Effects of nicotinic acid on glucose tolerance and glucose incorporation into adipose tissue in hypertriglyceridaemia.

Nicotinic acid 4 g daily was given to 28 weight-stable hypertriglyceridaemic patients. The aim was to study its effects on serum lipoprotein lipid levels, intravenous glucose tolerance (k-values) and glucose incorporation into subcutaneous adipose tissue (GLIAT) in vitro. The investigations were performed prior to the nicotinic acid therapy, after 6 weeks, and 6 months of drug treatment. Fasting blood glucose levels increased by 13%, whereas k-values fell by 26% after the nicotinic acid treatment. Decrease in k-values was predicted from the initial k-values (R2-value = 0.76). GLIAT increased by 76%, while in the subgroup of eight patients, treated for 6 months with nicotinic acid, GLIAT increased by 331%. The changes in k-values and GLIAT were not significantly interrelated. Serum triglyceride levels were strongly decreased. The most likely explanation for the decrease in intravenous glucose tolerance is that nicotinic acid stimulates glucose output from the liver and that this effect outweighs the stimulating effects of the drug on glucose utilization in extrahepatic tissues. The latter is reflected by the increased uptake of glucose in adipose tissue. A stimulated GLIAT, reflecting formation of alpha-glycerophosphate in adipose tissue, might contribute to the reduction of serum triglyceride levels induced by nicotinic acid, since alpha-glycerophosphate is the acceptor of fatty acids assumed to be liberated from circulating triglycerides by lipoprotein lipase.     

Mechanisms underlying regional differences in lipolysis in human adipose tissue.

Catecholamine-induced lipolysis was investigated in nonobese females and males. Isolated subcutaneous adipocytes were obtained from the abdominal and gluteal regions. The lipolytic effect of noradrenaline was four to fivefold more marked in abdominal adipocytes than in gluteal fat cells. This regional difference was more apparent in females than in males. No site differences were observed when lipolysis was stimulated with agents acting at different postreceptor levels. The beta-adrenergic lipolytic sensitivity was 10-20 times greater in abdominal adipocytes from both sexes than in gluteal adipocytes. Abdominal adipocytes from females showed a 40 times lower alpha 2-adrenergic antilipolytic sensitivity than did gluteal adipocytes, but the adenosine receptor sensitivity was similar in both sites. Beta-receptor affinity for agonists displayed no site or sex variation. Abdominal adipocytes showed a twofold increased beta-adrenoceptor density than did gluteal cells from both sexes. The alpha 2-adrenoceptor density was similar in all regions, but in females the affinity of clonidine for these sites was 10-15 times lower in the abdominal fat cells compared with gluteal cells. In conclusion, regional differences in catecholamine-induced lipolysis are regulated at the adrenoceptor level, chiefly because of site variations in beta-adrenoceptor density. Further variations in the affinity properties of alpha 2-adrenergic receptor in females may explain why the regional differences in catecholamine-induced lipolysis are more pronounced in women than in men.     

Catecholamine-induced lipolysis in adipose tissue of the elderly.

Age-dependent alterations in the effects of catecholamines on lipolysis were investigated in 25 young (21-35 yr) and 10 elderly (58-72 yr) healthy, nonobese subjects using isolated adipocytes obtained from abdominal subcutaneous tissue. Basal lipolysis was not affected by aging, while the rate of catecholamine-stimulated lipolysis was reduced by 50% in the elderly subjects (P less than 0.005). To elucidate the mechanisms behind this phenomenon lipolysis was stimulated with agents that act at well-defined steps in the lipolytic cascade, from the receptor down to the final step: the activation of the protein kinase/hormone-sensitive lipase complex. All agents stimulated lipolysis at a 50% lower rate in elderly as compared with young subjects (P less than 0.05 or less). However, half-maximum effective concentrations of the lipolytic agents were similar in both groups. The antilipolytic effects of alpha 2-adrenoceptor agonists were also the same in young and old subjects. Moreover, the stoichiometric properties of the beta- and alpha 2-receptors did not change with increasing age. In vivo studies performed on the same individuals likewise demonstrated an impaired lipolytic responsiveness, with 50% lower plasma glycerol concentrations during exercise in the elderly subjects (P less than 0.05), in spite of a normal rise in plasma norepinephrine. The plasma glycerol levels correlated strongly to the glycerol release caused by catecholamine-stimulated lipolysis in vitro in both young and elderly subjects (r = 0.8-0.9, P less than 0.001). In conclusion, a decreased activation of the hormone-sensitive lipase complex appears to be the mechanism underlying a blunted lipolytic response of fat cells to catecholamine stimulation in elderly subjects. This finding may, explain the age-dependent decreased lipolytic response to exercise in vivo.     

