipose tissue sites showed different expression patterns, hinting at relatively specialized functions in fat storage; thus, single site analyses cannot be extrapolated to whole WAT. Differences between mesenteric and the other sites suggest that 'visceral fat' should be reserved for this site only, and not applied to other abdominal fat depots (epididymal, retroperitoneal).

Chrysanthemum Leaf Ethanol Extract Prevents Obesity and Metabolic Disease in Diet-Induced Obese Mice via Lipid Mobilization in White Adipose Tissue.

This study aimed to elucidate the molecular mechanism of Chrysanthemum morifolium Ramat. against obesity and diabetes, by comparing the transcriptional changes in epididymal white adipose tissue (eWAT) with those of the bioactive compound in C. morifolium, luteolin (LU). Male C57BL/6J mice were fed a normal diet, high-fat diet (HFD), and HFD supplemented with 1.5% w/w chrysanthemum leaf ethanol extract (CLE) for 16 weeks. Supplementation with CLE and LU significantly decreased the body weight gain and eWAT weight by stimulating mRNA expressions for thermogenesis and energy expenditure in eWAT via lipid mobilization, which may be linked to the attenuation of dyslipidemia. Furthermore, CLE and LU increased uncoupling protein-1 protein expression in brown adipose tissue, leading to energy expenditure. Of note, CLE and LU supplements enhanced the balance between lipid storage and mobilization in white adipose tissue (WAT), in turn, inhibiting adipocyte inflammation and lipotoxicity of peripheral tissues. Moreover, CLE and LU attenuated hepatic steatosis by suppressing hepatic lipogenesis, thereby ameliorating insulin resistance and dyslipidemia. Our data suggest that CLE helps inhibit obesity and its comorbidities via the complex interplay between liver and WAT in diet-induced obese mice.

Sulfatides inhibit leukotriene synthesis in human polymorphonuclear granulocytes by a mechanism involving lipid rearrangement in intracellular membranes.

Sulfatides - sulfated derivatives of galactocerebroside - are endogenous ligands for P- and L-selectins and are able to induce intracellular signaling in neutrophils through a L-selectin dependent pathway. Sulfatides are implicated in a variety of physiological functions and have been found to suppress the synthesis of 5-lipoxygenase (5-LO) metabolites and impede 5-LO translocation to the nuclear envelope in adherent human polymorphonuclear leukocytes (PMNs) [Sud'ina, G. F., Brock, T. G., Pushkareva, M. A., Galkina, S. I., Turutin, D. V., Peters-Golden, M., et al. (2001). Sulphatides trigger polymorphonuclear granulocyte spreading on collagen-coated surfaces and inhibit subsequent activation of 5-lipoxygenase. The Biochemical Journal, 359, 621-629]. In this study we investigated the mechanism of the leukotriene (LT) synthesis inhibition by sulfatides. Sulfatides neither attenuated the ionophore-induced rise in [Ca(2+)](i) nor promoted PKA activation. We demonstrated that sulfatides directly inhibited 5-LO enzyme activity in a cell-free assay. BODIPY-labeled sulfatides were able to rapidly penetrate into the cells. Sulfatides induced rearrangement and redistribution of cytoskeletal components in adherent PMNs. The lipid incorporation as well as sulfatide-induced inhibition of LT synthesis were abolished by cytochalasin D, an inhibitor of actin polymerization and endocytosis. Importantly, sulfatides caused a prominent intracellular cholesterol redistribution, increasing its abundance at the uropod region. On the basis of these data, we suggest that increased cholesterol accumulation in cell compartments represents a novel mechanism by which sulfatides abrogate 5-LO translocation and activation.

Effects of chronic ethanol intake on mobilization and excretion of cholesterol in baboons.

