Lipolysis in brown adipose tissue of cold- and heat-acclimated hamsters

Rates of release of free fatty acids (FFA) and glycerol to the incubation medium by brown adipose tissue (BAT) slices isolated from heat-acclimated (H), cold-acclimated (C), and control (N) hamsters in the absence or presence of epinephrine (E) were studied. Rates of FFA and glycerol release by tissue slices isolated from H and N animals were similar. In tissue slices isolated from C animals rate of release of FFA and glycerol was three times as high. Addition of E to the incubation medium (200 microgram/ml) had no effect on the rate of FFA and glycerol release of slices from C animals, but tripled the rates of slices from N, resulting in similar values for the two groups. In slices from H animals the rate of release was lower than in the other two groups, increasing only 1.5-fold. Pretreatment of N animals with triiodothyronine (T3; 0.8 microgram/100 g daily for 7 days) doubled the rates of FFA and glycerol release. Addition of E to the medium affected both pretreated and nontreated slices similarly. Two possible mechanisms by which temperature acclimation controls the lipolytic rate of BAT are suggested by 1) the concentration of specific enzymes and 2) cellular metabolites and hormones which activate existing systems. It seems that both operate in temperature-acclimated hamsters.     

Splanchnic free fatty acid kinetics

These studies were conducted to assess the relationship between visceral adipose tissue free fatty acid (FFA) release and splanchnic FFA release. Steady-state splanchnic bed palmitate ([9,10-(3)H]palmitate) kinetics were determined from 14 sampling intervals from eight dogs with chronic indwelling arterial, portal vein, and hepatic vein catheters. We tested a model designed to predict the proportion of FFAs delivered to the liver from visceral fat by use of hepatic vein data. The model predicted that 15 +/- 2% of hepatic palmitate delivery originated from visceral lipolysis, which was greater (P = 0.004) than the 11 +/- 2% actually observed. There was a good relationship (r(2) = 0.63) between the predicted and observed hepatic palmitate delivery values, but the model overestimated visceral FFA release more at lower than at higher palmitate concentrations. The discrepancy could be due to differential uptake of FFAs arriving from the arterial vs. the portal vein or to release of FFAs in the hepatic circulatory bed. Splanchnic FFA release measured using hepatic vein samples was strongly related to visceral adipose tissue FFA release into the portal vein. This finding suggests that splanchnic FFA release is a good indicator of visceral adipose tissue lipolysis.     

Inhibition of free fatty acid mobilization by colchicine

Segments of epididymal adipose tissue from normal male rats were incubated with micromolar concentrations of colchicine for different periods of time up to 4 hr, and the mobilization of free fatty acids (FFA) was measured during a subsequent reincubation. Although pretreatment with colchicine did not alter basal unstimulated FFA release, mobilization of FFA in the presence of epinephrine or theophylline was reduced. However, neither lipolysis, as judged by glycerol production, nor cyclic AMP accumulation was impaired under the same conditions. To assess the possibility that colchicine might limit production of fatty acids by accelerating the entry and metabolism of glucose into adipocytes, the metabolism of glucose by adipose tissue was studied. Pretreatment with colchicine did not affect uptake of glucose nor its oxidation to CO(2), although colchicine-treated tissues did have slightly more [(14)C]glucose incorporated into the glyceride moiety of triglyceride. When adipose tissues pretreated with colchicine were incubated in an albumin-free medium, no reduction in FFA production by colchicine was observed. Because no FFA release occurs in albumin-free media, this experiment suggests that colchicine-induced inhibition of FFA mobilization results from impaired extrusion of FFA from adipose cells.     

A method for the determination of lipoprotein lipase in postheparin plasma and body tissues utilizing a triolein-coated Celite substrate

A simple and specific method for assaying lipoprotein lipase activity is described. Postheparin plasma, heart homogenates, or extracts of acetone powder of adipose tissue were incubated with a triolein-coated Celite substrate, and enzyme activity was determined from the rate of free fatty acid (FFA) release in the incubation system. FFA release was linear for 30 min, and was proportional to protein concentration in the incubation system. FFA release was decreased by addition of deoxycholate or Triton X-100. Increasing the concentration of heparin in the incubation system caused a gradual decrease in FFA release by postheparin plasma and increases in activity of heart homogenates and adipose tissue lipoprotein lipase. The Celite substrate was found to be satisfactory for assaying pancreatic lipase activity as well.     

