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.     

Fatty acid synthesis in vitro in the liver and adipose tissue of rats of various ages.

In white male Wistar rats aged 2, 4, 6, 9, 12, 18 and 32 months, receiving a standard laboratory diet (25 cal% proteins, 22 cal% fat and 53 cal% glycides) we assessed the total amount of fatty acids in the carcass and in vitro lipogenesis in liver and adipose tissue by the means of glucose-14C(U). The amount of body fat increased with age; the highest rate of increase in the percentage of body fat occurs 2 to 4 months after birth. Incorporation of 14C of glucose into total liver lipids attains maximum values in the group of rats age 2 to 4 months. By the age of 6 months lipogenesis decreased to a level observed in all the remaining age groups. In adipose tissue, the incorporation of 14C into total lipids decreased between the age of 2 to 4 months, a further decrease was observed around the age of 6 months. No appreciable changes were noted in any of the older age groups.     

Ketone body synthesis from leucine by adipose tissue from different sites in the rat

Leucine is catabolized to ketone bodies in adipose tissue, but the contribution of this output to overall ketone metabolism is not known. The intent of the present study was to determine the capacity of different adipose tissues to synthesize ketone bodies from leucine. The amino acid was readily converted into acetoacetate in epididymal, perirenal, and omental fat tissues. In rats fed ad libitum, the rate of acetoacetate synthesis in omental fat (about 2 mumol g tissue-1h-1) was at least 8 times higher than in epididymal or perirenal fat. In omental fat, the rates of acetoacetate formation from alpha-ketoisocaproic acid were 47-55% lower than from leucine at all concentrations examined. There was no significant synthesis of beta-hydroxybutyrate from leucine or alpha-ketoisocaproic acid. After oxidative decarboxylation, a greater proportion (about three-fourths) of leucine in omental fat was metabolized to acetoacetate than to CO2 production through the Krebs cycle. Although addition of glucose, pyruvate, or carnitine did not affect the production of acetoacetate, fasting for 24 h stimulated acetoacetate synthesis from leucine and alpha-ketoisocaproic acid in omental fat. The high rate of leucine conversion to acetoacetate in omental fat was related to high activities of leucine aminotransferase and branched-chain alpha-keto acid dehydrogenase. Moreover, protein content and cytochrome c oxidase activity of omental mitochondria were, respectively, 13 and 12 times higher than in epididymal mitochondria. In contrast, fat content of epididymal adipose tissue was 21 times that of omental adipose tissue. Epididymal depot consisted of 2.0% protein and 75.8% fat, whereas omental depot contains 17.2% protein and 3.6% fat, resembling that of liver and muscle. The results suggest that the high ketogenic capacity of omental fat stems in part from an augmented mitochondrial mass and high activity of branched-chain alpha-keto acid dehydrogenase.     

Kinetic properties of 5'-nucleotidase in the adipose tissue, liver and blood of rats of different ages

The kinetic characteristics (Vmax and Km) of membrane-bound and serum 5'-nucleotidases from rats of different age groups are studied. The age-related increase of Km parameter in plasma membranes of fatty tissue and liver demonstrated a lower enzyme affinity to the substrate in ageing membranes. The specific activity of 5'-nucleotidase in blood serum of aged animals was below that in young ones, while Km value remained stable. Tissue differentiation of the catalytic properties of the enzyme and their aged-related characteristics may be to a considerable extent determined by the physicochemical state of cell membranes.     

Changes in collagen content of the diaphragm in rats of different ages

Rat diaphragm showed a nine to ten-fold increase in the dry and wet weights and a four-fold increase in its surface area between 21 and 540 days of age. The increase was the most rapid between days 21 and 90, thereafter a non-significant trend of increase was maintained at least up to 360 days of age followed by a tendency to decrease. The total collagen content of the diaphragm paralleled the increase in surface area with progressing age. The collagen content significantly dropped when expressed relative to the dry weight of the diaphragm, between 21 and 90 days of age. This trend was maintained till 180 days of age but thereafter significantly rise was seen. These later changes may be due to elenated synthesis and/or reduced degradation of collagen, and may contribute to the decline in contractility of the diaphragm as age advances.     