Effects of uridine and inosine on glucose metabolism in skeletal muscle and activated lipolysis in adipose tissue.

In a first series of experiments, the effects of uridine and inosine on glucose metabolism in rat diaphragm muscle incubated in Krebs-bicarbonate buffer were studied. Uridine in concentrations of 10(-4) to 10(-6) M stimulated the uptake of glucose and increased the content of glycogen, but had no effect on the production of lactate. When diaphragm muscles were incubated in the buffer without glucose, uridine (10(-4)-10(-6) M) had no effects on the content of glycogen and on the production of lactate. On the other hand, inosine in concentrations of 10(-4) to 10(-6) M stimulated the uptake of glucose and the production of lactate, but had no effect on the content of glycogen in the muscle. In a second series of experiments, uridine (10(-4)-10(-5) M) and inosine (10(-4)-10(-7) M) inhibited the relase of glycerol from isolated rat epididymal adipose tissue in Krebs-bicarbonate buffer. Uridine and inosine in concentrations of 10(-4) M inhibited the epinephrine (10(-5) M)-, the norepinephrine (10(-5) M)- and the theophylline (10(-3) M)-stimulated lipolysis. Dibutyryl 3',5'-adenosine monophosphate-stimulated lipolysis was further activated in the presence of 10(-4) M uridine or inosine. Dose-response curves studies suggested that inosine, but not uridine, has a common receptor site with epinephrine in adipose tissue. These results demonstrated that both nucleosides stimulated the glucose uptake, but only uridine increased the synthesis of glycogen in the muscle. Both nucleosides also inhibited lipolysis in adipose tissue. The mechanism of antilipolytic action of these nucleosides is unknown, but one of the receptor sites for inosine might be adenylate cyclase.     

Effects of TNF-alpha on glucose metabolism and lipolysis in adipose tissue and isolated fat-cell preparations

We hypothesized that exposure to tumor necrosis factor-alpha (TNF-alpha) would significantly increase lactate production by adipose-tissue (AT) fragments and isolated adipocytes. We therefore examined the effects of TNF-alpha on the metabolism of epididymal AT explants during 24-hour tissue incubation. We also studied the effects of this 24-hour TNF-alpha tissue exposure on subsequent glucose metabolism and lipolysis by isolated adipocytes. Glycerol release into the medium was significantly increased (50%, P =.027) by exposure of the AT fragments to TNF-alpha (4 nmol/L) for 24 hours. During this time, glucose uptake from the medium and lactate release into the medium tended to increase, whereas leptin release into the medium tended to decrease, but these effects of TNF-alpha were not statistically significant. After the 24-hour AT-explant incubation, adipocytes were isolated by means of collagenase digestion from the AT fragments and subsequently tested in a short-term (60-minute) metabolic incubation. Prior exposure to TNF-alpha resulted in a significant increase in adipocyte glycerol release (P =.044), total glucose metabolism (P =.019), and lactate production (P =.037). With the exception of lactate, TNF-alpha produced no significant stimulation of the metabolites of glucose. The pattern of glucose metabolism elicited by TNF-alpha exposure differs from that usually attributed to a lipolytic hormone and suggests that the effects of TNF-alpha on glucose metabolism involve pathways separate from, or in addition to, its effects on lipolytic stimulation.     

Differential regulation of adipose tissue glucose transporters in genetic obesity (fatty rat). Selective increase in the adipose cell/muscle glucose transporter (GLUT 4) expression.

Adipocytes from young obese Zucker rats exhibit a hyperresponsive insulin-mediated glucose transport, together with a marked increase in cytochalasin B binding as compared with lean rat adipocytes. Here, we examined in these cells the expression of two isoforms of glucose transporter, the erythroid (GLUT 1) and the adipose cell/muscle (GLUT 4) types, in rats aged 16 or 30 d, i.e., before and after the emergence of hyperinsulinemia. GLUT 1 protein and mRNA levels were identical in the two genotypes at both ages. In contrast, the levels of GLUT 4 protein in obese rat adipocytes were 2.4- and 4.5-fold those of lean littermates at 16 and 30 d of age, respectively, in perfect agreement with the genotype effect on insulin-stimulated glucose transport activity. The levels of GLUT 4 mRNA per fat pad were increased 2.3- and 6.2-fold in obese vs. lean rats 16- and 30-d-old, indicating a pretranslational level of regulation. The obese phenotype was not associated with overexpression of GLUT 4 mRNA in gastrocnemius muscle. This work indicates that the fa gene exerts a differential control on the expression of GLUT 1 and GLUT 4 in adipose tissue and provides evidence that independent of hyperinsulinemia, genotype is a major regulatory factor of GLUT 4 expression in this tissue.     