To investigate the effects of chronic ethanol administration on the mobilization and excretion of cholesterol, turnover and balance studies were carried out in baboons pair-fed cholesterol-free diets containing 50% of energy either as ethanol or as additional carbohydrate for several years. Ethanol feeding increased free cholesterol in all plasma lipoprotein fractions, and esterified cholesterol in very low density lipoprotein, intermediate density lipoprotein, and high density lipoprotein (HDL). The major increase occurred in HDL, mainly as esterified cholesterol. The latter was associated with decreased transfer of esterified cholesterol from HDL to low density lipoprotein. By contrast, the smaller increase in HDL-free cholesterol was associated with increased turnover in the plasma, increased splanchnic uptake, and increased fecal excretion of plasma cholesterol, mainly as neutral steroids. Cholesterol extraction predominated over release in the splanchnic vascular bed, suggesting that the excess of cholesterol excreted in the feces originated in extrasplanchnic tissues. Thus, these findings indicate that alcohol consumption favors mobilization of tissue free cholesterol for hepatic removal and excretion. By contrast the increase in HDL-cholesterol (mainly esterified) appears to be a poor indicator of cholesterol mobilization.

Perifusion of isolated rat adipose cells. Modulation of lipolysis by adenosine.

Incubation of isolated rat epididymal fat cells is associated with the accumulation of adenosine in the incubation medium. To more clearly define the effect of adenosine on lipolysis, isolated rat epididymal adipocytes were studied with the perifusion system. Various combinations of epinephrine, adenosine, and adenosine deaminase were perifused through the adipocytes. Exogenous adenosine, 0.001-10.0 muM, had no discernible influence upon unstimulated lipolysis; but exogenous adenosine inhibited epinephrine-sensitive lipolysis in a concentration-dependent manner. Cells perifused with 0.3 muM epinephrine plus 0.001 muM adenosine did not show any impairment of the lipolytic response to 0.3 muM epinephrine alone. Adenosine, 0.01 muM, inhibited the response to epinephrine by 50%; response to 0.3 muM epinephrine plus 0.1 muM adenosine was similar to the basal rate. Perifusion with adenosine deaminase significantly increased basal lipolysis to 30% of the epinephrine response. Adenosine deaminase and epinephrine were synergistic in stimulating lipolysis to 180% of the response to epinephrine alone. Isolated fat cells were incubated for 30 min, and the cell-free used medium was perifused through fresh fat cells. Epinephrine in used medium was less effective in promoting lipolysis than epinephrine in fresh buffer. High-pressure liquid chromatography identified adenosine in the used medium. Bovine serum albumin possessed adenosine deaminase activity but accounted for negligible conversion of adenosine to inosine. Adenosine is shown to have a modulating effect upon basal and hormone-stimulated lipolysis in the perifusion system. Sufficient endogenous adenosine (<0.01 muM) is present to maximally affect basal lipolysis. Hormone-stimulated lipolysis, although inhibited somewhat by endogenous adenosine, requires the addition of exogenous adenosine for complete inhibition.

Effects of glucagon on lipolysis and ketogenesis in normal and diabetic men.

The effect of glucagon (50 ng/kg/min) on arterial glycerol concentration and net splanchnic production of total ketones and glucose was studied after an overnight fast in four normal and five insulin-dependent diabetic men. Brachial artery and hepatic vein catheters were inserted and splanchnic blood flow determined using indocyanine green. The glucagon infusion resulted in a mean circulating plasma level of 4,420 pg/ml. In the normal subjects, the glucagon infusion resulted in stimulation of insulin secretion indicated by rising levels of immunoreactive insulin and C-peptide immunoreactivity. Arterial glycerol concentration (an index of lipolysis) declined markedly and net splanchnic total ketone production was virtually abolished. In contrast, the diabetic subjects secreted no insulin (no rise in C-peptide immunoreactivity) in response to glucagon. Arterial glycerol and net splanchnic total ketone production in these subjects rose significantly (P=<0.05) when compared with the results in four diabetics who received a saline infusion after undergoing the same catheterization procedure.Net splanchnic glucose production rose markedly during glucagon stimulation in the normals and diabetics despite the marked rise in insulin in the normals. Thus, the same level of circulating insulin which markedly suppressed lipolysis and ketogenesis in the normals failed to inhibit the glucagon-mediated increase in net splanchnic glucose production. It is concluded (a) that glucagon at high concentration is capable of stimulating lipolysis and ketogenesis in insulin-deficient diabetic man; (b) that insulin, mole for mole, has more antilipolytic activity in man than glucagon has lipolytic activity; and (c) that glucagon, on a molar basis, has greater stimulatory activity than insulin has inhibitory activity on hepatic glucose release.

