Combination therapy with acipimox enhances the effect of growth hormone treatment on linear body growth in the normal and small-for-gestational-age rat

Growth hormone (GH) therapy is often associated with adverse side effects, including impaired insulin sensitivity. GH treatment of children with idiopathic short stature does not lead to an optimized final adult height. It has been demonstrated that FFA reduction induced by pharmacological antilipolysis can stimulate GH secretion per se in both normal subjects and those with GH deficiency. However, to date, no investigation has been undertaken to establish efficacy of combination treatment with GH and FFA regulators on linear body growth. Using a model of maternal undernutrition in the rat to induce growth-restricted offspring, we investigated the hypothesis that combination treatment with GH and FFA regulators can enhance linear body growth above that of GH alone. At postnatal day 28, male offspring of normally nourished mothers (controls) and offspring born with low birth weight [small for gestational age (SGA)] were treated with saline, GH, or GH (5 mg.kg(-1).day(-1)) in combination with acipimox (GH + acipimox, 20 mg.kg(-1).day(-1)) or fenofibrate (GH + fenofibrate, 30 mg.kg(-1).day(-1)) for 40 days. GH plus acipimox treatment significantly enhanced linear body growth in the control and SGA animals above that of GH, as quantified by tibial and total body length. Treatment with GH significantly increased fasting plasma insulin, insulin-to-glucose ratio, and plasma volumes in control and SGA animals but was not significantly different between saline and GH-plus-acipimox-treated animals. GH-induced lipolysis was blocked by GH plus acipimox treatment in both control and SGA animals, concomitant with a significant reduction in fasting plasma FFA and insulin concentrations. This is the first study to show that GH plus acipimox combination therapy, via pharmacological blocking of lipolysis during GH exposure, can significantly enhance the efficacy of GH in linear growth promotion and ameliorate unwanted metabolic side effects.     

High-fat diet elevates resting intramuscular triglyceride concentration and whole body lipolysis during exercise

This study determined the role of intramuscular triglyceride (IMTG) and adipose lipolysis in the elevated fat oxidation during exercise caused by a high-fat diet. In four separate trials, six endurance-trained cyclists exercised at 50% peak O2 consumption for 1 h after a two-day control diet (22% fat, CON) or an isocaloric high-fat diet (60% fat, HF) with or without the ingestion of acipimox, an adipose lipolysis inhibitor, before exercise. During exercise, HF elevated fat oxidation by 72% and whole body lipolysis [i.e., the appearance rate of glycerol in plasma (Ra glycerol)] by 79% compared with CON (P < 0.05), and this was associated with a 36% increase (P < 0.05) in preexercise IMTG concentration. Although acipimox lowered plasma free fatty acid (FFA) availability, HF still increased fat oxidation and Ra glycerol to the same magnitude above control as the increase caused by HF without acipimox (i.e., both increased fat oxidation 13-14 micromol.kg(-1).min(-1)). In conclusion, the marked increase in fat oxidation after a HF diet is associated with elevated IMTG concentration and whole body lipolysis and does not require increased adipose tissue lipolysis and plasma FFA concentration during exercise. This suggests that altered substrate storage in skeletal muscle is responsible for increased fat oxidation during exercise after 2 days of an HF diet.     

Determinants of intramyocellular triglyceride turnover: implications for insulin sensitivity

Increased intramyocellular triglyceride (IMTG) content is found in both insulin-sensitive endurance-trained subjects and insulin-resistant obese/type 2 diabetic subjects. A high turnover rate of the IMTG pool in athletes is proposed to reduce accumulation of lipotoxic intermediates interfering with insulin signaling. IMTG turnover is a composite measure of the dynamic balance between lipolysis and lipid synthesis; both are influenced by mitochondrial fat oxidation and plasma free fatty acid availability. Therefore, more attention should be given to the factors controlling the rate of turnover of IMTG. In this review, particular attention has been given to muscle oxidative capacity, plasma free fatty acid availability, and IMTG hydrolysis (lipolysis) and synthesis. A higher oxidative, lipolytic, and lipid storage capacity in the muscle of endurance-trained subjects reflects a higher fractional turnover of the IMTG pool. Thus the co-localization of intermyofibrillar lipid droplets and mitochondria allows for a fine coupling of lipolysis of the IMTG pool to mitochondrial beta-oxidation. Conversely, reduced oxidative capacity and a mismatch between IMTG lipolysis and beta-oxidation might be detrimental to insulin sensitivity by generating several lipotoxic intermediates in sedentary populations including obese/type 2 diabetic subjects. Further studies are clearly required to better understand the relationship between the rate of turnover of IMTG and the accumulation of lipotoxic intermediates in the pathophysiology of insulin resistance.     

