Plasma FFA Levels Research Articles

Role of the adipocyte, free fatty acids, and ectopic fat in pathogenesis of type 2 diabetes mellitus: peroxisomal proliferator-activated receptor agonists provide a rational therapeutic approach.

Type 2 diabetes mellitus (T2DM) is characterized by insulin resistance in liver and muscle and impaired insulin secretion. Considerable evidence also implicates deranged adipocyte metabolism and altered fat topography in the pathogenesis of glucose intolerance in T2DM. 1) Fat cells are resistant to insulin’s antilipolytic effect, leading to day-long elevated plasma FFA levels. Chronically increased plasma FFA stimulates gluconeogenesis, induces hepatic/muscle insulin resistance, and impairs insulin secretion in genetically predisposed individuals. These FFA-induced disturbances are referred to as lipotoxicity. 2) Dysfunctional fat cells produce excessive amounts of insulin resistance-inducing, inflammatory, and atherosclerotic-provoking cytokines and fail to secrete normal amounts of insulin-sensitizing adipocytokines. 3) Enlarged fat cells are insulin resistant and have diminished capacity to store fat. When adipocyte storage capacity is exceeded, lipid “overflows” into muscle, liver, and perhaps -cells, causing muscle/ hepatic insulin resistance and impaired insulin secretion. Thiazolidinediones enhance adipocyte insulin sensitivity, inhibit lipolysis, reduce plasma FFA, and favorably influence the production of adipocytokines. Thiazolidinediones also redistribute fat within the body (decreased visceral and hepatic fat; increased sc fat) and decrease intracellular concentrations of triglyceride metabolites in muscle, liver, and -cells, contributing to improvements in muscle/hepatic insulin sensitivity and pancreatic function in type 2 diabetics.

Left-ventricular diastolic dysfunction may be prevented by chronic treatment with PPAR-alpha or -gamma agonists in a type 2 diabetic animal model.

The aim of this study was to determine whether the peroxisome proliferator-activated receptor (PPAR) ligands could prevent left-ventricular diastolic dysfunction (LVDD) in rats with advanced diabetes. In addition, this study examined whether the activity of malonyl-CoA decarboxylase (MCD), which is an enzyme related to the degradation of malonyl-CoA that is known to regulate the fatty acid metabolism, is changed by the diabetic state itself or by treatment with the PPAR ligands. Male Otsuka Long-Evans Tokushima Fatty (OLETF) rats, a model of type 2 diabetes, aged 28 weeks, were divided into 3 groups: the untreated, pioglitazone-treated (10 mg/kg/d), and fenofibrate-treated (150 mg/kg/d) groups. The rats were treated for 10 weeks. Male Long-Evans Tokushima Otsuka (LETO) rats were used as nondiabetic control. Doppler echocardiography and measurements of the MCD activity at the myocardium were performed. At the age of 38 weeks, the OLETF rats treated with either pioglitazone or fenofibrate showed an improvement in the plasma glucose levels after glucose loading as well as an improvement in the fasting plasma insulin, triglyceride, and FFA levels compared to the untreated OLETF rats. The untreated OLETF rats showed a prolonged deceleration time of the E-wave (DTE) (74.3 +/- 3.7 vs LETO, 56.3 +/- 3.8 ms, P < 0.05) and a reduced ratio of the peak early diastolic velocity wave to the late diastolic wave (E/A ratio) (1.25 +/- 0.06 vs LETO 1.54 +/- 0.08, P < 0.05). Pioglitazone treatment in the OLETF rats improved the DTE (51.6 +/- 1.7 ms, P < 0.05), and the fenofibrate treatment also improved the DTE (61.4 +/- 4.3 ms, P < 0.05) and E/A ratio (1.57 +/- 0.05, P < 0.05) compared to the untreated OLETF rats. The parameters related to the systolic function did not change among the groups at both pre- and post-treatments. The MCD activity of the myocardium was remarkably lower in the OLETF rats compared to the LETO rats (3.26 +/- 0.38 vs 7.76 +/- 0.84 nmol/min/mg protein, P < 0.05). The pioglitazone and fenofibrate treatments resulted in an increase in the MCD activity compared to that in the untreated rats (7.20 +/- 0.74 and 8.33 +/- 0.83 nmol/min/mg protein, P < 0.05, respectively). The PPAR-alpha or -gamma agonists prevented LVDD in the advanced diabetic rat hearts, possibly through an improvement in the fatty acid metabolism in the myocardium or a correction of the hyperglycemia and/or hyperlipidemia.

