Acute pharmacologic blockade of lipolysis normalizes nocturnal growth hormone levels and pulsatility in obese subjects

Circadian Dysfunction in Adipose Tissue: Chronotherapy in Metabolic Diseases

Essential for survival and reproduction, the circadian timing system (CTS) regulates adaptation to cyclical changes such as the light/dark cycle, temperature change, and food availability. The regulation of energy homeostasis possesses rhythmic properties that correspond to constantly fluctuating needs for energy production and consumption. Adipose tissue is mainly responsible for energy storage and, thus, operates as one of the principal components of energy homeostasis regulation. In accordance with its roles in energy homeostasis, alterations in adipose tissue's physiological processes are associated with numerous pathologies, such as obesity and type 2 diabetes. These alterations also include changes in circadian rhythm. In the current review, we aim to summarize the current knowledge regarding the circadian rhythmicity of adipogenesis, lipolysis, adipokine secretion, browning, and non-shivering thermogenesis in adipose tissue and to evaluate possible links between those alterations and metabolic diseases. Based on this evaluation, potential therapeutic approaches, as well as clock genes as potential therapeutic targets, are also discussed in the context of chronotherapy.

Insulin Action, Glucose Homeostasis and Free Fatty Acid Metabolism: Insights From a Novel Model

Glucose and free fatty acids (FFA) are essential nutrients that are both partly regulated by insulin. Impaired insulin secretion and insulin resistance are hallmarks of aberrant glucose disposal, and type 2 diabetes (T2DM). In the current study, a novel model of FFA kinetics is proposed to estimate the role insulin action on FFA lipolysis and oxidation allowing estimation of adipose tissue insulin sensitivity (S_IFFA ). Twenty-five normal volunteers were recruited for the current study. To participate, volunteers had to be less than 40 years of age and have a body mass index (BMI) < 30 kg/m², and be free of medical comorbidity. The proposed model of FFA kinetics was used to analyze the data derived from the insulin-modified FSIGT. Mean fractional standard deviations of the parameter estimates were all less than 20%. Standardized residuals of the fit of the model to the FFA temporal data were randomly distributed, with only one estimated point lying outside the 2-standard deviation range, suggesting an acceptable fit of the model to the FFA data. The current study describes a novel one-compartment non-linear model of FFA kinetics during an FSIGT that provides an FFA metabolism insulin sensitivity parameter (S_IFFA ). Furthermore, the models suggest a new role of glucose as the modulator of FFA disposal. Estimates of S_IFFA confirmed previous findings that FFA metabolism is more sensitive to changes in insulin than glucose metabolism. Novel derived indices of insulin sensitivity of FFA (S_IFFA ) were correlated with minimal model indices. These associations suggest a cooperative rather than competitive interplay between the two primary nutrients (glucose and FFA) and allude to the FFA acting as the buffer, such that glucose homeostasis is maintained.

Rhythmic growth hormone secretion in physiological and pathological conditions: Lessons from rodent studies

Evolutionally conserved in all mammalians, the release of GH occurs in a rhythmic pattern, characterized by several dominant surges (pulsatile GH) with tonic low inter-pulse levels (tonic GH). Such pulsatile secretion pattern is essential for many physiological actions of GH on different tissues with defined gender dimorphism. Rhythmic release of pulsatile GH is tightly controlled by hypothalamic neurons as well as peripheral metabolic factors. Changes of GH pattern occur within a range of sophisticated physiological and pathological settings and significantly contribute to growth, ageing, survival and disease predispositions. Precise analysis of GH secretion pattern is vitally important for a comprehensive understanding of the function of GH and the components that regulate GH secretion pattern.

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

One day of overfeeding impairs nocturnal glucose but not fatty acid homeostasis in overweight men

OBJECTIVE

Overfeeding is associated with insulin resistance. Studies on animals suggest this is likely due to disruption of fatty acid metabolism and increased plasma free fatty acid (FFA) availability during the night. We tested the hypothesis that overfeeding induces insulin resistance and increases nocturnal but not daytime plasma FFA availability in human subjects.

DESIGN AND METHODS

We measured plasma glucose, insulin, and FFA concentrations hourly for 24 h during a day of isocaloric feeding and a day of hypercaloric feeding (30% calorie excess) in 8 overweight and obese, nondiabetic men (age: 38±3 years; body mass index: 34±2 kg/m²).