Release of free fatty acids by the rat kidney.

The authors studied the release of free fatty acids (FFA) by the rat kidney cortex. They found that the kidney cortex released FFA into the incubation medium like adipose tissue. The presence of Ca2+ ions did not affect FFA release. Glucose significantly inhibited it. It was further shown that the kidney cortex is sensitive to the akipokinetic action of adrenaline and the antilipolytic action of insulin, in the same way as adipose tissue. It is concluded from the results that the kidney cortex has a lipolytic system which seems to be subject to higher hormonal regulatory mechanisms.     

Adipose tissue metabolism -- an aspect we should not neglect?

Free fatty acids (FFAs) are the most metabolically important products of adipose tissue lipolysis. Experimentally creating high FFA concentrations can reproduce the metabolic abnormalities of obesity in lean, healthy persons and lowering FFA concentrations can improve the metabolic health of upper body obese individuals. FFA concentrations are determined by both the release of FFAs into the bloodstream and the clearance of FFAs from the bloodstream. Normal FFA release rates are different in men and women and total FFA release is closely linked to resting energy expenditure. Upper body subcutaneous fat, visceral fat, and leg fat depots contribute differently to the exposure of various tissues to FFAs. The implications of regional adipose tissue lipolysis to systemic FFA availability and the effect of different approaches to treatment of obesity are discussed.     

Higher Free Fatty Acid Uptake in Visceral Than in Abdominal Subcutaneous Fat Tissue in Men

Visceral adipose tissue has been shown to have high lipolytic activity. The aim of this study was to examine whether free fatty acid (FFA) uptake into visceral adipose tissue is enhanced compared to abdominal subcutaneous tissue in vivo. Abdominal adipose tissue FFA uptake was measured using positron emission tomography (PET) and [¹⁸F]‐labeled 6‐thia‐hepta‐decanoic acid ([¹⁸F]FTHA) and fat masses using magnetic resonance imaging (MRI) in 18 healthy young adult males. We found that FFA uptake was 30% higher in visceral compared to subcutaneous adipose tissue (0.0025 ± 0.0018 vs. 0.0020 ± 0.0016 µmol/g/min, P = 0.005). Visceral and subcutaneous adipose tissue FFA uptakes were strongly associated with each other (P < 0.001). When tissue FFA uptake per gram of fat was multiplied by the total tissue mass, total FFA uptake was almost 1.5 times higher in abdominal subcutaneous than in visceral adipose tissue. In conclusion, we observed enhanced FFA uptake in visceral compared to abdominal subcutaneous adipose tissue and, simultaneously, these metabolic rates were strongly associated with each other. The higher total tissue FFA uptake in subcutaneous than in visceral adipose tissue indicates that although visceral fat is active in extracting FFA, its overall contribution to systemic metabolism is limited in healthy lean males. Our results indicate that subcutaneous, rather than visceral fat storage plays a more direct role in systemic FFA availability. The recognized relationship between abdominal visceral fat mass and metabolic complications may be explained by direct effects of visceral fat on the liver.     

The hormonal regulation of premigratory fat deposition and winter fattening in red-winged blackbirds

Glucagon is a highly potent lipolytic agent in birds and a candidate for regulating premigratory and winter fattening. The seasonal role of glucagon in fat metabolism was determined by monitoring plasma glucagon, fatty acids and glucose in two groups of red-winged blackbirds; one group exposed to outside environmental conditions (September to May) and a second group maintained at summer conditions with respect to day length and temperature. The results of this investigation demonstrate significantly lower plasma glucagon (480.1 pg/ml) in birds exposed to outdoor conditions than in birds maintained at summer conditions (734.6 pg/ml) during September/October. The data are consistent with the view that low plasma glucagon in outdoor birds ensures the preservation of fat stores for autumn migration. Lower plasma free fatty acid (FFA) levels (0.35 mEq/l) in outdoor birds (vs. 0.54 mEq/l in indoor birds) in autumn may reflect the rapid transport of FFA to adipose tissue for lipogenesis resulting in a steady increase in body weight from September to January. The sharp decline in plasma FFA in indoor birds from 0.54 mEq/l in September/October to 0.28 mEq/l in January/February may be attributed to a marked decrease in food consumption, rather than a dramatic change in the rate of lipid transport from blood to muscle or adipose tissue. Glucagon injections caused a 600% increase in plasma FFA and a more modest (50%) increase in plasma glucose. This confirms the major role of glucagon in fat mobilization. Its lipolytic effects, however, can vary seasonally by way of down regulation of glucagon receptors. Down regulation of glucagon receptors in adipose tissue and the associated reduced sensitivity of adipocytes to the lipolytic action of glucagon would account for the progressive increase in weight of the birds throughout November/December when plasma glucagon levels were significantly higher (578.9 pg/ml) in outdoor birds as compared to indoor birds (436.9 pg/ml). Lower plasma glucagon levels (405.5 pg/ml) in outdoor birds in January/February (vs. 638.6 pg/ml in indoor birds) may reflect the same physiological conditions prevailing in September/October favoring the preservation of fat stores.     