Inhibition of adipose tissue lipolysis increases intramuscular lipid and glycogen use in vivo in humans

This study investigates the consequences of inhibition of adipose tissue lipolysis on skeletal muscle substrate use. Ten subjects were studied at rest and during exercise and subsequent recovery under normal, fasting conditions (control trial, CON) and following administration of a nicotinic acid analog (low plasma free fatty acid trial, LFA). Continuous [U-13C]palmitate and [6,6-2H2]glucose infusions were applied to quantify plasma free fatty acid (FFA) and glucose oxidation rates and to estimate intramuscular triacylglycerol (IMTG) and glycogen use. Muscle biopsies were collected to measure 1) fiber type-specific IMTG content; 2) allosteric regulators of hormone-sensitive lipase (HSL), glycogen phosphorylase, and pyruvate dehydrogenase; and 3) the phosphorylation status of HSL at Ser563 and Ser565. Administration of a nicotinic acid analog (acipimox) substantially reduced plasma FFA rate of appearance and subsequent plasma FFA concentrations (P < 0.0001). At rest, this substantially reduced plasma FFA oxidation rates, which was compensated by an increase in the estimated IMTG use (P < 0.05). During exercise, the progressive increase in FFA rate of appearance, uptake, and oxidation was prevented in the LFA trial and matched by greater IMTG and glycogen use. Differential phosphorylation of HSL or relief of its allosteric inhibition by long-chain fatty acyl-CoA could not explain the increase in muscle TG use, but there was evidence to support the contention that regulation may reside at the level of the glucose-fatty acid cycle. This study confirms the hypothesis that plasma FFA availability regulates both intramuscular lipid and glycogen use in vivo in humans.     

Inhibition of growth hormone-stimulated lipolysis by somatostatin, insulin, and insulin-like growth factors (somatomedins) in vitro

The effects of somatostatin, insulin, insulin-like growth factor I (IGF-I), and insulin-like growth factor II (IGF-II)/MSA on growth hormone (GH) (1 microgram/ml)-induced lipolysis were examined employing chicken adipose tissue in vitro. Basal and GH-stimulated glycerol release were inhibited by somatostatin (1 ng/ml) and by IGF-II/MSA (10 and 100 ng/ml). Insulin and IGF-I (10 and 100 ng/ml) completely inhibited the lipolytic response to GH without affecting basal glycerol release. Insulin and IGF-I were equipotent in inhibiting GH-induced lipolysis while IGF-II is only 16% as potent as insulin.     

Acipimox enhances spontaneous growth hormone secretion in obese women

We hypothesized that a high circulating free fatty acid (FFA) concentration is involved in the pathogenesis of hyposomatotropism associated with obesity. To evaluate this hypothesis, 10 healthy premenopausal women (body mass index 33.8 +/- 1.0 kg/m(2)) were studied in the follicular phase of their menstrual cycle at two occasions with a time interval of at least 8 wk, where body weight remained stable. Subjects were randomly assigned to treatment with either acipimox (an inhibitor of lipolysis, 250 mg orally 4 times daily) or placebo in a double-blind crossover design, starting 1 day before admission until the end of the blood sampling period. Blood samples were taken during 24 h with a sampling interval of 10 min for assessment of growth hormone (GH) concentrations, and GH secretion was estimated by deconvolution analysis. Identical methodology was used to study GH secretion in a historical control group of age-matched normal weight women. GH secretion was clearly blunted in obese women (total daily release 66 +/- 10 vs. lean controls: 201 +/- 23 mU x l(Vd)(-1) x 24 h(-1), P = 0.005, where l(Vd) is lite of distribution volume). Acipimox considerably enhanced total (113 +/- 50 vs. 66 +/- 10 mU x l(Vd)(-1) x 24 h(-1), P = 0.02) and pulsatile GH secretion (109 +/- 49 vs. 62 +/- 30 mU x l(Vd)(-1) x 24 h(-1), P = 0.02), but GH output remained lower compared with lean controls. Further analysis did not show any relationship between the effects of acipimox on GH secretion and regional body fat distribution. In conclusion, acipimox unleashes spontaneous GH secretion in obese women. It specifically enhances GH secretory burst mass. This might mean that lowering of systemic FFA concentrations by acipimox modulates neuroendocrine mechanisms that orchestrate the activity of the somatotropic ensemble.     