RESULTS

Overfeeding had no effect on daytime plasma glucose, insulin, and FFA concentrations compared to isocaloric feeding, but increased nocturnal glucose (P = 0.007) and insulin (P = 0.003) concentrations and decreased nocturnal FFA concentration (P = 0.006). The homeostasis model assessment of insulin resistance score was ∼30% greater the morning after hypercaloric than isocaloric feeding (P = 0.040).

CONCLUSIONS

One day of overfeeding has no effect on daytime plasma glucose and FFA concentrations but increases nocturnal plasma glucose and insulin concentrations, whereas nocturnal plasma FFA availability is reduced. The acute overfeeding-induced development of insulin resistant glucose metabolism therefore does not appear to be directly mediated by plasma FFA availability.

The decline in pulsatile GH secretion throughout early adulthood in mice is exacerbated by dietary-induced weight gain

The transition between puberty and adulthood is accompanied by a slowing in linear growth. Although GH is a key factor that drives somatic development into adulthood, early adulthood coincides with a reduction in circulating levels of GH. To this extent, a pathological decline in postpubertal GH secretion is detrimental to attainment of peak lean muscle mass and bone mass and promotes adiposity and increases susceptibility to the development of obesity in adulthood. Here we characterized pulsatile GH secretion in C57BL/6J mice at 12 and 16 wk of age. Deconvolution analysis of these measures reveals a reduction in pulsatile GH secretion between 12 and 16 wk of age. Dietary intervention with high-fat feeding at 8 wk of age results in a significant increase in adiposity, the development of glucose intolerance, and hyperinsulinemia. We show the exacerbation of the age-associated decline in pulsatile GH secretion in high-fat-fed mice after 4 wk of dietary intervention (at 12 wk of age), and a further suppression of pulsatile GH secretion by 8 wk of dietary intervention (at 16 wk of age). Suppressed pulsatile secretion of GH did not coincide with an elevation in circulating free fatty acids. Rather, we observed increased hepatic triglyceride content and an eventual decrease in circulating levels of IGF-I. Given the established role of GH in maintaining healthy aging, we anticipate that an advancing of the age-associated decline in pulsatile GH secretion as a consequence of dietary-induced weight gain may have long-term ramifications on adult health.

Nocturnal free fatty acids are uniquely elevated in the longitudinal development of diet-induced insulin resistance and hyperinsulinemia

Obesity is strongly associated with hyperinsulinemia and insulin resistance, both primary risk factors for type 2 diabetes. It has been thought that increased fasting free fatty acids (FFA) may be responsible for the development of insulin resistance during obesity, causing an increase in plasma glucose levels, which would then signal for compensatory hyperinsulinemia. But when obesity is induced by fat feeding in the dog model, there is development of insulin resistance and a marked increase in fasting insulin despite constant fasting FFA and glucose. We examined the 24-h plasma profiles of FFA, glucose, and other hormones to observe any potential longitudinal postprandial or nocturnal alterations that could lead to both insulin resistance and compensatory hyperinsulinemia induced by a high-fat diet in eight normal dogs. We found that after 6 wk of a high-fat, hypercaloric diet, there was development of significant insulin resistance and hyperinsulinemia as well as accumulation of both subcutaneous and visceral fat without a change in either fasting glucose or postprandial glucose. Moreover, although there was no change in fasting FFA, there was a highly significant increase in the nocturnal levels of FFA that occurred as a result of fat feeding. Thus enhanced nocturnal FFA, but not glucose, may be responsible for development of insulin resistance and fasting hyperinsulinemia in the fat-fed dog model.

Improved triglycerides and insulin sensitivity with 3 months of acipimox in human immunodeficiency virus-infected patients with hypertriglyceridemia

CONTEXT

Metabolic abnormalities such as hypertriglyceridemia remain a challenge for optimizing long-term health in HIV-infected patients.

OBJECTIVE

Elevation of free fatty acids (FFAs) may contribute to hyperlipidemia and insulin resistance in HIV. We evaluated the efficacy and safety of chronic inhibition of lipolysis in HIV-infected men and women with hypertrigyceridemia. We hypothesized that acipimox would lead to significant reductions in triglycerides and improved insulin sensitivity, compared with placebo.

