Stimulation of cyclic adenosine 3′,5′-monophosphate accumulation and lipolysis in fat cells by adenosine deaminase

Summary The basal lipolytic activity of isolated fat cells of the rat was greatly enhanced in the presence of 0.01 to 30 g adenosine deaminase (ADA) per ml. This effect was more pronounced in dilute (20000 cells/ml) than in concentrated cell suspensions (100000 cells/ml); this is possibly due to the presence, in the incubation medium, of a high concentration of inosine which is formed by the deamination of the large amounts of adenosine released from high concentrations of fat cells. Inosine, although less potent than adenosine as an antilipolytic agent, markedly inhibited ADA-induced lipolysis at concentrations between 10 to 100 M. The lipolytic effect of ADA was identical with the stimulation of lipolysis by 1 M noradrenaline or 1 mM theophylline, while 1 mM dibutyryl cyclic AMP yielded two-fold higher values. The effects of ADA and lipolytic agents at maximally stimulating concentrations were not additive.After 5 min of incubation maximally effective concentrations of ADA which were also maximal with respect to lipolysis caused a 3- to 6-fold elevation of cyclic AMP levels in fat cells. A similar increase was observed with maximally effective concentrations of theophylline, whereas noradrenaline produced a 100- to 200-fold elevation. This indicates that a small accumulation of cyclic AMP may be sufficient to trigger the full lipolytic response. Furthermore, ADA, like theophylline, acted synergistically with noradrenaline and prevented the fall of cyclic AMP levels during 30 min of incubation.Insulin (100 U/ml) and nicotinic acid (0.1 M) decreased cyclic AMP accumulation and glycerol production induced by ADA.The results support the hypothesis that adenosine is released from isolated fat cells and that this nucleoside may serve as an inhibitor of adenyl cyclase activity, thus regulating cyclic AMP-dependent processes in adipose tissue.     

Activation and inhibition of lipolysis in isolated fat cells by various inhibitors of cyclic AMP phosphodiesterase

Summary The effects of methylxanthines, papaverine, dipyridamole and imipramine on lipolysis and phosphodiesterase activity of rat adipose tissue were investigated. Lipolysis in isolated fat cells was stimulated by theophylline and caffeine whereas papaverine, dipyridamole and imipramine had no substantial effect on the basal lipolytic rate. Lipolysis induced by noradrenaline was potentiated by theophylline, but blocked by papaverine, dipyridamole and imipramine at concentrations between 0.02 to 0.2 mM. These agents also depressed lipolysis induced by theophylline and dibutyryl cyclic AMP and reduced the lipolytic activity of homogenates of adipose tissue. The activity of phosphodiesterase assayed over a wide range of substrate concentrations revealed two different Michaelis constants. Both types of phosphodiesterase were inhibited by theophylline, papaverine, dipyridamole and imipramine in a competitive manner, the low K _m enzyme being more sensitive for inhibition than the high K _m enzyme. On both types of phosphodiesterase papaverine and dipyridamole proved to be 10 to 100 times more potent inhibitors than theophylline and imipramine. To explain the antilipolytic effect of phosphodiesterase inhibitors it is assumed that they do not only affect substrate binding of cyclic AMP to phosphodiesterase but also displace cyclic AMP from the binding site on protein kinase, thus acting as inhibitors of the activation process within the lipolytic system.     

Effects of adenosine derivatives on cAMP accumulation and lipolysis in rat adipocytes and on adenylate cyclase in adipocyte plasma membranes