In situ studies of catecholamine-induced lipolysis in human adipose tissue using microdialysis

The effects of catecholamines on lipolysis in situ were investigated in humans. Subcutaneous adipose tissue was microdialyzed with solvents containing adrenergic agents. Norepinephrine caused a rapid increase in the glycerol level in adipose tissue (lipolysis index) that was further increased by the alpha adrenoreceptor blocker phentolamine. At 10(-11) mol/l of norepinephrine caused a 100% stimulation of lipolysis (P less than .025). In the presence of phentolamine the lipolytic effects of catecholamines at 10(-12) mol/l was isoproterenol greater than epinephrine greater than norepinephrine. All these three lipolytic catecholamines caused a transient increase in the adipose tissue dialysate glycerol level, which peaked after 20 to 30 min of catecholamine exposure and then declined. The apparent tachyphylaxia could not be overcome by a gradual increase of the catecholamine concentration from 10(-12) to 10(-8) mol/l. However, the selective alpha-2 adrenoreceptor agonist clonidine caused a continuous and dose-dependent decrease in the dialysate glycerol level; the minimum effective concentration was 10(-9) mol/l. In conclusion, catecholamines have a lipolytic effect in situ at much lower concentrations than those in the circulation. This effect is transient and is related to beta adrenoreceptors. In additio, catecholamines have alpha adrenoreceptor-mediated effects on lipolysis in situ.     

Interstitial fluid concentrations of glycerol, glucose, and amino acids in human quadricep muscle and adipose tissue. Evidence for significant lipolysis in skeletal muscle.

To determine the relationship between circulating metabolic fuels and their local concentrations in peripheral tissues we measured glycerol, glucose, and amino acids by microdialysis in muscle and adipose interstitium of 10 fasted, nonobese human subjects during (a) baseline, (b) euglycemic hyperinsulinemia (3 mU/kg per min for 3 h) and, (c) local norepinephrine reuptake blockade (NOR). At baseline, interstitial glycerol was strikingly higher (P < 0.0001) in muscle (3710 microM) and adipose tissue (2760 microM) compared with plasma (87 microM), whereas interstitial glucose (muscle 3.3, fat 3.6 mM) was lower (P < 0.01) than plasma levels (4.8 mM). Taurine, glutamine, and alanine levels were higher in muscle than in adipose or plasma (P < 0.05). Euglycemic hyperinsulinemia did not affect interstitial glucose, but induced a fall in plasma glycerol and amino acids paralleled by similar changes in the interstitium of both tissues. Local NOR provoked a fivefold increase in glycerol (P < 0.001) and twofold increase in norepinephrine (P < 0.01) in both muscle and adipose tissues. To conclude, interstitial substrate levels in human skeletal muscle and adipose tissue differ substantially from those in the circulation and this disparity is most pronounced for glycerol which is raised in muscle as well as adipose tissue. In muscle, insulin suppressed and NOR increased interstitial glycerol concentrations. Our data suggest unexpectedly high rates of intramuscular lipolysis in humans that may play an important role in fuel metabolism.     

Differential inhibition of lipolysis in human adipose tissue by adrenergic beta receptor blocking drugs.

The effects of various adrenergic beta receptor agonists and antagonists on lipolysis (measured as glycerol release) in human adipose tissue in vitro were studied. Of the agonists investigated, the potency rank order was isoproteronol greater than norepinephrine greater than salbutamol. Adrenergic beta receptor blocking drugs inhibited catecholamineinduced lipolysis competitively. Propranolol was the overall most effective compound, followed by metoprolol, alprenolol and practolol, whereas butoxamine and H35/25 were weak inhibitors. The results indicate that the adrenergic reciptor mediating lipolysis in human adipose tissue is of type beta-1. Basal and theophylline-induced lipolysis was reduced when higher concentrations of these drug were used.     