The leptin-like effects of 3-d peripheral administration of a melanocortin agonist are more marked in genetically obese Zucker (fa/fa) than in lean rats.

The effects of a 3-d peripheral administration of an alpha-MSH agonist, MTII, on body weight and the expression of uncoupling proteins (UCPs) and carnitine palmitoyltransferase-1 were determined in lean and genetically obese fa/fa rats by comparing MTII-treated animals with two different control groups, one being ad libitum fed, the other pair-fed to the amount of food consumed by MTII-treated rats. MTII treatment of lean and obese rats lowered food intake and body weight, the effects being more marked in obese than in lean rats. In both groups, MTII administration suppressed the increased plasma FFA levels brought about by food restriction. In lean rats, MTII prevented the decrease in brown adipose tissue UCP1, UCP2, and UCP3 expression and muscle UCP3 occurring during food restriction. In obese animals, MTII markedly increased brown adipose tissue (7-fold) and muscle (2.5-fold) UCP3 expression. The decrease in liver carnitine palmitoyltransferase-1 elicited by food restriction in lean and obese rats was prevented by MTII administration. In summary, the effects of MTII resemble those of leptin and are more marked in obese than in lean animals, in keeping with their reported reduced endogenous melanocortin tone. Melanocortin agonists may be useful in the treatment of obesity associated with impaired leptin signaling.

The Leptin-Like Effects of 3-d Peripheral Administration of a Melanocortin Agonist Are More Marked in Genetically Obese Zucker (fa/fa) than in Lean Rats

The effects of a 3-d peripheral administration of an α-MSH agonist, MTII, on body weight and the expression of uncoupling proteins (UCPs) and carnitine palmitoyltransferase-1 were determined in lean and genetically obese fa/fa rats by comparing MTII-treated animals with two different control groups, one being ad libitum fed, the other pair-fed to the amount of food consumed by MTII-treated rats. MTII treatment of lean and obese rats lowered food intake and body weight, the effects being more marked in obese than in lean rats. In both groups, MTII administration suppressed the increased plasma FFA levels brought about by food restriction. In lean rats, MTII prevented the decrease in brown adipose tissue UCP1, UCP2, and UCP3 expression and muscle UCP3 occurring during food restriction. In obese animals, MTII markedly increased brown adipose tissue (7-fold) and muscle (2.5-fold) UCP3 expression. The decrease in liver carnitine palmitoyltransferase-1 elicited by food restriction in lean and obese rats was prevented by MTII administration. In summary, the effects of MTII resemble those of leptin and are more marked in obese than in lean animals, in keeping with their reported reduced endogenous melanocortin tone. Melanocortin agonists may be useful in the treatment of obesity associated with impaired leptin signaling.

FFAs and QT intervals in obese women with visceral adiposity: effects of sustained weight loss over 1 year.

We evaluated 66 obese patients grouped by waist-to-hip ratio (WHR) into group A (WHR > 0.85, n = 30) and group B (WHR < or = 0.85, n = 36), before and after 1 yr of diet-induced weight loss compared with 25 nonobese women. Before diet, the longest values of QT intervals and the highest levels of FFA and catecholamines were in group A (P < 0.01). In obese women (both groups), the corrected QT (QTc); interval correlated with plasma FFA (P < 0.01) and catecholamine (P < 0.02) concentrations. After 1 yr of diet, at the same levels of body weight reduction, the decrement of the QTc interval (P < 0.02), FFA (P < 0.01) and catecholamine (P < 0.02) levels were significantly greater in-group A than group B. In multivariate analysis, the decline of the QTc interval after weight loss was associated with changes in plasma FFA independently of changes in WHR and plasma catecholamines. Our data suggest that the QTc interval is tightly correlated with plasma FFA levels; shortening of cardiac repolarization times in the course of long-lasting weight reduction may reduce the risk of ventricular electrical instability, especially in women with abdominal adiposity.