Continuous 24-h nicotinic acid infusion in rats causes FFA rebound and insulin resistance by altering gene expression and basal lipolysis in adipose tissue

Nicotinic acid (NA) has been used as a lipid drug for five decades. The lipid-lowering effects of NA are attributed to its ability to suppress lipolysis in adipocytes and lower plasma FFA levels. However, plasma FFA levels often rebound during NA treatment, offsetting some of the lipid-lowering effects of NA and/or causing insulin resistance, but the underlying mechanisms are unclear. The present study was designed to determine whether a prolonged, continuous NA infusion in rats produces a FFA rebound and/or insulin resistance. NA infusion rapidly lowered plasma FFA levels (>60%, P < 0.01), and this effect was maintained for ≥5 h. However, when this infusion was extended to 24 h, plasma FFA levels rebounded to the levels of saline-infused control rats. This was not due to a downregulation of NA action, because when the NA infusion was stopped, plasma FFA levels rapidly increased more than twofold (P < 0.01), indicating that basal lipolysis was increased. Microarray analysis revealed many changes in gene expression in adipose tissue, which would contribute to the increase in basal lipolysis. In particular, phosphodiesterase-3B gene expression decreased significantly, which would increase cAMP levels and thus lipolysis. Hyperinsulinemic glucose clamps showed that insulin's action on glucose metabolism was improved during 24-h NA infusion but became impaired with increased plasma FFA levels after cessation of NA infusion. In conclusion, a 24-h continuous NA infusion in rats resulted in an FFA rebound, which appeared to be due to altered gene expression and increased basal lipolysis in adipose tissue. In addition, our data support a previous suggestion that insulin resistance develops as a result of FFA rebound during NA treatment. Thus, the present study provides an animal model and potential molecular mechanisms of FFA rebound and insulin resistance, observed in clinical studies with chronic NA treatment.     

Effect of Acipimox on Plasma Lipids and Glucose/Insulin in Pregnant Rats

To determine how a reduction in maternal hypertriglyceridemia during late pregnancy may affect glucose/insulin relationships, pregnant and virgin rats were orally treated with acipimox, a potent antilipolytic agent. In 20-day pregnant rats receiving 80 mg of acipimox, plasma triglycerides (TG), free fatty acids (FFA), and glycerol decreased more than in virgin rats shortly after the drug (up to 7 hours), when compared with animals treated with distilled water, whereas plasma glucose level was unaffected by the treatment in either group of rats. When acipimox was given every 12 hours from day 17 to day 20 of pregnancy, plasma TG, FFA, and glycerol levels progressively increased, whereas they either decreased or did not change in virgin rats receiving the same treatment, with no effect in plasma glucose levels in either group. Fetal body weight was lower than in controls in 20-day pregnant rats that received acipimox for 3 days. On day 20 of pregnancy, 3 hours after receiving acipimox or distilled water, rats received a 2 g glucose/kg oral load and it was found that the change in plasma glucose was similar in both groups, whereas the increase in plasma insulin was greater in pregnant rats treated with acipimox. However, no difference was found in either variable after the oral glucose load in virgin rats receiving acipimox or distilled water. No differences in plasma glucose levels were found after intravenous (IV) administration of insulin in pregnant rats treated or not treated with acipimox. In conclusion, present results show that administration of acipimox during the last days of gestation inhibited lipolysis and decreased fetal weight. Over a short period of time, in pregnant rats, reductions of plasma FFA and TG after acipimox treatment improved the glucose-induced insulin release, but did not seem to have any effect in peripheral insulin resistance.     