DESIGN

A 3-month, randomized, double-blind, controlled trial of acipimox (250 mg thrice daily) vs. placebo was conducted in 23 HIV-infected men and women with hypertriglyceridemia (>150 mg/dl), abnormal fat distribution, and no current lipid-lowering therapy. The primary outcome variable was triglyceride concentration, and insulin sensitivity measured by hyperinsulinemic euglycemic clamp was a secondary outcome.

SETTING

The study was conducted at an academic medical center.

RESULTS

Acipimox resulted in significant reductions in FFAs [mean change -0.38 (0.06) vs. 0.08 (0.06) mEq/liter with placebo, -68 vs. +17% change from mean baseline, P < 0.0001], decreased rates of lipolysis (P < 0.0001), and a median triglyceride decrease from 238 mg/dl at baseline to 190 mg/dl, compared with an increase from 290 to 348 mg/dl in the placebo group (P = 0.01). Acipimox improved insulin sensitivity [acipimox +2.31 (0.74) vs. placebo -0.21 (0.90) mg glucose per kilogram lean body mass per minute, or +31 vs. -2% change from mean baseline values, P = 0.04]. Improvements in insulin sensitivity were significantly correlated with reductions in FFAs (r = -0.62, P = 0.003) and lipolysis (r = -0.59, P = 0.005).

CONCLUSIONS

Acipimox resulted in significant sustained reductions in lipolysis, improved glucose homeostasis, and significant but modest reductions in triglycerides in HIV-infected individuals with abnormal fat distribution and hypertriglyceridemia. Improvement in overall metabolic profile with acipimox suggests a potential clinical utility for this agent that requires further investigation.

Acute regulation of adiponectin by free fatty acids

Little is known about the acute regulation of adiponectin in humans. In animal studies, adiponectin increases the clearance of free fatty acids (FFA) from the circulation by increasing skeletal uptake and oxidation of lipid, thereby regulating the FFA concentration. However, it is unknown if FFA regulate adiponectin. The aim of the present study was to investigate the effect of an acute reduction in free fatty acids on adiponectin concentration in healthy subjects. Ten normal male subjects were admitted for 2 inpatient visits and randomized to receive either acipimox (500 mg orally at 2 am and again at 6 am) or placebo on the first visit and vice versa on the second visit. Adiponectin, FFA, insulin and glucose were measured at 7:45 am. FFA concentrations were significantly lower after acipimox than placebo administration (0.08 +/- 0.02 mEq/L v 0.35 +/- 0.53 mEq/L, P <.05). Adiponectin concentrations were also significantly lower after acipimox than placebo administration (7.4 +/- 1.2 microg/mL v 10.3 +/- 1.7 microg/mL, P <.05). The change in FFA between acipimox and placebo correlated significantly with the change in adiponectin (r = 0.66, P <.05), eg, the larger the reduction in FFA in response to acipimox, the larger the reduction in adiponectin. These results suggest that acute lowering of FFA is associated with decreased adiponectin concentrations.

Nutritional status in the neuroendocrine control of growth hormone secretion: the model of anorexia nervosa

Growth hormone (GH) plays a key role not only in the promotion of linear growth but also in the regulation of intermediary metabolism, body composition, and energy expenditure. On the whole, the hormone appears to direct fuel metabolism towards the preferential oxidation of lipids instead of glucose and proteins, and to convey the energy derived from metabolic processes towards the synthesis of proteins. On the other hand, body energy stores and circulating energetic substrates take an important part in the regulation of somatotropin release. Finally, central and peripheral peptides participating in the control of food intake and energy expenditure (neuropeptide Y, leptin, and ghrelin) are also involved in the regulation of GH secretion. Altogether, nutritional status has to be regarded as a major determinant in the regulation of the somatotropin-somatomedin axis in animals and humans. In these latter, overweight is associated with marked impairment of spontaneous and stimulated GH release, while acute dietary restriction and chronic undernutrition induce an amplification of spontaneous secretion together with a clear-cut decrease in insulin-like growth factor I (IGF-I) plasma levels. Thus, over- and undernutrition represent two conditions connoted by GH hypersensitivity and GH resistance, respectively. Anorexia nervosa (AN) is a psychiatric disorder characterized by peculiar changes of the GH-IGF-I axis. In these patients, low circulating IGF-I levels are associated with enhanced GH production rate, highly disordered mode of somatotropin release, and variability of GH responsiveness to different pharmacological challenges. These abnormalities are likely due not only to the lack of negative IGF-I feedback, but also to a primary hypothalamic alteration with increased frequency of growth hormone releasing hormone discharges and decreased somatostatinergic tone. Given the reversal of the above alterations following weight recovery, these abnormalities can be seen as secondary, and possibly adaptive, to nutritional deprivation. The model of AN may provide important insights into the pathophysiology of GH secretion, in particular as regards the mechanisms whereby nutritional status effects its regulation.