Lipolytic catecholamine resistance due to decreased beta 2-adrenoceptor expression in fat cells.

The existence of lipolytic beta-adrenoceptor (BAR) resistance was investigated in vivo and in isolated abdominal subcutaneous adipocytes in 65 healthy and drug-free subjects. The concentration of isoprenaline (nonselective BAR agonist) causing half-maximum lipolysis effect (ED50) varied bimodally and 10(6)-fold between individuals but was almost constant in the same subject when measured two times at rest or before and 30 min after exercise. The subjects were categorized as having either high or low isoprenaline sensitivity. The former group had a 50% reduced in vivo lipolytic response to exercise and mental stress, despite a 50% increased plasma noradrenaline response (P < 0.01) and a 350% increased plasma adrenaline response (P < 0.02). In fat cells the lipolytic ED50 values for noradrenaline and terbutaline (BAR2 agonist) were 10 times lower (P < 0.001) in low-sensitive subjects, but the maximum lipolytic actions of these agents (and of isoprenaline) were similar in both groups. The action on lipolysis of dobutamine (BAR1 agonist), forskolin (stimulating adenylate cyclase), dibutyryl cyclic AMP (activating protein kinase), clonidine (alpha 2-adrenergic agonist), or phenyl isopropyladenosine (adenosine receptor agonist) were almost identical in high- and low-sensitivity subjects. ED50 for isoprenaline correlated with ED50 for terbutaline (r = 0.75), but not with ED50 for dobutamine. In high-sensitivity subjects the number of BAR2 was almost three-fold increased (P < 0.002) and the steady-state adipocyte mRNA level for BAR2 was sixfold increased (P < 0.005). BAR2 affinity as well as BAR1 number, affinity and mRNA expression were similar in both groups. In 11 cholecystectomy patients (otherwise healthy) lipolytic ED50 for beta agonists correlated in omental and subcutaneous fat cells (r = 0.85 for isoprenaline; r = 0.95 for terbutaline). In conclusion, lipolytic resistance to catecholamines is present in vivo in apparently healthy subjects due to reduced expression of BAR2 in adipocytes.

Enhanced glycemic responsiveness to epinephrine in insulin-dependent diabetes mellitus is the result of the inability to secrete insulin. Augmented insulin secretion normally limits the glycemic, but not the lipolytic or ketogenic, response to epinephrine in humans.