Summary N⁶-monosubstituted adenosine (Ad)-derivatives and Ad-derivatives altered in the adenine-or ribose-moiety have been compared with Ad in their effects on noradrenaline (NA)-stimulated cAMP accumulation, on lipolysis stimulated by NA or theophylline (THEO) and on adenylate cyclase (AC) activity of adipocyte plasma membranes.In isolated adipocytes about 0.01 M Ad caused a 50% inhibition of cAMP accumulation stimulated maximally by 1M NA. Depending upon the structure of substituent, the Ad-N⁶-derivatives were up to one order of magnitude either more or less active than Ad itself. 2-fluoro-Ad was nearly as active as Ad, whereas 2,5-dideoxy-Ad and 2-deoxy-Ad were practically ineffective as inhibitors of NA-stimulated cAMP accumulation. All compounds showed the same order of potency relative to Ad, when tested against lipolysis stimulated maximally by 1 mM THEO or submaximally by 0.3 M NA.In adipocyte plasma membranes a 50% inhibition of AC activity stimulated by 10M NA was observed at about 10 M Ad. None of the N⁶-substituted derivatives had any effect on either basal or NA-stimulated AC activity, whereas 2,5-dideoxy-Ad proved to be about 40 times more potent than Ad. 2-deoxy-Ad and 2-fluoro-Ad were nearly equipotent to Ad. Similar results were obtained, if AC was stimulated with 5-guanylylimidodiphosphate or NaF. Neither the N⁶-derivatives nor THEO could reverse the inhibitory effect of Ad on AC in plasma membranes.It is concluded, that different mechanisms are involved in the inhibitory effects of Ad on cAMP accumulation and lipolysis in intact cells and on AC activity of adipocyte plasma membranes.     

Effects of short-term dietary change from high fat to high carbohydrate diets on the storage and utilization of glycogen and triacylglycerol in untrained rats

The effects of short-term diet change from high fat (F) to high carbohydrate (C) (or vice versa) on the storage and utilization of glycogen and triacylglycerol (TG) in muscle and liver were studied in untrained rats. Rats were fed on an F or C diet for 28 days. For an additional 3 days, half of the rats in both F and C groups were fed the same diets as before (F-F and C-C) and the other half of the rats were switched to the counterpart diets (F-C and C-F). On the final day of the experiment, half of the rats in each diet group were exercised by swimming for 1.5 h and the other half were rested. Short-term diet change from F to C diets increased, but the change from C to F diets decreased, glycogen stores of soleus and plantaris muscles and liver, resulting in no difference in glycogen stores between F-C and C-C, and between F-F and C-F. The dietary change also had an affect on TG stores of red gastrocnemius muscle and liver - however, muscle TG stores were still higher in F-C than in C-C and C-F, and there were no differences in liver TG stores between F-C and C-F. Exercise decreased muscle glycogen contents markedly in F-C and C-C, whereas, it decreased muscle TG concentrations in F-F and C-F. Liver glycogen depletion was lower in F-C than in other groups. Lipolytic activities of epididymal adipose tissue at rest and postexercise were no differences between F-F and F-C, and were higher in F-C than in C-C and C-F. -adrenergic receptor binding was determined with [¹²⁵I] iodocyanopindolol, and maximal numbers of -adrenergic receptor of plasma membrane from perirenal adipose tissue were approximately 170%–200% higher in F-C than in other groups at rest and postexercise. These results suggested that short-term C diet fed rats adapted to F diet enhanced not only glycogen stores of muscle and liver but also did not decrease lipolytic activity of adipose tissue with increased -adrenergic receptor density, resulting in the preservation of energy reserves (glycogen and TG) of muscle at rest, and liver glycogen sparing during exercise.     

Metabolic effects of glucose,medium chain triglyceride and long chain triglyceride feeding before prolonged exercise in rats

Summary The purpose of this study was to test the hypothesis that oral ingestion of lipids could increase endurance by slowing the rate of glycogen depletion. Trained rats were killed after a 2 h run on a rodent treadmill, following an intragastric infusion of water, glucose, medium chain triglycerides (MCT) or long chain triglycerides (LCT). Glucose and triglycerides were administered in equicaloric concentrations (50 kJ).The results show that oral ingestion of lipids (MCT or LCT) did not reduce glycogen depletion in liver, heart or skeletal muscle after exercise whereas the fat diet increased muscle and heart glycogen stores in resting conditions. In contrast, glucose feeding induced a significant sparing effect on endogenous carbohydrate utilization and reduced physical exercise lipolysis. These data indicated, firstly, that enhanced lipid availability induced by a single lipid meal before exercise was not able to modify the glycogen depletion occuring after exercise and, secondly, that the glucose/fatty acid cycle was not effective in these conditions. The comparison between lipids indicated that the effect on glycogen use of MCT did not differ from that of LCT, and did not seem to be of any particular importance during physical exercise.     