Importance of the cyclic AMP concentration for the rate of lipolysis in human adipose tissue

1. The activation of lipolysis on incubation of human subcutaneous adipose tissue was examined in terms of the relationship between the release of glycerol and the concentration of tissue cyclic AMP. 2. A strong positive correlation was obtained between the maximum concentration of cyclic AMP and the rate of glycerol release in the presence of noradrenaline (r = 0.9), whereas, in the basal state, these two parameters were only weakly correlated (r = 0.45). 3. It appears that the noradrenaline-induced rate of lipolysis depends upon the maximal concentration of cyclic AMP that is present in human adipose tissue.     

beta-Receptor response to noradrenaline in cluster headache. A study of adipose tissue lipolysis

We have previously shown decreased lipolysis in both phases of cluster headache (CH), as an indication of a sympathetic dysregulation. Reduced lipolysis could be a result of diminished beta-receptor sensitivity in adipose tissue. The aim of this study was to measure the lipolytic response to noradrenaline in 10 CH patients in remission and in 10 healthy subjects, to estimate beta-receptor function. Microdialysis technique was used to measure the increase of glycerol, the end-product of lipolysis, during infusion of noradrenaline into the adipose tissue. Noradrenaline infusion resulted in a distinct elevation of glycerol. The average glycerol increase was significantly higher in CH patients (121% +/- 48) than in healthy subjects (77% +/- 41) (P < 0.05), which indicates increased beta-receptor response to noradrenaline in CH patients in remission. This may be due to up-regulated beta-receptor sensitivity, secondary to reduced sympathetic outflow and a primary autonomic disturbance in CH.     

An optimized method for fatty acid analysis, including quantification of trans fatty acids, in human adipose tissue by gas-liquid chromatography.

Considering the need for a quick direct method for measurement of the fatty acid composition including trans isomers of human adipose tissue we have developed a procedure using gas-liquid chromatography (GLC) alone, which is thus suitable for validation of fatty acid status in epidemiological studies. Fatty acids ranging in carbon number from 12 to 22 and with 0-6 double bonds were resolved and identified by capillary column GLC with a temperature program starting at 150 degrees C. Following injection, the oven temperature was increased at a rate of 3 degrees C/min to 200 degrees C, then held constant for 25 min, and finally raised at 25 degrees C/min to 225 degrees C. The trans and cis isomers of 18:1 were well separated from each other, as shown by silver-ion thin-layer chromatography. Verification by standards showed that the trans 18:1 isomers with a double bond in position 12 or lower were separated from the cis 18:1 isomers with a double bond in position 6 or higher. As the adipose tissue samples contained only small amounts of the 13t-, 14t- and 15t-18:1 isomers and the 4c- and 5c-18:1 isomers the overlapping was found to be minimal. The GLC method may also be valuable for determining the fatty acid profiles including total trans in other tissues.     

Influence of insulin on glucose metabolism and lipolysis in adipose tissue in situ in patients with liver cirrhosis

Abstract. The influence of insulin on lipolysis and glucose metabolism in abdominal adipose tissue was studied in situ with the microdialysis technique during a euglycaemic insulin clamp (1 mUkg^-1 min-1)in nine cirrhotic patients and 10 controls. The cirrhotic patients displayed a 50% decrease in glucose utilization rate during the clamp (P< 0.001). Dialysate glucose levels decreased similarly by 20–30%, in patients and controls, which in the presence of unchanged local blood flow in the adipose tissue in response to insulin, is at hand with a glucose uptake into the adipocytes of similar magnitude in both groups. Before and during the clamp, the arterial and dialysate levels of glycerol were higher in the patients than in the control subjects (anova P =0.001 and 0.048 in arterial blood and dialysate, respectively). In relative terms, however, insulin induced a 70% reduction of arterial and dialysate glycerol in both groups. The concentrations of lactate and pyruvate in the dialysate and blood increased in a similar way in both groups during hyperinsulinaemia. The results suggest an increased rate of lipolysis in cirrhotic patients. Insulin cannot lower it to normal, although it is still capable of achieving a relative reduction. No explanation was found in the adipose tissue to the insulin resistance to whole-body glucose utilization that was noted in the patients with cirrhosis.     