To determine if the enhanced glycemic response to epinephrine in patients with insulin-dependent diabetes mellitus (IDDM) is the result of increased adrenergic sensitivity per se, increased glucagon secretion, decreased insulin secretion, or a combination of these, plasma epinephrine concentration-response curves were determined in insulin-infused (initially euglycemic) patients with IDDM and nondiabetic subjects on two occasions: once when insulin and glucagon were free to change (control study), and again when insulin and glucagon were held constant (islet clamp study). During the control study, plasma C-peptide doubled, and glucagon did not change in the nondiabetic subjects, whereas plasma C-peptide did not change but glucagon increased in the patients. The patients with IDDM exhibited threefold greater increments in plasma glucose, largely the result of greater increments in glucose production. This enhanced glycemic response was apparent with 30-min increments in epinephrine to plasma concentrations as low as 100-200 pg/ml, levels that occur commonly under physiologic conditions. During the islet clamp study (somatostatin infusion with insulin and glucagon replacement at fixed rates), the heightened glycemic response was unaltered in the patients with IDDM, but the nondiabetic subjects exhibited an enhanced glycemic response to epinephrine indistinguishable from that of patients with IDDM. In contrast, the FFA, glycerol, and beta-hydroxybutyrate responses were unaltered. Thus, we conclude the following: Short, physiologic increments in plasma epinephrine cause greater increments in plasma glucose in patients with IDDM than in nondiabetic subjects, a finding likely to be relevant to glycemic control during the daily lives of such patients as well as during the stress of intercurrent illness. Enhanced glycemic responsiveness of patients with IDDM to epinephrine is not the result of increased sensitivity of adrenergic receptor-effector mechanisms per se nor of their increased glucagon secretory response; rather, it is the result of their inability to augment insulin secretion. Augmented insulin secretion, albeit restrained, normally limits the glycemic response, but not the lipolytic or ketogenic responses, to epinephrine in humans.

Lipolysis during fasting. Decreased suppression by insulin and increased stimulation by epinephrine.

These studies were designed to determine whether the insulin resistance of fasting extends to its antilipolytic effects and whether fasting enhances the lipolytic effects of adrenergic stimulation independent of changes in plasma hormone and substrate concentrations. Palmitate flux was determined isotopically ([1-14C]palmitate) before and during epinephrine infusion in normal volunteers after a 14-h (day 1) and an 84-h (day 4) fast. Using a pancreatic clamp, constant plasma hormone and glucose concentrations were achieved on both study days in seven subjects. Six subjects were infused with saline and served as controls. During the pancreatic clamp, palmitate flux was greater (P less than 0.01) on day 4 than day 1, despite similar plasma insulin, glucagon, growth hormone, cortisol, epinephrine, norepinephrine, and glucose concentrations. The lipolytic response to epinephrine was greater (P less than 0.05) on day 4 than day 1 in both groups of subjects. In conclusion, lipolysis during fasting is less completely suppressed by insulin and more readily stimulated by epinephrine.

Multiple symmetric lipomatosis: a defect in adrenergic-stimulated lipolysis.

The cellularity of normal and lipomatous adipose tissue and its response to different lipolytic agents have been studied in a group of 10 patients with multiple symmetric lipomatosis (MSL). In MSL patients, fat cells from lipomatous tissue are smaller than normal, uninvolved adipocytes. Fat cells from lipomata show minimal variations in size following conspicuous increase of lipomatous masses. These findings suggest that the growth of lipomata can be attributed to the neoformation of adipocytes rather than to an enlargement in the single fat cells. The incidence of reduced glucose tolerance and of hyperlipoproteinemia is similar in MSL patients and in controls. A significant reduction in plasma free fatty acids was observed in MSL patients after a 24-h fast as well as after noradrenaline infusion. A specific insensitivity of lipomatous tissue to the lipolytic effect of noradrenaline and isoprenaline was observed in vitro, as indicated by glycerol release in the medium, whereas response to theophylline and to dibutyryl cyclic AMP was retained. The lipolytic response to catecholamines was retained. The lipolytic response to catecholamines was normal in the nonlipomatous adipose tissue of MSL patients. In basal conditions ATP concentrations were similar in normal and in lipomatous adipose tissue. However, incubation with noradrenaline induced a significant fall in intracellular ATP levels in normal tissue, whereas no variations were observed in lipomatous tissue. Theophylline, instead, induced a prompt and significant decrease in intracellular ATP levels in lipomatous tissue. These observations indicate that the block in catecholamine-stimulated lipolysis in lipomatous tissue of MSL patients can be localized at a level preceding the formation of cyclic AMP.

Studies of human adipose tissue in culture. 3. Influence of insulin and medium glucose concentration on cellular metabolism.