A single 1-h bout of evening exercise increases basal FFA flux without affecting VLDL-triglyceride and VLDL-apolipoprotein B-100 kinetics in untrained lean men

Our group (Magkos F, Wright DC, Patterson BW, Mohammed BS, Mittendorfer B, Am J Physiol Endocrinol Metab 290: E355-E362, 2006) has recently demonstrated that a single, prolonged bout of moderate-intensity cycling (2 h at 60% of peak oxygen consumption) in the evening increases basal whole-body free fatty acid (FFA) flux and fat oxidation, decreases hepatic VLDL-apolipoprotein B-100 (apoB-100) secretion, and enhances removal efficiency of VLDL-triglyceride (TG) from the circulation the following day in untrained, healthy, lean men. In the present study, we investigated the effect of a single, shorter-duration bout of the same exercise (1 h cycling at 60% of peak oxygen consumption) on basal FFA, VLDL-TG, and VLDL-apoB-100 kinetics in seven untrained, healthy, lean men by using stable isotope-labeled tracer techniques. Basal FFA rate of appearance in plasma and plasma FFA concentration were approximately 55% greater (P < 0.05) the morning after exercise than rest, whereas resting metabolic rate and whole-body substrate oxidation rates were not different after rest and exercise. Exercise had no effect on plasma VLDL-TG and VLDL-apoB-100 concentrations, hepatic VLDL-TG and VLDL-apoB-100 secretion rates, and VLDL-TG and VLDL-apoB-100 plasma clearance rates (all P > 0.05). We conclude that in untrained, healthy, lean men 1) the exercise-induced changes in basal whole-body fat oxidation, VLDL-TG, and VLDL-apoB-100 metabolism during the late phase of recovery from exercise are related to the duration of the exercise bout; 2) single sessions of typical recreational activities appear to have little effect on basal, fasting plasma TG homeostasis; and 3) there is a dissociation between systemic FFA availability and VLDL-TG and VLDL-apoB-100 secretion by the liver.     

The lowering of hepatic fatty acid uptake improves liver function and insulin sensitivity without affecting hepatic fat content in humans

Lipolysis may regulate liver free fatty acid (FFA) uptake and triglyceride accumulation; both are potential causes of insulin resistance and liver damage. We evaluated whether 1) systemic FFA release is the major determinant of liver FFA uptake in fasting humans in vivo and 2) the beneficial metabolic effects of FFA lowering can be explained by a reduction in liver triglyceride content. Sixteen healthy subjects were subdivided in two groups of similar characteristics to undergo positron emission tomography with [(11)C]acetate and [(11)C]palmitate to quantify liver FFA metabolism (n = 8), or magnetic resonance spectroscopy (MRS) to measure hepatic fat content (n = 8), before and after the acute lowering of circulating FFAs by using the antilipolytic agent acipimox. MRS was again repeated after a 1-wk treatment period. Acipimox suppressed FFA levels while stimulating hepatic fractional extraction of FFAs (P < 0.05). As a result, fasting liver FFA uptake was decreased by 79% (P = 0.0002) in tight association with lipolysis (r = 0.996, P < 0.0001). The 1-wk treatment induced a significant improvement in systemic (+30%) and liver (+70%) insulin sensitivity (P < 0.05) and decreased circulating triglycerides (-20%, P = 0.06) and liver enzymes (ALT -20%, P = 0.03). No change in liver fat content was observed after either acute or sustained FFA suppression. We conclude that acute and sustained inhibitions of lipolysis and liver FFA uptake fail to deplete liver fat in healthy human subjects. Liver FFA uptake was decreased in proportion to FFA delivery. As a consequence, liver and systemic insulin sensitivity were improved, together with liver function, independently of changes in hepatic triglyceride accumulation.     

Effect of nicotinic acid administration on hepatic very low density lipoprotein-triglyceride production

Our objective was to examine very low density lipoprotein-triglyceride (VLDL-TG) kinetics after chronic and acute administration of nicotinic acid (NA). Incorporation of [1,2,3,4-(13)C(4)]palmitate and [2-(13)C(1)]glycerol into VLDL-TG was measured in five healthy, normolipidemic women. Each subject was studied twice; the 4-day hospital stays were separated by 1 mo, during which time doses of NA were increased to 2 g/day (500 mg, 4 times/day). During posttreatment study, 500 mg of NA were administered acutely at 0800. Under baseline postabsorptive conditions, incorporation curves from (13)C-labeled free fatty acid (FFA) and (13)C-labeled glycerol were superimposable, and VLDL-TG kinetics were in agreement (t(1/2) = 1.4 +/- 0.3 and 1.3 +/- 0.3 h, and production rates = 27.2 +/- 6.1 and 28.5 +/- 5.3 g/day, respectively). In the postabsorptive state after chronic NA therapy, VLDL-TG concentrations and production rates were lower despite a trend toward elevated plasma FFA concentrations and fluxes. After the acute dose of NA, plasma FFA concentrations and flux fell dramatically, and there was a virtual halt to VLDL-TG production, which continued throughout the 6-h period after NA, despite a marked rebound overshoot in serum FFA concentrations and flux after hour 2. Plasma homocysteine concentrations increased 68% (P < 0.001) in the NA phase, consistent with chronic increased transmethylation demand. We conclude that 1) NA acutely and chronically decreases VLDL-TG production rate in normal women; 2) the acute effect on VLDL-TG production is associated with an initial suppression of lipolysis but persists for several hours after the antilipolytic action of NA has abated and is observed in the basal postabsorptive state, when lipolytic rates are not reduced; and 3) the effect of NA on VLDL-TG production, therefore, cannot be completely explained by its antilipolytic actions.     

