Effect of growth hormone on oral glucose tolerance and circulating metabolic fuels in man

Altered metabolism and resistance to obesity in long-lived mice producing reduced levels of IGF-I

The extension of lifespan due to reduced insulin-like growth factor 1 (IGF-I) signaling in mice has been proposed to be mediated through alterations in metabolism. Previously, we showed that mice homozygous for an insertion in the Igf1 allele have reduced levels of IGF-I, are smaller, and have an extension of maximum lifespan. Here, we tested whether this specific reduction of IGF-I alters glucose metabolism both on normal rodent chow and in response to high-fat feeding. We found that female IGF-I-deficient mice were lean on a standard rodent diet but paradoxically displayed an insulin-resistant phenotype. However, these mice gained significantly less weight than normal controls when placed on a high-fat diet. In control animals, insulin response was significantly impaired by high-fat feeding, whereas IGF-I-deficient mice showed a much smaller shift in insulin response after high-fat feeding. Gluconeogenesis was also elevated in the IGF-I-deficient mice relative to controls on both normal and high-fat diet. An analysis of metabolism and respiratory quotient over 24 h indicated that the IGF-I-deficient mice preferentially utilized fatty acids as an energy source when placed on a high-fat diet. These results indicate that reduction in the circulating and tissue IGF-I levels can produce a metabolic phenotype in female mice that increases peripheral insulin resistance but renders animals resistant to the deleterious effects of high-fat feeding.

The physiology of growth hormone and sport

The growth hormone (GH)/ insulin-like growth factor-I (IGF-I) axis exerts short-and long-term metabolic effects that are potentially important during exercise. Exercise is a potent stimulus to GH release and there is some evidence that the acute increase in GH is important in regulating substrate metabolism post-exercise. Regular exercise also increases 24-hour GH secretion rates, which potentially contributes to the physiologic changes induced by training. The effects of GH replacement in GH-deficient adults provide a useful model with which to study the effects of the more long-term effects of the GH/ IGF-I axis. There is convincing evidence that GH replacement increases exercise capacity. Measures of exercise performance including maximal oxygen uptake (VO2max) and ventilatory threshold (VeT) are impaired in GH deficiency and improved by GH replacement, probably through some combination of increased oxygen delivery to exercising muscle, increased fatty acid availability with glycogen sparing, increased muscle strength, improved body composition and improved thermoregulation. Administration of supraphysiologic doses of GH to athletes increases fatty acid availability and reduces oxidative protein loss particularly during exercise, and increases lean body mass. It is not known whether these effects translate to improved athletic performance, although recombinant human GH is known to be widely abused in sport. The model of acromegaly provides evidence that long-term GH excess does not result in improved performance but it is possible that a "window" exists in which the protein anabolic effects of supraphysiologic GH might be advantageous.

Liver-specific deletion of the growth hormone receptor reveals essential role of growth hormone signaling in hepatic lipid metabolism

Growth hormone (GH) plays a pivotal role in growth and metabolism, with growth promotion mostly attributed to generation of insulin-like growth factor I (IGF-I) in liver or at local sites of GH action, whereas the metabolic effects of GH are considered to be intrinsic to GH itself. To distinguish the effects of GH from those of IGF-I, we developed a Cre-lox-mediated model of tissue-specific deletion of the growth hormone receptor (GHR). Near total deletion of the GHR in liver (GHRLD) had no effect on total body or bone linear growth despite a >90% suppression of circulating IGF-I; however, total bone density was significantly reduced. Circulating GH was increased 4-fold, and GHRLD displayed insulin resistance, glucose intolerance, and increased circulating free fatty acids. Livers displayed marked steatosis, the result of increased triglyceride synthesis and decreased efflux; reconstitution of hepatic GHR signaling via adenoviral expression of GHR restored triglyceride output to normal, whereas IGF-I infusion did not correct steatosis despite restoration of circulating GH to normal. Thus, with near total absence of circulating IGF-I, GH action at the growth plate, directly and via locally generated IGF-I, can regulate bone growth, but at the expense of diabetogenic, lipolytic, and hepatosteatotic consequences. Our results indicate that IGF-I is essential for bone mineral density, whereas hepatic GH signaling is essential to regulate intrahepatic lipid metabolism. We propose that circulating IGF-I serves to amplify the growth-promoting effects of GH, while simultaneously dampening the catabolic effects of GH.

The growth hormone/insulin-like growth factor-I axis in exercise and sport

The syndrome of adult GH deficiency and the effects of GH replacement therapy provide a useful model with which to study the effects of the GH/IGF-I axis on exercise physiology. Measures of exercise performance including maximal oxygen uptake and ventilatory threshold are impaired in adult GH deficiency and improved by GH replacement, probably through some combination of increased oxygen delivery to exercising muscle, increased fatty acid availability with glycogen sparing, increased muscle strength, improved body composition, and improved thermoregulation. In normal subjects, in addition to the long-term effects of GH/IGF-I status, there is evidence that the acute GH response to exercise is important in regulating substrate metabolism after exercise. Administration of supraphysiological doses of GH to athletes increases fatty acid availability and reduces oxidative protein loss, particularly during exercise, and increases lean body mass. Despite a lack of evidence that these metabolic effects translate to improved performance, GH abuse by athletes is widespread. Tests to detect GH abuse have been developed based on measurement in serum of 1) indirect markers of GH action, and 2) the relative proportions of the two major naturally occurring isoforms (20 and 22kDa) of GH. There is evidence that exercise performance and strength are improved by administration of GH and testosterone in combination to elderly subjects. The potential benefits of GH in these situations must be weighed against potential adverse effects.