Metabolism of adipose tissue in the fat tail of the sheep in vivo

The metabolism of the large mass of adipose tissue constituting the fat tail of the Syrian sheep has been investigated by measuring arteriovenous concentration (A-V) differences. The tail in situ in the intact anesthetized animal, as well as the isolated tail perfused with blood through a constant flow pump oxygenator, was used. In fed animals, the adipose tissue took up glucose and ketone bodies and released lactate and free fatty acids (FFA), although in some animals uptake of FFA also occurred. After 48-144 hr of fasting, uptake of glucose and ketone bodies continued and the FFA release increased. Total lipid esters and phospholipids were not released even after food had been withheld for 6 days. Insulin increased the A-V difference and the uptake of glucose, and reduced the FFA release. Adrenaline increased the A-V difference and uptake of glucose; the simultaneous increase in serum FFA was not accompanied by an increase in A-V difference for FFA in most experiments, which suggests that this adipose tissue is relatively insensitive to the lipolytic effect of the hormone. The effect of noradrenaline was similar to that of adrenaline. Glucagon hyperglycemia was not accompanied by increase in glucose uptake in most experiments.     

Fatty acid cycling in the fasting rat

Adipose tissue lipolysis and fatty acid reesterification by liver and adipose tissue were investigated in rats fasted for 15 h under basal and calorigenic conditions. The fatty acid flux initiated by adipose fat lipolysis in the fasted rat is mostly futile and is characterized by reesterification of 57% of lipolyzed free fatty acid (FFA) back into adipose triglycerides (TG). About two-thirds of FFA reesterification are carried out before FFA release into plasma, whereas the rest consists of plasma FFA extracted by adipose tissue. Thirty-six percent of the fasting lipolytic flux is accounted for by oxidation of plasma FFA, whereas only a minor fraction is channeled into hepatic very low density lipoprotein-triglycerides (VLDL-TG). Total body calorigenesis induced by thyroid hormone treatment and liver-specific calorigenesis induced by treatment with beta, beta'-tetramethylhexadecanedioic acid (Medica 16) are characterized by a 1.7- and 1.3-fold increase in FFA oxidation, respectively, maintained by a 1.5-fold increase in adipose fat lipolysis. Hepatic reesterification of plasma FFA into VLDL-TG is negligible under both calorigenic conditions. Hence, total body fatty acid metabolism is regulated by adipose tissue as both source and sink. The futile nature of fatty acid cycling allows for its fine tuning in response to metabolic demands.     

Hormone-sensitive lipase deficiency in mice causes diglyceride accumulation in adipose tissue, muscle, and testis.

Hormone-sensitive lipase (HSL) is expressed predominantly in white and brown adipose tissue where it is believed to play a crucial role in the lipolysis of stored triglycerides (TG), thereby providing the body with energy substrate in the form of free fatty acids (FFA). From in vitro assays, HSL is known to hydrolyze TG, diglycerides (DG), cholesteryl esters, and retinyl esters. In the current study we have generated HSL knock-out mice and demonstrate three lines of evidence that HSL is instrumental in the catabolism of DG in vivo. First, HSL deficiency in mice causes the accumulation of DG in white adipose tissue, brown adipose tissue, skeletal muscle, cardiac muscle, and testis. Second, when tissue extracts were used in an in vitro lipase assay, a reduced FFA release and the accumulation of DG was observed in HSL knock-out mice which did not occur when tissue extracts from control mice were used. Third, in vitro lipolysis experiments with HSL-deficient fat pads demonstrated that the isoproterenol-stimulated release of FFA was decreased and DG accumulated intracellularly resulting in the essential absence of the isoproterenol-stimulated glycerol formation typically observed in control fat pads. Additionally, the absence of HSL in white adipose tissue caused a shift of the fatty acid composition of the TG moiety toward increased long chain fatty acids implying a substrate specificity of the enzyme in vivo. From these in vivo results we conclude that HSL is the rate-limiting enzyme for the cellular catabolism of DG in adipose tissue and muscle.     