FFAs and QT Intervals in Obese Women with Visceral Adiposity: Effects of Sustained Weight Loss Over 1 Year

We evaluated 66 obese patients grouped by waist-to-hip ratio (WHR) into group A (WHR > 0.85, n = 30) and group B (WHR ≤ 0.85, n = 36), before and after 1 yr of diet-induced weight loss compared with 25 nonobese women. Before diet, the longest values of QT intervals and the highest levels of FFA and catecholamines were in group A (P < 0.01). In obese women (both groups), the corrected QT (QTc); interval correlated with plasma FFA (P < 0.01) and catecholamine (P < 0.02) concentrations. After 1 yr of diet, at the same levels of body weight reduction, the decrement of the QTc interval (P < 0.02), FFA (P < 0.01) and catecholamine (P < 0.02) levels were significantly greater in-group A than group B. In multivariate analysis, the decline of the QTc interval after weight loss was associated with changes in plasma FFA independently of changes in WHR and plasma catecholamines. Our data suggest that the QTc interval is tightly correlated with plasma FFA levels; shortening of cardiac repolarization times in the course of long-lasting weight reduction may reduce the risk of ventricular electrical instability, especially in women with abdominal adiposity.

Adrenergic Regulation of Lipid Mobilization in Fishes; a Possible Role in Hypoxia Survival

Lipids are for fish the major energy source. The nutritional conditions of most species vary throughout the year considerably. Thus storage and mobilization of lipids have to be tightly controlled, yet little is known about its control. Though several hormones are known to have a lipolytic effect, short term regulation of lipolysis is known for catecholamines only. Catecholamines are usually released under stress conditions, including hypoxia. In mammals these hormones have a strong lipolytic action, causing high plasma fatty acids levels during hypoxia. In contrast to mammals, several fish species show a decrease of plasma FFA-levels during hypoxia and anoxia. However, some studies gave contrasting results when catecholamines were administrated to different fish species. The reason for this may be due to opposing effects of catecholamines on lipolysis in different tissues. From catecholamine administration experiments in cannulated carp there is evidence that norepinephrine inhibits lipolysis via β1- and β3- adrenoceptors while β2-adrenoceptors are involved in stimulation of lipolysis. Thus the opposite responses of different β-adrenoceptors may explain the conflicting in-vivo results obtained with fish. In vitro studies with adipocytes from different fish species confirm that activation of β-adrenoceptors suppresses lipolysis, while the opposite occurs in hepatocytes. Inhibiting β1- and β3- adrenoceptors in adipocytes were shown to be involved. Under hypoxia β-oxidation is inhibited, resulting in accumulation of fatty acids together with intermediates of the β-oxidation. This process may cause severe cellular damage in mammalian tissues, which ‘apparently’ does not occur in fishes. Fishes encounter (environmental) hypoxia on a regular basis, while for mammals hypoxia/anoxia is a pathological phenomenon. Hence the suppression of lipolysis in fishes under hypoxia (by β1- and β3- adrenoceptors) may be considered as a survival mechanism lost in higher vertebrates.