Energy expenditure, sex, and endogenous fuel availability in humans

Adipose tissue lipolysis supplies circulating FFAs, which largely meet lipid fuel needs; however, excess FFAs, can contribute to the adverse health consequences of obesity. Because "normal" FFA release has not been well defined, average (mean of 4 days) basal FFA release and its potential regulation factors were measured in 50 lean and obese adults (25 women). Resting energy expenditure (REE), but not body composition, predicted most of the interindividual variation in FFA release. There was a significant, positive linear relationship between palmitate release and REE; however, women released approximately 40% more FFA than men relative to REE. Neither plasma palmitate concentrations nor respiratory quotient by indirect calorimetry differed between men and women. Glucose release rates were not different in men and women whether related to REE or fat free mass. These findings indicate that nonoxidative FFA clearance is greater in women than in men. This could be an advantage at times of increased fuel needs. We conclude that "normal" adipose tissue lipolysis is different in men and women and that the fuel export role of adipose tissue in obesity will need to be reassessed.

Somatostatin and somatostatin receptor physiology

Since the discovery of somatostatin (SST) over three decades ago, its ubiquitous distribution and manifold functions are still being documented. SST is synthesized in the hypothalamus and transported to the anterior pituitary gland where it tonicaly inhibits GH and TSH secretion as well as being responsible for GH pulsatile release. Several internal feedback loops, sleep, exercise, and chemical agents control and influence SST release. SST also impacts the function of a wide variety of cells and organ systems throughout the body. Knowledge of the structures of the SSTs has resulted in recognition of the essential four core conserved residues responsible for their actions. The SSTs act through six separate SST cell surface receptors (SSTRs), members of the family of G protein-coupled receptors. Receptor ligand binding (SST/SSTR) results in cellular activities specific for each receptor, or receptor combinations, and their tissue/cell localization. Understanding the structure/function relationship of the SSTs and their receptors, including the internalization of SST/SSTR complexes, has facilitated the development of a variety of novel pharmacologic agents for the diagnosis and treatment of neuroendocrine tumors and unfolding new applications.

Inhibition of lipolysis improves insulin sensitivity in protease inhibitor-treated HIV-infected men with fat redistribution

BACKGROUND

Fatty acid concentrations are increased in patients with HIV and fat redistribution and may contribute to insulin resistance in this population.

OBJECTIVE

We determined the effects of acute inhibition of lipolysis on insulin sensitivity in HIV-infected patients with fat redistribution who were receiving a protease inhibitor.

DESIGN

Seven HIV-infected men [age: 45 +/- 2 y; body mass index (in kg/m(2)): 28.8 +/- 1.9] with a fasting insulin concentration > or= 104 pmol/L (15 micro IU/mL), combined visceral adiposity and peripheral lipoatrophy, and receiving a protease inhibitor were studied. Tolbutamide-modified frequently sampled intravenous-glucose-tolerance tests (FSIGTTs) were performed after randomized double-blind administration of acipimox (500 mg at -90 and 0 min), a potent inhibitor of lipolysis, and placebo. The subjects completed 2 FSIGTTs separated by 3-7 d.

RESULTS

At baseline, fasting insulin and fatty acid concentrations were 27.6 +/- 5.0 micro IU/mL and 0.83 +/- 0.08 mmol/L (normal range: 0.1-0.6 mmol/L), respectively. Fatty acid concentrations were significantly reduced after acipimox compared with placebo (fatty acid area under the curve: acipimox = 73 +/- 8 compared with placebo = 122 +/- 12 mmol x 270 min/L, P = 0.002). Acipimox treatment resulted in a significant increase in the insulin sensitivity index (acipimox = 1.63 +/- 0.5 compared with placebo = 0.88 +/- 0.3 x 10(-4) x min(-1) x micro IU/mL, P = 0.015).