Measuring VLDL-triglyceride turnover in humans using ex vivo-prepared VLDL tracer

There has been more interest in VLDL-triglyceride (TG) kinetics during the last decade. Unfortunately, robust measurement methods are elaborate and not readily available. Here, we describe a method using unique, ex vivo labeling of the fatty acid moiety of VLDL-TG followed by intravenous bolus infusion in the same person. We found that plasma disappearance of ex vivo-labeled VLDL-TG was comparable to that of in vivo-labeled VLDL-TG and that turnover rates can be safely estimated from the log linear decay of VLDL-TG specific activity. We found minor labeling of the plasma FFA (oleate) pool, which was largely attributable to coinfusion of free [14C]triolein; VLDL-TG did not contribute substantially to the plasma FFA pool. The plasma decay curve of VLDL-TG was not affected by the presence of tracer in the FFA pool, provided that the data from 2 h after the VLDL tracer bolus infusion was used. The FFA contamination problem was circumvented by minor modification of the VLDL-TG tracer preparation. The approach we describe should expand the opportunity to study processes that cannot be assessed if the FFA precursor pool is labeled. This method for VLDL-TG tracer preparation can allow measurement of VLDL turnover, tissue uptake of VLDL-TG, and oxidation of VLDL-TG.     

Reduced plasma FFA availability increases net triacylglycerol degradation, but not GPAT or HSL activity, in human skeletal muscle

Intramuscular triacylglycerols (IMTG) are proposed to be an important metabolic substrate for contracting muscle, although this remains controversial. To test the hypothesis that reduced plasma free fatty acid (FFA) availability would increase IMTG degradation during exercise, seven active men cycled for 180 min at 60% peak pulmonary O(2) uptake either without (CON) or with (NA) prior ingestion of nicotinic acid to suppress adipose tissue lipolysis. Skeletal muscle and adipose tissue biopsy samples were obtained before and at 90 and 180 min of exercise. NA ingestion decreased (P < 0.05) plasma FFA at rest and completely suppressed the exercise-induced increase in plasma FFA (180 min: CON, 1.42 +/- 0.07; NA, 0.10 +/- 0.01 mM). The decreased plasma FFA during NA was associated with decreased (P < 0.05) adipose tissue hormone-sensitive lipase (HSL) activity (CON: 13.9 +/- 2.5, NA: 9.1 +/- 3.0 nmol.min(-1).mg protein(-1)). NA ingestion resulted in decreased whole body fat oxidation and increased carbohydrate oxidation. Despite the decreased whole body fat oxidation, net IMTG degradation was greater in NA compared with CON (net change: CON, 2.3 +/- 0.8; NA, 6.3 +/- 1.2 mmol/kg dry mass). The increased IMTG degradation did not appear to be due to reduced fatty acid esterification, because glycerol 3-phosphate activity was not different between trials and was unaffected by exercise (rest: 0.21 +/- 0.07; 180 min: 0.17 +/- 0.04 nmol.min(-1).mg protein(-1)). HSL activity was not increased from resting rates during exercise in either trial despite elevated plasma epinephrine, decreased plasma insulin, and increased ERK1/2 phosphorylation. AMP-activated protein kinase (AMPK)alpha1 activity was not affected by exercise or NA, whereas AMPKalpha2 activity was increased (P < 0.05) from rest during exercise in NA and was greater (P < 0.05) than in CON at 180 min. These data suggest that plasma FFA availability is an important mediator of net IMTG degradation, and in the absence of plasma FFA, IMTG degradation cannot maintain total fat oxidation. These changes in IMTG degradation appear to disassociate, however, from the activity of the key enzymes responsible for synthesis and degradation of this substrate.     