Hydrogenated fat consumption affects acylation-stimulating protein levels and cholesterol esterification rates in moderately hypercholesterolemic women

To determine whether hydrogenated fat consumption alters triglyceride metabolism and cholesterol esterification rates, 14 women (65–71 years of age) were provided with each of four diets for 5-week periods according to a randomized cross-over design. The experimental diets contained either soybean oil (SO), low trans squeeze (SQM), medium trans tub (TM), or high trans stick (SM) margarines. Triglyceride uptake by adipose tissue was determined by measuring plasma acylation-stimulating protein (ASP), FFA, glucose, and insulin levels, while rates of transfer and esterification rate of newly synthesized cholesterol (ER) were derived by using plasma CETP levels and the deuterium incorporation methodology. Plasma ASP levels were lowest (P < 0.05) in subjects on the SM diet (33.4 ± 12.7 nM) compared with the SO (48.7 ± 17.0 nM) and SQM (50.7 ± 15.7 nM) diets. Conversely, FFA were highest (P < 0.05) on the SM diet (0.86 ± 0.45 mM) relative to all the other diets. No differences were observed in plasma glucose and insulin levels among diets. A trend toward higher CETP levels after consumption of the SM diet was observed. However, the ER was lowest (P < 0.05) after the SM (0.111 ± 0.062 g·day−1) diet and highest after consumption of the SQM (0.216 ± 0.123 g·day−1) diet. In addition, ASP levels were negatively correlated with FFA (r = −0.63, P < 0.05), LDL cholesterol (r = −0.56, P < 0.05), and TG (r = −0.41, P < 0.05), whereas FFA was positively correlated with apolipoprotein B-containing lipoproteins (r = 0.58 and 0.47, for VLDL and LDL cholesterol, P < 0.05), and negatively correlated with HDL cholesterol (r = −0.51, P < 0.05). The ER was found to positively correlate with HDL cholesterol and HDL2 subfraction (r = 0.53 and 0.45, respectively, P < 0.05). Taken together, these data demonstrate that the alterations in circulating lipid levels, commonly observed with consumption of hydrogenated fat-rich diets, can be explained in part by changes in ASP activity as well as newly synthesized cholesterol ER.—Matthan, N. R., K. Cianflone, A. H. Lichtenstein, L. M. Ausman, M. Jauhiainen, and P. J. H. Jones. Hydrogenated fat consumption affects acylation-stimulating protein levels and cholesterol esterification rates in moderately hypercholesterolemic women.

Effects of meal intake on materno-foetal exchanges of energetic substrates in the pig.

Catheters were implanted in 18 gilts at 99 days of pregnancy to study the effects of meal intake on uterine and umbilical uptake of energetic substrates in the conscious pig. Blood samples were withdrawn at 105 days of pregnancy from 10 min before and up to 90 min after feeding of a 2.5-kg meal. Plasma glucose was 2.2 to 2.5 times lower and blood lactate 2 to 3 times higher in the foetus than in the sow. Glucose and lactate increased after the meal. Their umbilical uptake amounted to 0.32 and 0.26 mmol x L(-1), respectively. Fructose was found in large amounts in foetal plasma (4.3 mmol x L(-1)), but it did not seem to be metabolised by the foetus. Meal intake decreased plasma levels of FFA and glycerol in the sows, whereas they increased in the foetuses. A small FFA and glycerol umbilical uptake was recorded (14 and 6 micromol L(-1), respectively). Most features of the materno-foetal exchanges in the porcine species resemble those of other species, especially ruminants.

Food deprivation limits insulin secretory capacity in postpubertal rats.