Efeito do hormônio de crescimento sobre parâmetros antropométricos e metabólicos na obesidade andróide

O objetivo do estudo foi avaliar os efeitos do GH sobre peso, composição corporal, metabolismo de repouso e fatores de risco cardiovascular na obesidade visceral. O estudo foi prospectivo randomizado duplo-cego em 40 homens não diabéticos de 20 a 50 anos com RAQ (relação abdômen-quadril) > 1 tratados com GH (0,050 U/kg/dia) ou placebo por três meses. Foram avaliados peso, composição corporal por bioimpedância e DEXA, metabolismo de repouso através da calorimetria indireta e exames de fatores de risco cardiovasculares no início e fim do tratamento. O grupo de obesos tratados com GH teve reduções de peso (3,5 ± 2,9 kg), IMC (1,2 ± 1,0 kg/m²), RAQ (0,04 ± 0,01) e massa adiposa (2,4 ± 1,0 kg). As reduções porcentuais foram significantemente diferentes das observadas no grupo placebo. Também houve diminuição nos níveis de colesterol total (4,0 ± 3,3 mg/dL) e LDL-colesterol (5,7 ± 2,7 mg/dL) no grupo GH, em relação ao grupo placebo. Os outros fatores de risco não se alteraram significantemente. Concluímos que obesos tratados com GH por três meses apresentaram uma redução significante de peso corporal, gordura visceral e massa adiposa, e melhora do perfil lipídico. O benefício/risco do GH a longo prazo em obesos é desconhecido.

Effects of high-dose growth hormone on glucose and glycerol metabolism at rest and during exercise in endurance-trained athletes

CONTEXT

Recombinant human-GH (r-hGH), in supraphysiological doses, is self-administered by athletes in the belief that it is performance enhancing.

OBJECTIVE

The objective of this study was to determine whether r-hGH alters whole-body glucose and glycerol metabolism in endurance-trained athletes at rest and during and after exercise.

DESIGN

This was a 4-wk double-blind placebo-controlled trial.

SETTING

This study was conducted at St. Thomas Hospital (London, UK).

PARTICIPANTS

Twelve endurance-trained male athletes were recruited and randomized to r-hGH (0.2 U/kg.d) (n = 6) or identical placebo (n = 6) for 4 wk. One (placebo group) withdrew after randomization.

INTERVENTION

Intervention was conducted by randomization to r-hGH (0.2 U/kg x d) or identical placebo for 4 wk.

MAIN OUTCOME MEASURES

Whole-body rates of appearance (Ra) of glucose and glycerol (an index of lipolysis) and rate of disappearance of glucose were measured using infusions of d-[6-6-2H2]glucose and 2H5-glycerol.

RESULTS

Plasma levels of glycerol and free fatty acids and glycerol Ra at rest and during and after exercise increased during r-hGH treatment (P < 0.05 vs. placebo). Glucose Ra and glucose rate of disappearance were greater after exercise during r-hGH treatment (P < 0.05 vs. placebo). Resting energy expenditure and fat oxidation were greater under resting conditions during r-hGH treatment (P < 0.05 vs. placebo).

CONCLUSIONS

r-hGH in endurance-trained athletes increased lipolysis and fatty acid availability at rest and during and after exercise. r-hGH increased glucose production and uptake rates after exercise. The relevance of these effects for athletic performance is not known.

Fat metabolism in exercise - with special reference to training and growth hormone administration

Despite abundance of fat, exclusive dependency on fat oxidation can only sustain a metabolic rate corresponding to 50-60% of VO(2max) in humans. This puzzling finding has been subject to intense research for many years. Lately, it has gained renewed interest as a consequence of increased obesity and physical inactivity imposed by Western lifestyle. Why are humans so poor at metabolizing fat? Can fat metabolism be manipulated by exercise, training, diet and hormones? And why is fat stored in specialized adipose tissue and not just as lipid droplets inside muscle cells? In the present review, human fat metabolism is discussed in relation to how human fat metabolism is designed. Limitations in this design are explored and examples of different designs for fat metabolism from animal physiology are included to illustrate these limitations. Various means of manipulating fat metabolism are discussed with special emphasis on exercise, training, growth hormone (GH) physiology and GH administration. It is concluded that fat stores, non-esterified fatty acids (NEFAs) availability and enzymes for fat oxidation can be increased substantially. However, it is almost impossible to increase fat oxidation during endurance exercise at higher intensities. It seems that, for some reason, the human being is far from optimally designed for fat oxidation during exercise. Acute GH administration has several unexpected effects on fat and carbohydrate metabolism during aerobic exercise, and future research in this area is likely to provide valuable information with respect to GH physiology and the regulation of fat and carbohydrate metabolism during aerobic exercise.