CONCLUSIONS

Acute inhibition of lipolysis and reduction in fatty acid concentrations are associated with improved insulin sensitivity in patients with HIV lipodystrophy and hyperinsulinemia. Further studies are needed to determine whether long-term antilipolytic strategies to reduce fatty acid concentrations may be useful in treating the metabolic disturbances associated with HIV lipodystrophy.

Diagnosis of growth hormone deficiency after pituitary surgery: the combined acipimox/GH-releasing hormone test

OBJECTIVE

Reduction of plasma free fatty acids leads to enhanced GH response after stimulation by GH-releasing hormone (GHRH). We studied the clinical usefulness of combined administration of acipimox and GHRH for the diagnosis of GH deficiency.

DESIGN

We evaluated 35 patients [mean age 53.0 years; mean body mass index (BMI) 26.7 kg/m2] after pituitary surgery. We compared GH responses after acipimox and GHRH with the GH response during an insulin tolerance test (ITT) and, in a subgroup of 12 patients, with the GHRH/arginine test. The acipimox/GHRH test was additionally performed in 21 control subjects (mean age 53.8 years; mean BMI 24.7 kg/m2).

RESULTS

In the patients, the mean (+/- SEM) peak GH was almost four-fold higher after acipimox/GHRH (6.94 +/- 1.07 microg/l, range 0.46-23.1; P < 0.001) and after GHRH/arginine (8.32 +/- 1.23 microg/l, range 1.1-49.2; P < 0.001) than after ITT (1.84 +/- 0.46 microg/l, range 0.01-11.9). According to the ITT, 29 patients were severely GH deficient (peak GH < 3.0 microg/l). Peak GH levels after acipimox/GHRH in controls ranged from 7.5 to 78.4 microg/l (mean 29.3 +/- 3.5). GH peak values during the acipimox/GHRH test were significantly correlated with values from the ITT (r = 0.63, P < 0.01) and GHRH/arginine test (r = 0.87, P < 0.001). Areas under the curve were also correlated. According to generally accepted cut-off peak GH levels for the ITT and GHRH/arginine test, a GH peak exceeding 11.2 micro g/l excludes severe GH deficiency after acipimox/GHRH. Our control data indicate that the cut-off level is lower at older age.

CONCLUSIONS

The acipimox/GHRH test leads to GH responses similar to those of the GHRH/arginine test, and to higher peak GH values if compared with the ITT. The acipimox/GHRH test is a potential additional tool to detect GH deficiency in patients with pituitary disease, in particular in patients with a perturbation of fatty acid metabolism.

Glucagon administration elicits blunted GH but exaggerated ACTH response in obesity

Reduction in both spontaneous and stimulated GH secretion in obesity has been clearly demonstrated. Mild hyperactivity of hypothalamus-pituitary-adrenal (HPA) axis has been also reported. Glucagon, at least after im administration, induces clear increase in either GH or ACTH and F levels but its effect on somatotroph and corticotroph secretion in obesity has never been studied. In 7 patients with abdominal obesity (OB, aged 24-42 yr, BMI: 29.1-43.9 kg/m2, waist/hip ratio [WHR]: 0.86-1.00) we studied the GH, ACTH and F responses to the im administration of glucagon (0.017 mg/kg at 0 min). The results in OB were compared with those in a group of 6 age-matched controls normal subjects (Ns aged 26-32 yr, BMI 19.7-22.5 kg/m2). In Ns glucagon administration induced clear increase in GH (peak vs baseline, mean+/-SE: 11.6+/-3.4 vs 3.3+/-0.7 microg/l, p<0.02), and ACTH (52.9+/-15.2 vs 19.0+/-1.5 pg/ml, p<0.02) levels which peaked at +150 and +165 min, respectively. Increase in F levels (222.3+/-23.8 vs 158.3+/-7.0 ng/ml, p<0.05) was also recorded but peaked at +180 min. In OB glucagon administration induced GH response (7.4+/-2.3 vs 0.8+/-0.6 microg/l) lower (p<0.05) than that recorded in Ns; when the GH responses were evaluated by co-variance analysis, a significant difference between the 2 groups was recorded in term of peaks but not of AUCs. On the other hand, the ACTH response to glucagon in OB was higher than that in Ns (11452.6+/-2447.7 vs 4892.2+/-719.4 pg/ml x min, p<0.05). The F response to glucagon in OB and Ns was, however, similar (24057.9+/-4109.1 vs 29835.9+/-1566.0 ng/ml x min). In conclusion, this study demonstrates that in obese patients the im administration of glucagon elicits blunted GH response but exaggerated ACTH increase which is uncoupled with the adrenal response. These findings agree with the existence of concomitant GH insufficiency and altered corticotroph function in obesity.