ABSTRACT Because prenatal and perinatal undernutrition are associated with type 2 diabetes later in life, we posed the question whether nutrient deprivation during puberty would also result in a decreased ability to secrete insulin. Chronically catheterized, unstressed Sprague Dawley rats, fed ad libitum, were studied before puberty (Pre, n = 14) and after puberty (Post, n = 8). Moderately caloric-restricted rats (fed 70% of the control diet, n = 9), were studied after puberty. Insulin secretion was assessed using a hyperglycemic clamp at a glucose concentration of 300 mg/dL, or with a primed continuous infusion of intralipid (plasma FFA levels approximately 1.5 mM) at a plasma glucose concentration of 200 mg/dL. Stimulated insulin levels increased in Post rats by 3- to 4-fold compared with Pre rats (from 4.6 +/- 0.4 ng/mL Pre to 12.8 +/- 0.7 ng/mL Post, and from 4.5 +/- 0.4 ng/mL Pre to 15.8 +/- 0.7 ng/mL Post, respectively, p < 0.001, at a glucose concentration of 300 mg/dL, and 200 mg/dL with intralipid). Caloric restriction prevented any rise in insulin secretion (3.8 +/- 0.5 and 4.6 +/- 0.5 ng/mL in the caloric-restricted rats at glucose concentrations of 300 mg/dL and 200 mg/dL with intralipid, respectively). A semiquantitative reverse-transcriptase PCR procedure was used to assess basal and stimulated insulin mRNA levels. Caloric restriction did not compensate by enhancing insulin mRNA levels in response to glucose stimulation. Moderate food deprivation during puberty reduced the capacity of the pancreas to secrete insulin in response to different nutrient stimuli. We hypothesize that puberty has an important role in beta-cell maturation and any major nutrient modification may have deleterious consequences later in life.

Low adipocyte IRS-1 expression is associated with both insulin resistance and increased plasma FFA levels in response to stress

Low adipocyte IRS-1 expression is associated with both insulin resistance and increased plasma FFA levels in response to stress

Malonyl-CoA content and fatty acid oxidation in rat muscle and liver in vivo.

Malonyl-CoA acutely regulates fatty acid oxidation in liver in vivo by inhibiting carnitine palmitoyltransferase. Thus rapid increases in the concentration of malonyl-CoA, accompanied by decreases in long-chain fatty acyl carnitine (LCFA-carnitine) and fatty acid oxidation have been observed in liver of fasted-refed rats. It is less clear that it plays a similar role in skeletal muscle. To examine this question, whole body respiratory quotients (RQ) and the concentrations of malonyl-CoA and LCFA-carnitine in muscle were determined in 48-h-starved rats before and at various times after refeeding. RQ values were 0.82 at baseline and increased to 0.93, 1. 0, 1.05, and 1.09 after 1, 3, 12, and 18 h of refeeding, respectively, suggesting inhibition of fat oxidation in all tissues. The increases in RQ at each time point correlated closely (r = 0.98) with increases (50-250%) in the concentration of malonyl-CoA in soleus and gastrocnemius muscles and decreases in plasma FFA and muscle LCFA-carnitine levels. Similar changes in malonyl-CoA and LCFA-carnitine were observed in liver. The increases in malonyl-CoA in muscle during refeeding were not associated with increases in the assayable activity of acetyl-CoA carboxylase (ACC) or decreases in the activity of malonyl-CoA decarboxylase (MCD). The results suggest that, during refeeding after a fast, decreases in fatty acid oxidation occur rapidly in muscle and are attributable both to decreases in plasma FFA and increases in the concentration of malonyl-CoA. They also suggest that the increase in malonyl-CoA in this situation is not due to changes in the assayable activity of either ACC or MCD or an increase in the cytosolic concentration of citrate.

Metabolic adaptations in pregnancy and their implications for the availability of substrates to the fetus.

During the first two-thirds of gestation, the mother is in an anabolic condition, increasing her fat depots thanks to both hyperphagia and enhanced lipogenesis. During the last third of gestation, the mother switches to a catabolic condition. Glucose is the most abundant nutrient crossing the placenta, which causes maternal hypoglycemia despite an increase in the gluconeogenetic activity. Adipose tissue lipolytic activity becomes enhanced, increasing plasma levels of FFA and glycerol that reach the liver; consequently there is an enhanced production of triglycerides that return to the circulation in the form of very low density lipoproteins (VLDL). Glycerol is also used as a preferential gluconeogenetic substrate, saving other more essential substrates, like amino acids, for the fetus. Under fasting conditions, fatty acids are converted into ketone bodies throughout the beta-oxidation pathway, and these compounds easily cross the placental barrier and are metabolized by the fetus. An enhanced liver production of VLDL-triglycerides together with a decrease in adipose tissue lipoprotein lipase (LPL) and an increase in plasma activity of cholesterol ester transfer protein causes both an intense increment in these lipoproteins and a proportional enrichment of triglycerides in both low and high density lipoproteins. Maternal triglycerides do not cross the placenta, but the presence of LPL and other lipases allows their hydrolysis, releasing fatty acids to the fetus. Under fasting conditions, the maternal liver uses circulating triglycerides as ketogenic substrates. Around parturition there is an induction of LPL activity in the mammary glands, driving circulating triglycerides to this organ for milk synthesis, allowing essential fatty acids derived from the mother's diet to become available to the suckling newborn.