Adaptations in lipid metabolism of bovine adipose tissue in lactogenesis and lactation

The timing and magnitude of metabolic adaptations in adipose tissue during lactogenesis and lactation were determined in first lactation bovines. In vitro rates of lipogenesis and palmitate esterification were measured to estimate in vivo synthesis. Lipolysis was measured in the basal state and as maximally stimulated by norepinephrine or epinephrine to estimate physiological adaptations as well as the changes in catecholamine responsiveness. Subcutaneous adipose tissue was biopsied at -1, -0.5, +0.5, 1, 2, and 6 months from parturition. From 1 to 0.5 months prepartum there was a 54% reduction in lipogenesis, a 16% reduction in esterification, a 54 and 77% increase in norepinephrine- and epinephrine-stimulated free fatty acid (FFA) release, respectively, and a 28% increase in epinephrine-stimulated glycerol release. The immediate postpartum period (0.5 and 1 month) was marked by a decrease in lipogenesis to 5% and esterification to 50% of -1 month rates. During this period, norepinephrine-stimulated FFA release increased 50% above -1 month rates, epinephrine-stimulated FFA release increased 128%, and norepinephrine- and epinephrine-stimulated glycerol release increased 30 and 87%, respectively. Midlactation (2 and 6 months) was marked by a dramatic rebound in lipogenesis and esterification to 14-fold and 2.5-fold prepartum rates, respectively. Basal glycerol release doubled during this period, while basal FFA release declined to near prepartum levels. Catecholamine-stimulated FFA and glycerol release decreased from the peak during midlactation, but remained elevated compared to prepartum levels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lipolytic activity of purified pituitary and bacterially derived growth hormone on chicken adipose tissue in vitro

The ability of growth hormone (GH) to stimulate lipolysis was examined using chicken abdominal adipose tissue explants incubated in vitro and purified pituitary and bacterially derived chicken and bovine GH. Consistently in the fourth hour of incubation, lipolysis (as determined by glycerol release) was increased by the presence of GH (1 micrograms/ml), irrespective of pituitary or bacterial derivation or of chicken or bovine origins. This effect of GH was observed with adipose tissue originating from young (6-8 weeks old) intact and hypophysectomized chicks and adult (6-9 months old) male chickens. Glycerol release was also enhanced by lower doses of GH (10 ng/ml with tissue from young and 100 ng/ml with tissue from adult chickens).     

Sex difference in triglyceride/fatty acid substrate cycling of rat adipose tissue: indirect regulation by androgens.

Male Sprague-Dawley rats displayed significantly higher rates of triglyceride/fatty acid (TG/FFA) substrate cycling in subcutaneous, perigenital, and mesenteric white adipose tissue, compared to females. To investigate possible regulation via androgens and estrogens, male rats were treated with the androgen antagonist, cyproterone acetate (10 mg daily in subcutaneous injections), or estradiol polyphosphate (0.3 mg intramuscularly, given as a single dose). Estradiol treatment did not affect TG/FFA cycling. Treatment with cyproterone acetate significantly decreased TG/FFA cycling in perigenital (epididymal) tissue. This effect could however largely be ascribed to concomitant inhibition of food intake by cyproterone acetate. The effects of cyproterone acetate on the two axes of TG/FFA cycling (lipolysis and re-esterification) were further studied in vitro. Norepinephrine-stimulated glycerol release from perigenital adipocytes was inhibited, whereas activities of esterification enzymes (GPAT and PPH) was essentially unaffected. We conclude that androgens seem to affect TG/FFA cycling indirectly via the lipolytic axis.     