Impact of the GH-cortisol ratio on the age-dependent changes in body composition

Aging is associated with a decrease in GH levels and this is paralleled by changes in body composition, i.e., increased visceral fat, and decreased lean body mass and bone mineral density. Similar changes in body composition are seen in the state of hypercortisolism. Increasing age has been shown to be associated with elevated evening cortisol levels in men. An increased exposure of several tissues to glucocorticoids with aging, i.e., visceral fat cells, in combination with the reduction of the lipolytic effects of declining GH levels, may contribute to the age-dependent increase of visceral fat accumulation. We hypothesize that the age-dependent changes in body fat are the result of an age-dependent decrease of the GH/cortisol ratio at the level of the adipocyte. This is caused by the decline in GH concentrations and the increase in cortisol levels and/or metabolism at the adipocyte.

Effect of withdrawal of somatostatin plus GH-releasing hormone as a stimulus of GH secretion in obesity

OBJECTIVES

Somatostatin (SS) may not merely be inhibitory to GH secretion but, under appropriate temporal conditions, may act in a paradoxically positive manner to sensitize somatotroph responsiveness to GH-releasing hormone (GHRH). SS infusion withdrawal (SSIW) produces a rebound GH rise in humans and increases GHRH-induced GH release. Theoretically, SSIW leaves the somatotroph cell in a situation of low endogenous SS. In obesity, there is markedly decreased GH secretion. In both children and adults, the greater the body mass index (BMI), the lower the GH response to provocative stimuli. It has been postulated that increased hypothalamic somatostatin secretion is the main mechanism responsible for the blunted GH secretion of obesity. There are no data evaluating GH responsiveness to SSIW plus GHRH in obese adults. The aim of the present study was to evaluate the GH response to SSIW plus GHRH in a group of control subjects and a group of obese patients.

PATIENTS AND MEASUREMENTS

Seven obese patients (six female, one male) with a BMI of 36.1 +/- 7.7 kg/m2 were studied. As a control group, seven normal subjects (six female, one male) with a BMI of 20.3 +/- 0.9 kg/m2 were also studied. Two tests were performed. On one day, somatostatin (SS) i.v. infusion (500 microg from 0-90 min) was performed followed by a placebo i.v. bolus 90 min after SS withdrawal (SSIW). On another day, SS i.v. infusion (500 microg from 0-90 min) was performed followed by a GHRH (100 microg) i.v. bolus 90 min after SS withdrawal. A second group of seven obese patients (six female, one male) with a BMI of 32.2 +/- 2.3 kg/m2 were studied. As a second control group, seven normal healthy subjects (six female, one male) with a BMI of 20.1 +/- 0.6 kg/m2 were also studied. On one day, saline infusion was performed followed by a placebo i.v. bolus at 90 min. On another day, saline infusion was performed followed by a GHRH (100 microg) i.v. bolus at 90 min. Blood samples were taken at appropriate intervals for determination of GH. Serum GH was measured by chemiluminescent immunometric assay. Statistical analysis was performed by Wilcoxon and Mann-Whitney tests.

RESULTS

GHRH-induced GH secretion in normal subjects showed a mean peak of 15.8 +/- 2.1 microg/l. Normal control subjects had a mean peak of 3.1 +/- 1.5 microg/l after SSIW-induced GH secretion. When GHRH was administered after SSIW there was an increased GH secretion with a mean peak of 23.3 +/- 4.4 microg/l, significantly greater than the response after SSIW alone (P < 0.05) and GHRH alone (P < 0.05). GHRH-induced GH secretion in obese patients was decreased with a mean peak of 3.9 +/- 1.5 microg/l. In obese patients, GH secretion after SSIW was markedly decreased with a mean peak of 1.0 +/- 0.4 microg/l. When GHRH was administered after SSIW, an increase in GH secretion was observed with a mean peak of 4.3 +/- 0.9 microg/l, significantly greater than SSIW alone (P < 0.05) but not GHRH alone (P = NS), and significantly less than in normal subjects (P < 0.05).