Effects of Phosphate Injection on Metabolic and Hormonal Responses to Exercise in Fructose-Injected Rats

GHANBARI–NIAKI, A., F. DÉSY AND J.-M. LAVOIE. Effects of phosphate injection on metabolic and hormonal responses to exercise in fructose-injected rats. PHYSIOL BEHAV 67(5) 747–752, 1999.—The purpose of the present study was to evaluate the effects of an intraperitoneal injection of sodium phosphate on the metabolic and hormonal responses to exercise. Fructose-injected rats were either injected with sodium phosphate (Na 2HPO 4) or NaCl, either in a fed or in a food-restricted state (24 h), and evaluated at rest or after a 30-min exercise period (26 m/min; 0% grade). Liver ATP, phosphate (Pi), and glycogen concentrations were, on the whole, significantly ( p < 0.05) higher in Na 2HPO 4 than in NaCl groups. Exercise resulted in a significant ( p < 0.01) decrease in liver ATP and glycogen levels in fed and food-restricted rats whether injected with NaCl or Na 2HPO 4. Exercise, after NaCl and Na 2HPO 4 injection, resulted in a significant ( p < 0.01) increase in liver phosphate and Pi/ATP ratio, and in a decrease in glucose and an increase in glucagon levels in food-restricted rats only. The normal exercise-induced increase in plasma FFA, glycerol, and norepinephrine levels ( p < 0.05), observed in both fed and food-restricted NaCl-injected rats, was abolished by the injection of phosphate. The data are in line with the new concept that in addition to blood glucose levels, the increase in liver Pi/ATP ratio could also contribute to the increase in glucagon response during exercise.

Effect of heparin-stimulated plasma lipolytic activity on VLDL APO B subclass metabolism in normal subjects

Heparin given intravenously enhances lipolysis, although fasting lipids are not markedly altered in long-term administration. In the present study we investigated heparin-induced acute perturbation of VLDL subclass metabolism. Eight men were examined during a control study and during an 8.5 h infusion of heparin. 2H 3-leucine was used as tracer and kinetic constants derived using a non-steady-state model. Heparin infusion increased both plasma lipoprotein and hepatic lipase activity and raised plasma FFAs two-fold ( P<0.001). The fractional catabolic rate (FCR) of VLDL1 apo B increased on heparin (25.7±4.2 and 10.8±1.7 pools/d, heparin vs. control, P<0.02). The FCR of VLDL2 apo B increased to 12.6±1.9 pools/d on heparin vs. 8.8±1.1 pools/d during the control (NS). Total VLDL apo B production was not significantly changed (824±45 and 692±91 mg/d, heparin vs. control, NS). We conclude that during heparin infusion, the catabolism of especially large triglyceride-rich VLDL1 apo B is greatly increased. However, although the FFA levels were high during the heparin study, the production of total VLDL apo B did not rise. These findings are consistent with the known action of heparin on lipoprotein lipase but indicate that acute increase in plasma FFA levels does not lead to a rise in VLDL apo B production.

Free fatty acids and insulin levels--relationship to leptin levels and body composition in various patient groups from South Africa.