Metabolic regulation of growth hormone by free fatty acids, somatostatin, and ghrelin in HIV-lipodystrophy

Human immunodeficiency virus (HIV)-lipodystrophy is a syndrome characterized by changes in fat distribution and insulin resistance. Prior studies suggest markedly reduced growth hormone (GH) levels in association with excess visceral adiposity among patients with HIV-lipodystrophy. We investigated mechanisms of altered GH secretion in a population of 13 male HIV-infected patients with evidence of fat redistribution, compared with 10 HIV-nonlipodystrophic patients and 11 male healthy controls similar in age and body mass index (BMI). Although similar in BMI, the lipodystrophic group was characterized by increased visceral adiposity, free fatty acids (FFA), and insulin and reduced extremity fat. We investigated ghrelin and the effects of acute lowering of FFA by acipimox on GH responses to growth hormone-releasing hormone (GHRH). We also investigated somatostatin tone, comparing GH response to combined GHRH and arginine vs. GHRH alone with a subtraction algorithm. Our data demonstrate an equivalent number of GH pulses (4.1 +/- 0.6, 4.7 +/- 0.8, and 4.5 +/- 0.3 pulses/12 h in the HIV-lipodystrophic, HIV-nonlipodystrophic, and healthy control groups, respectively, P > 0.05) but markedly reduced GH secretion pulse area (1.14 +/- 0.27 vs. 4.67 +/- 1.24 ng.ml(-1).min, P < 0.05, HIV-lipodystrophic vs. HIV-nonlipodystrophic; 1.14 +/- 0.27 vs. 3.18 +/- 0.92 ng.ml(-1).min, P < 0.05 HIV-lipodystrophic vs. control), GH pulse area, and GH pulse width in the HIV-lipodystrophy patients compared with the control groups. Reduced ghrelin (418 +/- 46 vs. 514 +/- 37 pg/ml, P < 0.05, HIV-lipodystrophic vs. HIV-nonlipodystrophic; 418 +/- 46 vs. 546 +/- 45 pg/ml, P < 0.05, HIV-lipodystrophic vs. control), impaired GH response to GHRH by excess FFA, and increased somatostatin tone contribute to reduced GH secretion in patients with HIV-lipodystrophy. These data provide novel insight into the metabolic regulation of GH secretion in subjects with HIV-lipodystrophy.     

Insulin inhibits growth hormone signaling via the growth hormone receptor/JAK2/STAT5B pathway

Insulin is important for maintaining the responsiveness of the liver to growth hormone (GH). Insulin deficiency results in a decrease in liver GH receptor (GHR) expression, which can be reversed by insulin administration. In osteoblasts, continuous insulin treatment decreases the fraction of cellular GHR localized to the plasma membrane. Thus, it is not clear whether hyperinsulinemia results in an enhancement or inhibition of GH action. We asked whether continuous insulin stimulation, similar to what occurs in hyperinsulinemic states, results in GH resistance. Our present studies suggest that insulin treatment of hepatoma cells results in a time-dependent inhibition of acute GH-induced phosphorylation of STAT5B. Whereas total protein levels of JAK2 were not reduced after insulin pretreatment for 16 h, GH-induced JAK2 phosphorylation was inhibited. There was a concomitant decrease in GH binding and a reduction in immunoreactive GHR levels following pretreatment with insulin for 8-24 h. In summary, continuous insulin treatment in rat H4 hepatoma cells reduces GH binding, immunoreactive GHR, GH-induced phosphorylation of JAK2, and GH-induced tyrosine phosphorylation of STAT5B. These findings suggest that hepatic GH resistance may develop when a patient exhibits chronic hyperinsulinemia, a condition often observed in patients with obesity and in the early stage of Type 2 diabetes.     

High lipolytic activity and dyslipidemia in a Spontaneous Hypertensive/NIH Corpulent (SHR/N-cp) rat: a genetic model of obesity and type 2 diabetes mellitus