Lipoprotein lipase and lipogenic enzyme activities in adipose tissue from rats fed different lipid sources

This work was designed to study the effect of different lipid sources on the activities of lipoprotein lipase and lipogenic enzymes in adipose tissue from rats fedad libitum or energy-controlled diets. Male Wistar rats were fed diets containing 40% of energy as fat (olive oil, sunflower oil, palm oil or beef tallow), for 4 wk. Underad libitum feeding no differences were found among dietary fat groups in final body weight, adipose tissue weights and total body fat. Under energy-controlled feeding, despite isoenergetic intake, rats fed the beef tallow diet gained significantly less weight than rats fed the other three diets. Beef tallow fed rats showed the lowest values for adipose tissue weights and total body fat. When rats had free access to food no effect of dietary lipid source on lipogenic enzyme activities was found. In contrast, under energy-controlled feeding rats fed the beef tallow diet showed significantly higher activities of glucose-6-phosphate dehydrogenase and fatty acid synthase than rats fed the other three diets. Heparin-releasable lipoprotein lipase activity in perirenal and subcutaneous adipose tissues was not different among rats fed olive oil, safflower oil, palm oil or beef tallow. When comparing both adipose tissue anatomical locations, significantly higher activities were found in subcutaneous than in perirenal fat pad independently of dietary fat. In conclusion, under our experimental protocol, lipogenesis in rat adipose tissue does not seem to be affected by dietary fat type.     

Glutathion content, rate of apoptosis, and brown adipose tissue mass in rats exposed to different ambient temperatures

1. Amount of glutathione (GSH) in the interscapular brown adipose tissue (IBAT) of cold-adapted (5±1 °C), and 1, 3 and 7 days re-adapted (22±1 °C) rats were examined.

2. The higher amount of GSH was found in the IBAT of cold-adapted rats compared to control animals (22±1 °C).

3. After 1 and 7 days of re-adaptation, the level of GSH was still significantly higher compared to control group.

4. On the third day of re-adaptation, the quantity of GSH in the tissue was significantly lower compared to the cold-adapted group.

5. In cold-adapted animals, intensity of apoptosis in IBAT was lower, whereas in re-adapted animals (3rd and 7th day) it was higher than in control group.

6. This study indicates that GSH depletion is associated with an enhanced apoptosis in IBAT during re-adaptation.

Serotonin content of the interscapular brown adipose tissue of growing rats.

In brown fat of newborn rats the serotonin (5HT) content is high during the first five days of life. This may play a part in the lipid repletion of the tissue, lipids being the main fuel for nonshivering thermogenesis. Subsequently 5HT increases more in young rats reared at 16 degrees C than in those reared at 28 degrees C but to a lesser extent than norepinephrine (NE) content. A possible role of 5HT in thermoregulation of the rat during the early postnatal period is discussed.     

Pharmacokinetics of lithium in rats of various ages

The kinetics of lithium in serum was determined in rats aged 5 to 240 days, after 2 days of pretreatment with 0.15 mval Li+/100 g and a load of 0.3 mval Li+/100 g body weight at the day of experiments. In rats age differences in distribution and elimination of lithium can be described by a two-compartment model. The kinetic parameters were calculated (half-life(serum), apparent volume of distribution, rate constants of distribution and elimination, total plasma clearance) and, additionally, age differences in renal elimination were determined (renal clearance, half-life (urine)). The ability to excrete lithium is not fully developed in 5-day-old rats: t1/2serum (23 h) and especially t1/2urine (72 h) are much longer than in adult rats (t1/2serum = 12 h; t1/2urine = 6.4 h). The influence of administered lithium on the regulation of the electrolyte balance in the organism (sodium, potassium) occurred differently in dependence on age. Consequences of the investigations are discussed for the therapy with lithium in different age periods.     

