Utilization of glycerol by rat adipose tissue in vitro

Metabolic effects of L-citrulline in type 2 diabetes

The prevalence of type 2 diabetes (T2D) is increasing worldwide. Decreased nitric oxide (NO) bioavailability is involved in the pathophysiology of T2D and its complications. L-citrulline (Cit), a precursor of NO production, has been suggested as a novel therapeutic agent for T2D. Available data from human and animal studies indicate that Cit supplementation in T2D increases circulating levels of Cit and L-arginine while decreasing circulating glucose and free fatty acids and improving dyslipidemia. The underlying mechanisms for these beneficial effects of Cit include increased insulin secretion from the pancreatic β cells, increased glucose uptake by the skeletal muscle, as well as increased lipolysis and β-oxidation, and decreased glyceroneogenesis in the adipose tissue. Thus, Cit has antihyperglycemic, antidyslipidemic, and antioxidant effects and has the potential to be used as a new therapeutic agent in the management of T2D. This review summarizes available literature from human and animal studies to explore the effects of Cit on metabolic parameters in T2D. It also discusses the possible mechanisms underlying Cit-induced improved metabolic parameters in T2D.

Enhanced utilization of glycerol for glyceride synthesis in isolated adipocytes from early pregnant rats

Adipose tissue normally has low glycerol kinase activity, but its expression is enhanced under conditions of augmented insulin sensitivity and/or obesity. Since these conditions occur during early pregnancy, the comparative utilization of glucose or glycerol by isolated adipocytes from rats at 0, 7, 14, or 20 days of pregnancy was studied. Incubations were carried out in the presence of [U(14)C]-glucose or -glycerol in medium supplemented or not with 5 mM glucose and 100 nM insulin. The conversion of glucose into esterified fatty acids and glyceride glycerol was greatest in adipocytes from 7-day pregnant rats, the effect being further enhanced by insulin. Both the amount of aquoporin 7 and the in vitro conversion of glycerol into glyceride glycerol were greatest in adipocytes of 7-day pregnant rats, the later being unaltered by insulin. In the presence of glucose, the overall glycerol utilization was lower than in its absence and glycerol conversion into glyceride glycerol was further decreased by insulin, the effect only being significant in adipocytes from 7-day pregnant rats. It is proposed that the enhanced utilization of glycerol for glyceride glycerol synthesis in adipose tissue contributes to the net accumulation of fat depots that normally takes place in early pregnancy.

Decreased lipolysis and enhanced glycerol and glucose utilization by adipose tissue prior to development of obesity in monosodium glutamate (MSG) treated-rats

OBJECTIVE

To determine the metabolic alterations that lead to the neonatal administration of monosodium glutamate (MSG), which results in arrested growth and obesity. ANIMALS AND DESIGN: Wistar rats were injected 5 times, every other day, with 4 g of MSG/kg b.w. or with hyperosmotic saline (controls), within the first 10 days of life, and were studied at the age of 30 days.

RESULTS

Body weight was lower, whereas adipocyte lipid content, cell diameter, surface area and volume were higher in MSG rats than in controls. Plasma glucose, insulin, NEFA, glycerol and triglyceride levels, and in vitro production of NEFA by lumbar fat pad pieces incubated under basal conditions or in the presence of epinephrine and epinephrine plus glucose in the media were lower in MSG than in control rats. In the same fat pad pieces, the conversion of 1-14C-glycerol into fatty acids was always enhanced and its conversion into glyceride glycerol was enhanced when incubations were carried out in the presence of epinephrine or glucose. Both the hormone sensitive lipase activity and mRNA expression were lower in adipose tissue from MSG rats. Besides, the number of insulin receptors, lipid synthesis from U14C glucose, 3H-2-deoxy D-glucose uptake and cellular GLUT4 translocation index were higher in adipocytes from MSG rats than from the controls.

CONCLUSION

It is proposed that an enhanced insulin sensitivity in 1 month old MSG rats is responsible for the decreased lipolytic activity and enhanced glucose uptake. In addition, the enhanced lipogenesis and glycerol reutilization seen in their adipose tissue, disturbs the normal balance between fat depots breakdown and accumulation in favor of the latter.

Amino acids and glucose utilization by different metabolic pathways in ascites-tumour cells

The utilization of amino acids and glucose by ascites tumour cells has been studied in order to elucidate which are their relative roles as energy substrates or building blocks for biosynthetic purposes, as well as the quantitative contribution of the different metabolic pathways involved. 1. Glucose is utilized at a rate of 1.1 mumol x min-1 x g cells-1. 93% is transformed into lactate, 0.7% used by the pentose phosphate pathway, 1.5% by the tricarboxylic acid cycle and 2% is for lipid synthesis. 2. ATP production is derived: 78% from glucose conversion into lactate, 1% from glucose oxidation and 19% from glutamine oxidation. 3. Glucose starvation, in the presence of all amino acids, leads to a 70% decrease in the rate of protein synthesis, due to the drop in ATP levels. 4. Pentose phosphate pathway flux increases by 75% when glycolysing cells are incubated in the presence of all amino acids. 5. Pyruvate is decarboxylated at a rate of 66 nmol x min-1 x g cells-1, 45-80% of it is incorporated into lipids instead of being oxidized, depending on the incubation conditions. 6. Non-essential amino acids (aspartate and glutamate) are oxidized at a low rate. Glutamine is oxidized at a rate 20-times and 35-times that of glucose and glutamate respectively. Glutamine can not replace glucose as the main energy source. 7. Leucine utilization, 28 nmol x min-1 x g cells-1, is very high compared with normal cells, due to the high rate of lipid and protein synthesis. Its oxidation is similar to that of non-tumoural cells. 8. Sterols account for 80% of the lipids synthesized either from leucine or glucose.