Explants of human adipose tissue were maintained in culture for 1 wk in different glucose concentrations with or without the addition of insulin. After this period of time the explants were carefully washed and then subjected to short-term incubations in the same glucose concentration and in the absence of insulin. With this experimental design the influence of long-term exposure to insulin and different glucose concentrations on adipose tissue metabolism could be studied. The results of these studies show that an increase in the glucose concentration of the culture medium enhanced the basal as well as the catecholamine-stimulated lipolysis in the short-term incubations. The presence of insulin in the culture medium enhanced the lipolytic process as well. Analogous results were obtained with the cellular rate of glucose conversion to triglycerides in the short-term incubations. The stimulating effects of insulin and glucose were most pronounced in the larger adipose cells possibly due to their enlarged surface areas. The data suggest that the metabolism of adipose tissue as revealed by short-term studies may be profoundly influenced by the antecedent biochemical environment.

Inhibition of lipolysis by adenosine is potentiated with age.

The ability of a variety of hormones to activate cells declines with age. We have investigated the mechanism for the reduced ability of beta adrenergic stimulation to activate lipolysis in fat cells from older rats. Previously, we have found that these cells have an intact lipolytic response to a cAMP analogue but diminished cAMP accumulation after isoproterenol stimulation, suggesting that the blunted cAMP response is rate limiting. In the present study we have tested the hypothesis that enhanced inhibition of lipolysis by endogenously released adenosine accounts for the diminished lipolysis. Adenosine deaminase was added to media containing the adipocytes from older rats to remove endogenous adenosine. Under these conditions beta adrenergic stimulation of lipolysis is intact in fat cells from older rats. The adenosine analogue N6-phenylisopropyladenosine more effectively inhibited lipolysis in the older group (77 +/- 6%) than in the younger group (46 +/- 5%), suggesting that enhanced efficacy of endogenous adenosine may account for the reduced lipolytic response to catecholamines. When pertussis vaccine was used to functionally inactivate adenosine receptors in adipocytes from the younger and older rats, the ability of isoproterenol to activate lipolysis was restored in the older group. All the data are consistent with the hypothesis that enhanced inhibitory effects of adenosine explain the diminished ability of beta adrenergic agonists to activate lipolysis. It is possible that enhanced inhibitory pathways may be involved in blunting responses to stimulatory hormones in other tissues from older animals.

Role of vascular alpha-2 adrenoceptors in regulating lipid mobilization from human adipose tissue.

The role of alpha-2 adrenoceptors in lipid mobilization and blood flow was investigated in situ using microdialysis of subcutaneous adipose tissue in nonobese healthy subjects. The alpha-2 agonist clonidine caused dose-dependent biphasic response with increased glycerol levels at low clonidine concentrations and decreased glycerol levels at concentrations > 10(-7) mol/liter. Similar results were observed with epinephrine plus propranolol. Clonidine action was unaffected in the presence of labetalol (beta-/alpha-1 antagonist) but completely blunted by the presence of yohimbine (alpha-2 antagonist). The pseudolipolytic effect of clonidine was significantly more pronounced in gluteal as compared with abdominal adipose tissue. When clonidine was added together with the vasodilating agents nitroprusside or hydralazine, the pseudolipolytic effect was abolished and a dose-dependent decrease in dialysate glycerol was observed at all clonidine concentrations (10(-10)-10(-4) mol/liter). When ethanol was added to the perfusate to monitor blood flow, the escape of alcohol from the dialysate was accelerated by 30% with hydralazine or nitroprusside (P < 0.01) and 30% retarded (P < 0.05) by clonidine (10(-10) mol/liter). Thus, the results demonstrate an important role of blood flow for regulating lipid mobilization from adipose tissue in vivo. Alpha-2 adrenoceptor activation causes marked retention of lipids in adipose tissue due to vasoconstriction in combination with antilipoiysis.