In order to better understand the link between obesity and type 2 diabetes, lipolysis and its adrenergic regulation was investigated in various adipose depots of obese adult females SHR/N-cp rats. Serum insulin, glucose, free fatty acids (FFA), triglycerides (TG) and glycerol were measured. Adipocytes were isolated from subcutaneous (SC), parametrial (PM) and retroperitoneal (RP) fat pads. Total cell number and size, basal lipolysis or stimulated by norepinephrine (NE) and BRL 37344 were measured in each depot. Obese rats were hyperinsulinemic and hyperglycemic, suggesting high insulin resistance. They presented a marked dyslipidemia, attested by increased serum FFA and TG levels. High serum glycerol levels also suggest a strong lipolytic rate. Obese rats showed an excessive development of all fat pads although a more pronounced effect was observed in the SC one. The cellularity of this depot was increased 8 fold when compared to lean rats, but these fat cells were only 1.5 to 2-fold larger. SC adipocytes showed a marked increase in their basal lipolytic activity but a lack of change in responsiveness to NE or BRL 37344. The association between high basal lipolysis and increased cellularity yields to a marked adipose cell lipolytic rate, especially from the SC region. SHR/N-cp rats were characterized by a hyperplasic type of obesity with an excessive development of the SC depot. The dyslipidemia, attested by an altered serum lipid profile could be attributed to excessive lipolysis that contributes to increased FFA levels, and to early development of insulin resistance through a lipotoxicity effect.     

Fatty acid cycling in the fasting rat

Adipose tissue lipolysis and fatty acid reesterification by liver and adipose tissue were investigated in rats fasted for 15 h under basal and calorigenic conditions. The fatty acid flux initiated by adipose fat lipolysis in the fasted rat is mostly futile and is characterized by reesterification of 57% of lipolyzed free fatty acid (FFA) back into adipose triglycerides (TG). About two-thirds of FFA reesterification are carried out before FFA release into plasma, whereas the rest consists of plasma FFA extracted by adipose tissue. Thirty-six percent of the fasting lipolytic flux is accounted for by oxidation of plasma FFA, whereas only a minor fraction is channeled into hepatic very low density lipoprotein-triglycerides (VLDL-TG). Total body calorigenesis induced by thyroid hormone treatment and liver-specific calorigenesis induced by treatment with beta, beta'-tetramethylhexadecanedioic acid (Medica 16) are characterized by a 1.7- and 1.3-fold increase in FFA oxidation, respectively, maintained by a 1.5-fold increase in adipose fat lipolysis. Hepatic reesterification of plasma FFA into VLDL-TG is negligible under both calorigenic conditions. Hence, total body fatty acid metabolism is regulated by adipose tissue as both source and sink. The futile nature of fatty acid cycling allows for its fine tuning in response to metabolic demands.     

Effects on insulin secretion and insulin action of a 48-h reduction of plasma free fatty acids with acipimox in nondiabetic subjects genetically predisposed to type 2 diabetes

Elevated plasma FFA cause beta-cell lipotoxicity and impair insulin secretion in nondiabetic subjects predisposed to type 2 diabetes mellitus [T2DM; i.e., with a strong family history of T2DM (FH+)] but not in nondiabetic subjects without a family history of T2DM. To determine whether lowering plasma FFA with acipimox, an antilipolytic nicotinic acid derivative, may enhance insulin secretion, nine FH+ volunteers were admitted twice and received in random order either acipimox or placebo (double-blind) for 48 h. Plasma glucose/insulin/C-peptide concentrations were measured from 0800 to 2400. On day 3, insulin secretion rates (ISRs) were assessed during a +125 mg/dl hyperglycemic clamp. Acipimox reduced 48-h plasma FFA by 36% (P < 0.001) and increased the plasma C-peptide relative to the plasma glucose concentration or DeltaC-peptide/Deltaglucose AUC (+177%, P = 0.02), an index of improved beta-cell function. Acipimox improved insulin sensitivity (M/I) 26.1 +/- 5% (P < 0.04). First- (+19 +/- 6%, P = 0.1) and second-phase (+31 +/- 6%, P = 0.05) ISRs during the hyperglycemic clamp also improved. This was particularly evident when examined relative to the prevailing insulin resistance [1/(M/I)], as both first- and second-phase ISR markedly increased by 29 +/- 7 (P < 0.05) and 41 +/- 8% (P = 0.02). There was an inverse correlation between fasting FFA and first-phase ISR (r2 = 0.31, P < 0.02) and acute (2-4 min) glucose-induced insulin release after acipimox (r2 =0.52, P < 0.04). In this proof-of-concept study in FH+ individuals predisposed to T2DM, a 48-h reduction of plasma FFA improves day-long meal and glucose-stimulated insulin secretion. These results provide additional evidence for the important role that plasma FFA play regarding insulin secretion in FH+ subjects predisposed to T2DM.