To investigate the relationship between leptin concentrations, various metabolic indices and body composition in six different groups. Anthropometric measurements, fasting plasma glucose, serum insulin, C-peptide, FFA and leptin levels were performed. In the obese and diabetic subjects, body composition was analysed with bio-impedance equipment and as a 5 level CT scan. Five lipoatrophic diabetes mellitus (LDM) patients, five normal subjects (N), nine white and nine black obese women (WW, BW), and nine white and nine black diabetic women (DWW, DBW) were investigated after an overnight fast. In both ethnic groups there was a positive correlation between leptin and BMI (black group: r=0.8; P<0.0001, white group: r=0.7, P<0.002) and leptin and SC fat mass (black group: r=0.6; P<0.005, white group: r=0.6; P<0.004). Across the groups, there were positive linear correlations between leptin concentrations, BMI, SC fat mass and FFA levels. Leptin and FFA concentrations are higher and insulin levels lower in both groups of black women compared to the two groups of white women, despite a similar BMI and body fat mass. In the DBW the large increase in visceral fat mass may be indicative of a more complex relationship between compensatory insulin resistance, elevated FFA levels and leptin secretion.

Obesity and insulin resistance in human growth hormone transgenic rats.

A line of transgenic rats (heterozygotes) carrying a chimeric gene comprising a regulatory portion of murine whey acidic protein and a structural portion of human GH (hGH) genes developed severe obesity with age. To characterize physiological mechanisms that lead to fat accumulation, an array of parameters related to obesity were studied. Blood hGH levels were continuously low, endogenous rat GH secretion was suppressed, and the pulsatility in peripheral GH levels was absent. Plasma glucose, insulin, triglyceride, and FFA levels in the male transgenic rats significantly exceeded those in nontransgenic littermates at 12 and 17 weeks, but not at 7 weeks, of age. All symptoms except hyperlipidemia were restored to normal by treatment with an antidiabetic agent, thiazolidinedione (troglitazone), for 1 week from 17 weeks of age. As phenotypic expression of obesity was already evident before aberration of physiological parameters, it was assumed that animals had a condition in which obesity or hyperlipidemia caused hyperinsulinemia. Gene expression and enzymatic activity of lipoprotein lipase in the adipose tissue in the transgenic rats were not different from those in normal rats. In contrast, the gene expression level of glycerol-3-phosphodehydrogenase was markedly elevated, suggesting that glycerol synthesis was much enhanced in the adipocytes of the transgenic rats. In an i.p. glucose tolerance test, the transgenic rats were not hyperglycemic at 7 weeks of age; however, the animal became hyperglycemic at 15-17 weeks of age. Finally, treatment with recombinant hGH for 1 week to produce pulsatile secretion reduced the size of epididymal and kidney fat pads and restored normal weight gain. These observations suggest that continuously low peripheral GH levels with the lack of pulsatile secretion resulted in obesity and noninsulin-dependent diabetes mellitus.

A fatty acid- dependent step is critically important for both glucose- and non-glucose-stimulated insulin secretion.

Lowering of the plasma FFA level in intact fasted rats by infusion of nicotinic acid (NA) caused essentially complete ablation of insulin secretion (IS) in response to a subsequent intravenous bolus of arginine, leucine, or glibenclamide (as previously found using glucose as the beta-cell stimulus). However, in all cases, IS became supranormal when a high FFA level was maintained by co-infusion of lard oil plus heparin. Each of these secretagogues elicited little, if any, IS from the isolated, perfused "fasted" pancreas when tested simply on the background of 3 mM glucose, but all became extremely potent when 0.5 mM palmitate was also included in the medium. Similarly, IS from the perfused pancreas, in response to depolarizing concentrations of KCl, was markedly potentiated by palmitate. As was the case with intravenous glucose administration, fed animals produced an equally robust insulin response to glibenclamide regardless of whether their low basal FFA concentration was further reduced by NA. In the fasted state, arginine-induced glucagon secretion appeared to be independent of the prevailing FFA concentration. The findings establish that the essential role of circulating FFA for glucose-stimulated IS after food deprivation also applies in the case of nonglucose secretagogues. In addition, they imply that (i) a fatty acid-derived lipid moiety, which plays a pivotal role in IS, is lost from the pancreatic beta-cell during fasting; (ii) in the fasted state, the elevated level of plasma FFA compensates for this deficit; and (iii) the lipid factor acts at a late step in the insulin secretory pathway that is common to the action of a wide variety of secretagogues.