Effects of acipimox, a nicotinic acid derivative, on lipolysis in human adipose tissue and on cholesterol synthesis in human jejunal mucosa

The mode of action of acipimox (5-methyl-pyrazine carboxylic acid 4-oxide), an hypotriglyceridaemic agent, was examined in human adipose tissue and intestinal mucosa. The rates of release of fatty acids and glycerol from human adipose tissue were measured in vitro. The release of fatty acids and glycerol from adipose tissue maximally stimulated by isoprenaline (10(-5) mol/l) fell by 40 and 25% respectively (P less than 0.025 and P less than 0.025) in the presence of acipimox (10(-5) mol/l). In submaximally stimulated adipose tissue (isoprenaline 10(-7) mol/l) acipimox (10(-4) mol/l) fully inhibited release of fatty acids (P less than 0.05) and glycerol (P less than 0.025) to basal rates. In unstimulated adipose tissue acipimox (10(-3) mol/l) reduced the rate of glycerol release (P less than 0.05), but not the rate of fatty acid release. Cholesterol synthesis in jejunal mucosa was measured in vitro by the incorporation of [2-14C]-acetate into sterols. Addition of cholesterol to the incubation reduced [2-14C]acetate incorporation into sterols from 8.7 +/- 2.1 (mean +/- standard error) to 3.7 +/- 1.0 pmol h-1 mg-1 of tissue (P less than 0.01). Acipimox at 10(-4)-10(-2) mmol/l had no consistent effect on cholesterol synthesis. Acipimox appears to exert its main hypolipidaemic effect by reducing lipolysis and free fatty acid flux to the liver, thereby reducing the precursor pool size of very low density lipoprotein (VLDL)-triglyceride and VLDL synthesis.     

Effects of nicotinic acid treatment on glyceride formation and lipolysis in adipose tissue of hyperlipidemic patients

Summary Thirty-one weight-stable patients with different types of hyperlipoproteinemia were treated daily with 4g nicotinic acid for 6 weeks. Effects of this therapy on adipose tissue metabolism were evaluated. By using biopsy specimens of subcutaneous adipose tissue, fatty acid and glucose incorporation into adipose tissue glycerides were measured in vitro as well as glycerol and fatty acid release, which allowed us to estimate adipose tissue lipolysis. The amount of fatty acids produced by lipolysis and thereafter utilized within adipose tissue without being released (fatty acid retention) was estimated. Fatty acid and glucose incorporation into adipose tissue, glycerol release and fatty acid retention values increased, but serum triglyceride levels decreased (allP<0.001) after nicotinic acid treatment. The change in fatty acid incorporation was positively correlated with changes in glucose incorporation into adipose tissue (r=0.53,P<0.01) and fatty acid retention (r=0.76,P<0.001). Although adipose tissue lipolysis, measured as glycerol release, increased, the lipolyzed fatty acids were retained in adipose tissue, suggesting an enhanced synthesis of glycerides both from exogenous and endogenous sources. The increase in fatty acid incorporation into adipose tissue indicates that the decrease in serum triglyceride levels produced by nicotinic acid treatment may partly be due to the fact that this drug promotes incorporation of fatty acids, derived from lipoprotein-carried triglycerides in the blood, into adipose tissue glycerides.     

Effects of nicotinic acid treatment on fatty acid composition of plasma lipids and adipose tissue in hyperlipidaemia.

Effects of 6-week treatment with 4 g daily of nicotinic acid on fatty acid composition in different serum lipids and in adipose tissue glycerides were studied in 31 hyperlipidemic patients. The percentages of eight fatty acids in triglycerides, phospholipids and cholesteryl esters of whole plasma as well as in subcutaneous adipose tissue glycerides were measured. Nicotinic acid treatment produced decreases in triglyceride and total cholesterol concentrations of VLDL and LDL, whereas HDL total cholesterol levels in serum increased after drug therapy, all p less than 0.01. There were reductions in the relative contents of myristic acid in plasma phospholipids (from 0.4% to 0.3%; p less than 0.05) and cholesteryl esters (from 0.9% to 0.7%; p less than 0.001). There were decreases in the percentages of stearic acid in plasma phospholipids (from 17.0% to 15.0%) and cholesteryl esters (from 1.2% to 1.0%; both p less than 0.001). The relative contents of polyunsaturated fatty acids, mainly linoleic acid, in plasma phospholipids were increased (from 32% to 33.5%; p less than 0.05). There were reductions in the linolenic acid contents of adipose tissue (from 1.5% to 1.1%) and plasma triglycerides (from 1.1% to 0.8%), both p less than 0.05, possibly indicating increased conversion of linolenic acid to prostaglandins. There was no relationship between changes in the percentages of individual fatty acids and changes in triglyceride or total cholesterol levels of whole serum and its VLDL, LDL and HDL fractions.(ABSTRACT TRUNCATED AT 250 WORDS)