Effect of heparin on the utilization in vitro of labelled glycerides from triglyceride-rich lipoproteins in rat adipose tissue

Triglyceride-rich lipoproteins from adult rat plasma were labelled in vivo with 3H in the esterified fatty acids and 14C in the labelled glyceride glycerol of neutral lipids by injecting i.v. sodium 9-10 (n)-[3H] palmitate and [U-14C] glycerol, after which the prelabelled lipoproteins were purified by ultracentrifugation and dialysis. The lipoproteins were incubated in vitro, in the presence or not of heparin, with pieces of epididymal fat pads or isolated adipocytes from fed rats. The disappearance of both [3H]- and [14C] lipids from the media was greater when incubations were performed with adipocytes than with fat-pad pieces and it increased with heparin in both preparations. More 3H-label than 14C was found in the tissue lipids, a higher percentage being present in adipocytes than in fat-pad pieces, and the amount of label in tissue lipids was always enhanced by heparin. Some 14C-label appeared as esterified fatty acids in both tissue preparations and it also was enhanced by the presence of heparin. These findings are in agreement with the recognized influence of heparin on the release of lipoprotein lipase and show the direct relationship between heparin action and tissue ability to take up products of lipoprotein triglyceride breakdown. They also demonstrate the ability of adipose tissue to metabolize glycerol coming from the hydrolysis of lipoprotein glycerides.

Comparative utilization in vivo of [U-14C] glycerol, [2-3H] glycerol, [U-14C] glucose and [1-C14] palmitate in the rat

Female rats were injected i.v. with comparable trace amounts of [U-14C] glycerol, [2-3H] glycerol, [U-14C] glucose, or [1-14C] palmitate, and killed 30 min afterwards. The radioactivity remaining in plasma at that time was maximal in animals receiving [U-14C] glucose while the appearance of radioactive lipids was higher in the [U-14C] glycerol animals than in other groups receiving hydrosoluble substrates. The carcass, more than the liver, was the tissue where the greatest proportion of radioactivity was recovered, while the greatest percentage of radioactivity appeared in the liver in the form of lipids. The values of total radioactivity found in different tissues were very similar when using either labelled glucose or glycerol but the amount recovered as lipids was much greater in the latter. The maximal proportion of radioactive lipids appeared in the fatty-acid form in the liver, carcass, and lumbar fat pads when using [U-14C] glycerol as a hydrosoluble substrate, and the highest lipidic fraction appeared in adipose tissue as labelled, esterified fatty acids. In the spleen, heart, and kidney, most of the lipidic radioactivity from any of the hydrosoluble substrates appeared as glyceride glycerol. The highest proportion of radioactivity from [1-14C] palmitate appeared in the esterified fatty acid in adipose tissue, being followed in decreasing proportion by the heart, carcass, liver, kidney, and spleen. Thus at least in part, both labelled glucose and glycerol are used throughout different routes for their conversion in vivo to lipids. A certain proportion of glycerol is directly utilized by adipose tissue. The fatty acids esterification ability differs among the tissues and does not correspond directly with the reported activities of glycerokinase, suggesting that the alpha-glycerophosphate for esterification comes mainly from glucose and not from glycerol.

The effect of glucose, insulin and adrenaline on glycerol metabolism in vitro in rat adipose tissue

The uptake and utilization of [1-14C]glycerol was determined in pieces of rat epididymal fat-pads incubated in Krebs--Ringer bicarbonate buffer containing albumin. Insulin (200 muunits/ml), adrenaline (epinephrine; 0.5 mug/ml) and glucose (0, 5, 15 and 20 mM) were added to the medium. Changes in the specific radioactivity of the tracer during the incubation were taken into account in calculating the rate of glycerol utilization. Adrenaline decreased glycerol uptake, whereas insulin plus adrenaline increased it. The rate of incorporation of glycerol into glycerides was decreased by adrenaline and insulin, singly or together. Insulin increased the rate of formation of CO2 and fatty acids from glycerol. The formation of CO2 and fatty acids was further enhanced by insulin plus adrenaline. The decrease in glycerol uptake induced by adrenaline, the decrease in incorporation of glycerol into glycerides induced by insulin and insulin plus adrenaline and the synthesis of fatty acids were dependent on the presence of glucose in the medium. Thus insulin and adrenaline act on glycerol utilization in adipose tissue and some of their effects are mediated by action on glucose metabolism, but others are independent of this.

Glycerokinase activity and lipolysis regulation in brown adipose tissue of cold acclimated rats

Glycerol release by brown adipocytes from constant cold adapted rats was not stimulated by norepinephrine. On the contrary, the release was stimulated in rats adapted to a nycthemeral fluctuatiing temperature from 5 degrees to 28 degrees C. Glycerokinase activity was greatly increased in brown adipose tissue by cold adptation ; there was no change in the liver. However this increased activity cannot entirely explain the lack of norepinephrine stimulation of glycerol release in the brown adipose tissue of cold adapted rats.

Effect of lactation on lipolysis in rat adipose tissue

The concentrations of nonesterified fatty acids and glycerol in rat parametrial adipose tissue increased at peak lactation. Adipose tissue from lactating rats showed higher rates of release of nonesterified fatty acids and glycerol when incubated in vitro than did tissue from nonlactating rats, but there was a substantial increase in the esterification of fatty acids during involution. These results support earlier evidence that fat reserves were mobilized during lactation.