Effects of pterostilbene in brown adipose tissue from obese rats

In recent years, much attention has been paid by the scientific community to phenolic compounds as active biomolecules naturally present in foods. Pterostilbene is a resveratrol dimethylether derivative which shows higher bioavailability. The aim of the present study was to analyze the effect of pterostilbene on brown adipose tissue thermogenic markers in a model of genetic obesity, which shows reduced thermogenesis. The experiment was conducted with 30 Zucker (fa/fa) rats that were distributed in three experimental groups: control and two groups orally administered with pterostilbene at 15 and 30 mg/kg body weight/day for 6 weeks. Gene expression of uncoupling protein 1 (Ucp1), peroxisome proliferator-activated receptor γ co-activator 1 α (Pgc-1α), carnitine palmitoyl transferase 1b (Cpt1b), peroxisome proliferator-activated receptor α (Pparα), nuclear respiratory factor 1 (Nfr1), and cyclooxygenase-2 (Cox-2); protein expression of PPARα, PGC-1α, p38 mitogen-activated protein kinase (p38 MAPK), UCP1 and glucose transporter (GLUT4); and enzyme activity of CPT 1b and citrate synthase (CS) were assessed in interscapular brown adipose tissue. With the exception of Pgc-1α expression, all these parameters were significantly increased by pterostilbene administration. These results show for the first time that pterostilbene increases thermogenic and oxidative capacity of brown adipose tissue in obese rats. Whether these effects effectively contribute to the antiobesity properties of these compound needs further research.     

Invitro glycerol metabolism in adipose tissue from fasted pregnant rats

Lumbar fat pad pieces taken from fed and 48 h starved 19-day pregnant rats and virgin controls were incubated for different times with [U-¹⁴C] glycerol, albumin and glucose. The glycerol conversion rates to either CO₂, saponified lipids and glyceride glycerol were higher in the pregnant rat tissue than in the controls. Starvation produces a greater decline in these parameters in pregnant rat tissue than in controls. The lipolysis rate was elevated in pregnant rat tissue. The augmented glycerol utilization by adipose tissue in the mother would contribute to the net deposition of fat, despite augmented lipolysis. In the starved state the enhanced lipolysis of the mother is potenciated by a decreased reutilization of glycerol, allowing a maximal net mobilization of the fat stores.     

Protein turnover in adipose tissue from fasted or diabetic rats

Protein synthesis and degradation in vitro were compared in epididymal fat pads from animals deprived of food for 48 h or treated 6 or 12 days prior with streptozotocin to induce diabetes. Although both fasting and diabetes led to depressed (-24% to -57%) protein synthesis, the diminution in protein degradation (-63% to -72%) was even greater, so that net in vitro protein balance improved dramatically. Insulin failed to inhibit protein degradation in fat pads of these rats as it does for fed animals. Although insulin stimulated protein synthesis in fat pads of fasted and 12 day diabetic rats, the absolute change was much smaller than that seen in the fed state. The inhibition of protein degradation by leucine also seems to be less in fasted animals, probably because leucine catabolism is slower in fasting. These results show that fasting and diabetes may improve protein balance in adipose tissue but diminish the regulatory effects of insulin.     

Norepinephrine and dopamine content in the brown adipose tissue of developing eu- and hypothyroid rats.

The norepinephrine and dopamine content of interscapular brown adipose tissue of developing rats was studied at intervals from birth to 50 days. Throughout this period, neonatal hypothyroidism is associated with a decreased norepinephrine content and with an increased dopamine content.     

Fatty acid synthetase activity in the liver and adipose tissue of rats fed with various carbohydrates

1. The inclusion of sucrose in the diet of rats led to an increase in hepatic fatty acid synthetase activity compared with that of rats fed with starch as the sole carbohydrate. The higher activity occurred within 18h of the introduction of sucrose and persisted with fluctuations for the 30 days of the experiment. Reversal of the diets in some rats after 21 days led to changes in the enzyme activity to values appropriate to the second diet. The plasma triglyceride concentration followed a similar pattern. 2. A comparison of the effects of diets with starch, glucose, maltose, sucrose or fructose showed that fructose gave the highest values of triglyceride content and of fatty acid synthetase activity in liver, but the lowest values of the synthetase activity in adipose tissue and the lowest values of plasma insulin concentration. These effects may perhaps be attributed to the low insulin response to fructose and to the high affinity of the liver for this sugar. 3. When the diet contained fructose or sucrose there was a correlation between hepatic synthetase activity and plasma triglyceride concentration. Neither of these, however, was related to plasma insulin concentration. On the other hand, there was a correlation between plasma insulin concentration and fatty acid synthetase activity in adipose tissue. 4. When rats were starved and then re-fed the differences in enzyme activities induced by fructose or glucose were minimized. This, together with the varying degree of difference during the course of the experiments, may explain why other workers, using the starvation-re-feeding technique and making measurements on one day only, have failed to observe differences in the activities of lipogenic enzymes in animals fed with either fructose or glucose.