Plasma arginine vasotocin and angiotensin II in the water deprived Kelp gull (Larus dominicanus), Cape gannet (Sula capensis) and Jackass penguin (Spheniscus demersus)

1. The basal levels of the osmoregulatory hormones, arginine vasotocin (AVT) and angiotensin II (AII) were measured (by radioimmunoassay) in the plasma of conscious Kelp gulls, Cape gannets and Jackass penguins. 2. The responses of the hormones to similar degrees of hypertonicity and hypovolemia caused by water deprivation have also been determined. 3. Dehydration elevated plasma AVT and plasma AII in all three species. 4. The AVT concentration was increased by 2-4 fold and although in each case the correlation between plasma osmolality and plasma AVT was highly significant (2P less than 0.01), the sensitivity of release was greater in the gull (1.13 pg/ml per mOsm/kg) than in the gannet (0.36 pg/ml per mOsm/kg) or penguin (0.44 pg/ml per mOsm/kg). 5. Dehydration increased plasma AII 3-fold in the three bird types.     

Age influence on the lipolytic effect of glucagon in geese.

The lipolytic effect of glucagon was measured in vitro with adipose tissue of "young" (4-8 wk) and "old" (over 1 yr) geese. The response of the young geese tissue was about twice that observed with tissue of old geese, for glucagon concentrations of 0.05, 0.5, and 5.0 mug/ml. Our estimates indicate that the number of adipose cells per g of adipose tissue of young geese was three times that of the old geese tissue. This suggests that the greater lipolytic response to glucagon, observed in young geese adipose tissue, may possibly be due to its greater cellularity, rather than to a greater lipolytic response of the individual adipocyte. The lipolytic effect of glucagon in vivo, for each of the doses between 1.0 and 20.0 mug/kg, was significantly greater in the old than in the young geese. The slope of the linear equation relating log10 of glucagon dose and elevation of plasma FFA 5 min after injection, was significantly greater for the old than for the young geese. In the goose, therefore, the influence of age on the adipokinetic effect of glucagon appears to be mediated by factors operating in the whole animal, more than by changes in the adipose cell itself. A slower removal rate of circulating FFA by the old geese, could be one of these factors.     

Certain characteristic features of the lipolytic factor from rat submaxillary salivary glands

The aqueous extract of rat salivary submaxillary gland was found to contain three protein fractions activating the release of free fatty acids (FFA) and glycerol from rat epididymal adipose tissue in vitro. Physico-chemical investigations of these proteins demonstrated certain common features: all three fractions were albumins having a common isoelectric point, and their aqueous solutions absorbed light at the same wavelength. The use of lipolysis activators and inhibitors (theophylline, propranolol, insulin) for investigating their effects on FFA and glycerol release produced by these protein fractions explained the mechanism of the lipolytic action of the protein fractions from rat submaxillary glands.     

Arginine vasotocin-induced prostaglandin synthesis in vitro by the reproductive tract of the viviparous lizard Sceloporus jarrovi

Two experiments were performed to determine whether arginine vasotocin (AVT) stimulates synthesis of prostaglandins (PGs) in reptilian oviducts. Homogenized oviducal tissue from female Sceloporus jarrovi in early and late pregnancy were cultured with radiolabeled (14C) prostaglandin precursor, arachidonic acid (AA). In late pregnancy, oviducts exposed to AVT exhibited a greater conversion of AA to PGF2 alpha than did controls, whereas in early pregnancy there was no difference. The conversion of AA to other prostaglandins (PGA2, PGD2, PGE2, PGI2) was not influenced by AVT. The second experiment examined whether endogenous in vitro synthesis of PGF and PGE2 from intact, pregnant oviducts was stimulated by AVT (50 ng/ml; 100 ng/ml). Both doses of AVT induced a similar, significant rise in PGF concentrations within 30 min whereas no significant increase was noted in PGE2 concentrations until 90 min after treatment. Indomethacin pretreatment blocked synthesis of both PGF and PGE2 for 30 min following AVT treatment. These data indicate that AVT induces a highly specific rise in the synthesis of PGF from the oviduct of female S. jarrovi in late pregnancy. Furthermore, the prostaglandin-stimulating effect of AVT in reptiles appears homologous with the effect of oxytocin in mammals and AVT in birds. We hypothesize that this interaction is an evolutionarily conserved relationship found in all amniote vertebrates.