CONCLUSIONS

This study demonstrates a significantly blunted peak GH response to somatostatin infusion withdrawal plus GHRH in obese patients compared to normal subjects. In this theoretical situation of decreased somatostatinergic tone, there is persistence of GH hyposecretion in obesity, suggesting the existence of multiple defects responsible for decreased GH secretion in obesity. We also found that in obese patients, in contrast to normal subjects, SSIW did not increase GHRH-induced GH secretion.

Elevated insulin levels contribute to the reduced growth hormone (GH) response to GH-releasing hormone in obese subjects

We have recently presented experimental evidence indicating that insulin has a physiologic inhibitory effect on growth hormone (GH) release in healthy humans. The aim of the present study was to determine whether in obesity, which is characterized by hyperinsulinemia and blunted GH release, insulin contributes to the GH defect. To this aim, we used a simplified experimental protocol previously used in healthy humans to isolate the effect of insulin by removing the interference of free fatty acids (FFAs), which are known to block GH release. Six obese subjects (four men and two women; age, 30.8 +/- 5.2 years; body mass index, 36.8 +/- 2.8 kg/m2 [mean +/- SE]) and six normal subjects (four men and two women; age, 25.8 +/- 1.9 years; body mass index, 22.7 +/- 1.1 kg/m2) received intravenous (i.v.) GH-releasing hormone (GHRH) 0.6 microg/kg under three experimental conditions: (1) i.v. 0.9% NaCl infusion and oral placebo, (2) i.v. 0.9% NaCl infusion and oral acipimox, an antilipolytic agent able to reduce FFA levels (250 mg at 6 and 2 hours before GHRH), and (3) euglycemic-hyperinsulinemic clamp (insulin infusion rate, 0.4 mU x kg(-1) x min(-1)). As expected, after placebo, the GH response to GHRH was lower for obese subjects versus normals (488 +/- 139 v 1,755 +/- 412 microg/L x 120 min, P < .05). Acipimox markedly reduced FFA levels and produced a mild reduction of insulin levels; under these conditions, the GH response to GHRH was increased in both groups, remaining lower in obese versus normal subjects (1,842 +/- 360 v 4,871 +/- 1,286 microg/L x 120 min, P < .05). In both groups, insulin infusion yielded insulin levels usually observed under postprandial conditions and reduced circulating FFA to the levels observed after acipimox administration. Again, the GH response to GHRH was lower for obese subjects versus normals (380 +/- 40 v 1,075 +/- 206 microg/L x 120 min, P < .05), and in both groups, it was significantly lower than the corresponding response after acipimox. In obese subjects, as previously reported in normals, the GH response to GHRH was inversely correlated with the mean serum insulin (r = -.70, P < .01). In conclusion, our data indicate that in the obese, as in normal subjects, the GH response to GHRH is a function of insulin levels. The finding that after both the acipimox treatment and the insulin clamp the obese still show higher insulin levels and a lower GH response to GHRH than normal subjects suggests that hyperinsulinemia is a major determinant of the reduced GH release associated with obesity.

Serum insulin but not leptin is associated with spontaneous and growth hormone (GH)-releasing hormone-stimulated GH secretion in normal volunteers with and without weight loss