Metabolic adaptation to prolonged exercise

Summary A study was undertaken to evaluate and to examine the role of substrate supply in 50 healthy subjects after long distance events, such as 10 km, 25 km, and marathon races. The metabolic, variables of carbohydrate metabolism were greatest in 10-km runners, with the highest increase in glucose, lactate, and pyruvate, while in marathon runners only moderate changes were observed. Marathon competitors gave the greatest decrease in insulin concentration whereas glucagon and cortisol showed a contrary tendency. As for lipid concentrations, the most remarkable point was that after the marathon competition the best runners had the highest increase in free fatty acids; the longer the race, the higher were the -hydroxybutyrate and acetoacetate levels after the competition.It is important to emphasize that the limiting factor up to 90 min duration is the competitor's ability to deplete the stores of glycogen. Beyond 90 min (or 25 km) the decrease in insulin, the rise in cortisol and the higher concentration of ketnne bodies found indicate a change in metabnlic response.     

ACTH-Induced lipolysis in rat adipocytes: Structure-activity relationships

Summary The lipolytic action of natural porcine ACTH_1–39 and of a number of highly purified synthetic ACTH peptide fragments was studied using rat adipocytes. Of the analogues tested, only ACTH_1–24 exhibited full lipolytic activity with respect to intrinsic activity and affinity. Several shorter fragments appeared to be full agonists but had lower affinity. Fragments ACTH_5–10 and ACTH_7–10 were inactive. No antagonistic effects against the lipolytic action of ACTH could be demonstrated with substimulatory doses of ACTH_1–16, ACTH_1–10, ACTH_7–24 and ACTH_11–24. Based on the relative potency derived from dose-response curves, a more refined model with respect to the active centers being encoded in various sequences of the hormone, is proposed.     

Deletion of adipose triglyceride lipase abolishes blood flow increase after β3-adrenergic stimulation in visceral adipose tissue of mice

Dynamic changes in adipose tissue blood flow (ATBF) with nutritional status play a role in the regulation of metabolic and endocrine functions. Activation of the sympathetic nervous system via β-adrenergic receptors (β-AR) contributes to the control of postprandial enhancement of ATBF. Herein, we sought to identify the role of each β-AR subtype in the regulation of ATBF in mice. We monitored the changes in visceral epididymal ATBF (VAT BF), induced by local infusion of dobutamine, salbutamol, and CL316,243 (a selective β1-, β2-, and β3-AR agonist, respectively) into VAT of lean CD-1 mice and global adipose triglyceride lipase (ATGL) knockout (KO) mice, using laser Doppler flowmetry. Administration of CL316,243, known to promote lipolysis in adipocytes, significantly increased VAT BF of CD-1 mice to a greater extent compared to that of the vehicle, whereas administration of dobutamine or salbutamol did not produce significant differences in VAT BF. The increase in VAT BF induced by β3-AR stimulation disappeared in ATGL KO mice as opposed to their wild-type (WT) littermates, implying a role of ATGL-mediated lipolysis in the regulation of VAT BF. Different vascular reactivities occurred despite no significant differences in vessel density and adiposity between the groups. Additionally, the expression levels of the angiogenesis-related genes were significantly higher in VAT of ATGL KO mice than in that of WT, implicating an association of ATBF responsiveness with angiogenic activity in VAT. Our findings suggest a potential role of β3-AR signaling in the regulation of VAT BF via ATGL-mediated lipolysis in mice.