Free fatty acids (FFA), a link between obesity and insulin resistance.

Evidence, gained from human studies, is reviewed showing that elevation of plasma FFA levels produce peripheral and probably also hepatic insulin resistance in obese healthy and diabetic subjects. First, plasma FFA levels are elevated in most obese subjects. Second, physiological elevations of plasma FFA inhibit acutely as well as chronically insulin stimulated glucose uptake in a dose dependent fashion. Responsible for this inhibition is a FFA induced defect in insulin stimulated glucose transport and/or phosphorylation which develops after 3-4 hours of raising plasma FFA and a second defect, consisting of inhibition of glycogen synthase, the rate limiting enzyme of glycogen synthesis, which develops after 4-6 hours. FFA induced inhibition of fatty acid oxidation (Randle effect) does not affect insulin stimulated glucose uptake or glycogen synthesis and thus does not cause insulin resistance. Elevated plasma FFA levels also modestly increase insulin suppressed endogenous glucose production (EGP) although this effect has not been found by all investigators. The reasons why it has been difficult to demonstrate unequivocal effects of FFA on EGP include 1) the fact that FFA promote insulin secretion which counteracts its effect on EGP (FFA increase, while insulin decreases EGP); 2) the recognition that FFA induced increase in gluconeogenesis may be compensated by intrahepatic downregulation of EGP (i.e., by a decrease in glycogenolysis). The FFA induced insulin resistance is physiologically important during starvation by preserving carbohydrate for oxidation in the central nervous system and during pregnancy, where the well recognized accelerated starvation pattern provides carbohydrate for the growing fetus. In obesity, however, there is no need to spare carbohydrate and the FFA induced insulin resistance may result in type 2 diabetes and other cardiovascular risk factors.

Effects of prolonged hyperinsulinemia on serum leptin in normal human subjects.

We have studied the effect of prolonged hyperinsulinemia and hyperglycemia on serum leptin levels in young nonobese males during 72-h euglycemic-hyperinsulinemic and hyperglycemic ( approximately 8.5 and 12.6 mM) clamps. Hyperinsulinemia increased serum leptin concentrations (by RIA) dose-dependently. An increase in serum insulin concentration of > 200 pM for > 24 h was needed to significantly increase serum leptin. An increase of approximately 800 pM increased serum leptin by approximately 70% over 72 h. Changes in plasma glucose concentrations (from approximately 5.0 to approximately 12.6 mM) or changes in plasma FFA concentrations (from < 100 to > 1,000 microM) had no effect on serum leptin. Serum leptin concentrations changed with circadian rhythmicity. The cycle length was approximately 24 h, and the cycle amplitude (peak to trough) was approximately 50%. The circadian leptin cycles and the circadian cycles of total body insulin sensitivity (i.e., GIR, the glucose infusion rates needed to maintain euglycemia during hyperinsulinemic clamping) changed in a mirror image fashion. Moreover, GIR decreased between Days 2 and 3 (from 11.4+/-0.2 to 9. 8+/-0.2 mg/kg min, P< 0.05) when mean 24-h leptin levels reached a peak. In summary, we found (a) that 72 h of hyperinsulinemia increased serum leptin levels dose-dependently; (b) that hyperglycemia or high plasma FFA levels did not affect leptin release; (c) that leptin was released with circadian rhythmicity, and (d) that 24-h leptin cycles correlated inversely with 24-h cycles of insulin sensitivity. We speculate that the close positive correlation between body fat and leptin is mediated, at least in part, by insulin.