Weight loss in humans is associated with elevated hypothalamic-pituitary growth hormone (GH) secretion. This study evaluates the effects of weight loss on the hypothalamic-pituitary (GH-releasing hormone [GHRH]-GH) axis in 14 normal-weight (body mass index [BMI], 25+/-1 Kg/m2) subjects, of whom half had undergone a diet-induced weight loss of 14%+/-2% (mean+/-SEM). Insulin-like growth factor-1 (IGF-1), insulin, oral glucose tolerance, leptin, and GH pulse patterns were determined in both groups after weight maintenance for 1 week. Of note, we tested the effects of recent weight loss (3 months) and not a recent dietary intake, since both groups ingested a normal calorie diet for 2 days in the Clinical Research Center (CRC) prestudy. Serum insulin (3.8+/-0.7 v 9.0+/-0.9 microU/mL, P < .01) and C-peptide (0.44+/-0.06 v 0.59+/-0.04 ng/mL, P < .05) were significantly lower in the weight loss group. Serum leptin was not different. Endogenous GH pulse height (11.9+/-4.8 v 1.3+/-0.1 microg/L, P < .05), area per GH pulse ([AUC] 57+/-28 v 6+/-1 microg/L, P < .05), and mean GH (3.91+/-0.76 v 0.85+/-0.16 microg/L, P < .01) were increased in the weight loss group. The serum insulin level was inversely associated with the mean GH concentration (r=-.678, P < .01) and GH pulse height (r=-.733, P < .01). In addition to spontaneous GH secretion, the GHRH-stimulated GH pulse height (41.8+/-18.1 v7.1+/-1.6 microg/L, P < .05) and AUC (161+/-35 v46+/-13 microg/L/min, P < .05) were also increased in the weight loss group. The insulin concentration was also inversely correlated with the GHRH-stimulated GH pulse height (r=-.718, P < .01). The leptin concentration was correlated with the BMI (r=.554, P < .05) and body fat (r=.744, P < .01), but not with GH secretion. In summary, even though these patients were on a normal calorie diet, a history of recent weight loss in young men and women of normal weight and health can be associated with a significant increase in spontaneous GH pulse height and GHRH-stimulated pulse height. Weight loss was also associated with a reduced serum insulin level. The observed increase in GH secretion may be secondary to the reduction in insulin or alterations of other factors acting at the site of the pituitary.

The endocrinology of obesity

Numerous endocrine alterations are associated with obesity (Table 1). The majority of the alterations are secondary to obesity and must be considered simply associated and potentially in the pathogenesis of the complications of obesity. The discovery of new endocrine peptides such as leptin that signal body fat content will increase our understanding of the regulation of body fat content. As a result, therapies will most certainly be developed that are directly targeted at the alterations in endocrine function.

Effects of acipimox, an antilipolytic drug, on the growth hormone (GH) response to GH-releasing hormone alone or combined with arginine in obesity

Increased free fatty acid (FFA) levels of obese patients are likely involved in the pathogenesis of the growth hormone (GH) hyposecretion of obesity. To clarify their role, we studied the influence of inhibition of plasma FFA levels, induced by 500 mg oral acipimox (ACX), an antilipolytic drug, on the GH response to GH-releasing hormone (GHRH) alone or combined with arginine ([ARG] study A) in six normal women ([NS] aged 24 to 37 years; body mass index, 22.4 +/- 0.9 kg/m2) and six obese women ([OB] aged 21 to 40 years; body mass index 39.5 +/- 3.2 kg/m2). In a group of seven OB patients (aged 18 to 58 years; body mass index, 35.8 +/- 1.3 kg/m2), the effect of ACX on either GHRH- or GHRH+ARG-stimulated GH increase was also studied after a 4-day treatment with the same drug at 250 mg three times daily (study B). OB patients had baseline FFA levels higher than NS (0.77 +/- 0.06 v 0.44 +/- 0.09 mmol/L, P<.05). In study A, ACX reduced FFA levels to the same nadir in both groups (0.11 +/- 0.02 and 0.12 +/- 0.03 mmol/L, NS and OB subjects, respectively). In NS, ACX failed to significantly potentiate the GH response to either GHRH (1,371.9 +/- 425.2 v 1,001.8 +/- 229.0 micrograms/L x min) or GHRH+ARG (3558.4 +/- 1,513.7 v 3,045.9 +/- 441.8 micrograms/L x min), while in OB patients it increased the GH response to GHRH (797.6 +/- 277.3 v 353.8 +/- 136.7 micrograms/L x min, P<.01) and did not modify the response to ARG+GHRH (1,010.5 +/- 253.1 v 821.1 +/- 222.0 micrograms/L x min). In study B, ACX reduced FFA levels in OB patients (nadir, 0.09 +/- 0.04 mmol/L). This treatment strikingly increased the GH response to GHRH (1,734.0 +/- 725.4 v 271.5 +/- 112.8 micrograms/L x min, P<.01) and significantly potentiated that to ARG+GHRH (2,371.9 +/- 571.3 v 1,020.0 +/- 343.2 micrograms/L x min, P<.05). In conclusion, our present findings indicate that an acute reduction of plasma FFA levels in OB patients restores their somatotrope responsiveness, whereas it does not affect GH secretion in lean subjects. After prolonged treatment, ACX further improves GHRH-stimulated GH secretion in OB patients, suggesting that elevated FFA levels play a leading role in the GH hyposecretory state of obesity.