Effects of GH on urea, glucose and lipid metabolism, and insulin sensitivity during fasting in GH-deficient patients

Fasting-related states of distress pose major health problems, and growth hormone (GH) plays a key role in this context. The present study was designed to assess the effects of GH on substrate metabolism and insulin sensitivity during short-term fasting. Six GH-deficient adults underwent 42.5 h of fasting on two occasions, with and without concomitant GH replacement. Palmitate and urea fluxes were measured with the steady-state isotope dilution technique after infusion of [9,10-3H]palmitate and [13C]urea. During fasting with GH replacement, palmitate concentrations and fluxes increased by 50% [palmitate: 378 +/- 42 (GH) vs. 244 +/- 12 micromol/l, P < 0.05; palmitate: 412 +/- 58 (GH) vs. 276 +/- 42 microM, P = 0.05], and urea turnover and excretion decreased by 30-35% [urea rate of appearance: 336 +/- 22 (GH) vs. 439 +/- 43 micromol. kg-1. h-1, P < 0.01; urea excretion: 445 +/- 43 (GH) vs. 602 +/- 74 mmol/24 h, P < 0.05]. Insulin sensitivity (determined by a euglycemic hyperinsulinemic clamp) was significantly decreased [M value: 1.26 +/- 0.06 (GH) vs. 2.07 +/- 0.22 mg. kg-1. min-1, P < 0.01] during fasting with GH replacement. In conclusion, continued GH replacement during fasting in GH-deficient adults decreases insulin sensitivity, increases lipid utilization, and conserves protein.

Absence of effects of long-term growth hormone replacement therapy on insulin sensitivity in adults with growth hormone deficiency of childhood-onset (GHDA-CO)

In order to assess long-term efficacy and safety of GH therapy in GHDA-CO, we studied 20 patients (8 female, 12 male; mean age 24.6+/-6.2 years) treated with GH for up to 24 months. The assessment (IGF-1, IGFBP3, lipid profile, body composition, glycated hemoglobin, oral glucose tolerance test, ISI-HOMA and ISI-composite derived from OGTT) was carried out before GH and every 3 months during the first year of treatment, and then every 6 months. We observed a significant increase of IGF-1, lean mass and HDL levels and a decrease in LDL levels. Fasting glucose presented a significant increase, within the normal range, after 6 months, returning to pre-treatment levels at 9 months with no further alteration. Fasting insulin, the areas under the glucose and insulin curves, ISI-HOMA and ISI-composite did not vary significantly. We conclude that long-term GH therapy improved body composition and lipid profile, without altering ISI in this cohort of patients with GHDA-CO.

Diabetes control deteriorates in girls at cessation of growth: relationship with body mass index

AIM

Diabetic patients, particularly girls, often experience poor metabolic control during puberty and adolescence. The aim of this study was to investigate metabolic control during adolescence, especially in relation to pubertal stages, growth, insulin treatment and body mass index (BMI).

METHODS

We studied the records of 38 (consecutive) girls with prepubertal onset of Type 1 diabetes mellitus. Data from the age of 10 to 18-20 years were obtained with regard to glycaemic control, growth, age at menarche, final height and BMI, and analysed in relation to both chronological age and age at menarche.

RESULTS

HbA1c was lowest 3 years before menarche; mean (+/- sd) 7.6 (+/- 1.2). After the pubertal growth spurt, there was a marked impairment of metabolic control, the highest level of HbA1c occurring 3 years after menarche. Mean age at menarche was 13.3 (+/- 1.1) years and mean linear growth after menarche only 4.7 cm, giving a final height of 164.9 (+/- 5.3) cm which is 2.7 cm below the Swedish mean. During adolescence the degree of correlation between BMI and HbA1c continuously increased, pointing out the effect of body fat on metabolic control in this age group. The level of HbA1c at 10 years of age could not predict the metabolic control after cessation of puberty, but prepubertal BMI appears to be a risk factor for both obesity and poor glycaemic control in late adolescence.

CONCLUSIONS

The highest HbA1c was found after cessation of growth. Prepubertal BMI is a possible predictor of metabolic control in adolescent diabetic girls.

Interrelationship between insulin, leptin and growth hormone in growth hormone-treated children

OBJECTIVES

The aim of the study was to examine insulin homeostasis during growth hormone (GH) therapy, and to investigate the effect of GH treatment on insulin and leptin concentration in obese children.

SUBJECTS

Nineteen obese children (8 with Prader-Willi Syndrome (PWS)) were treated with GH 0.1 IU/kg/day dose for 3 months and were compared with 29 non-treated age and sex matched obese children (9 PWS) and 49 GH treated non-obese short children. Mean age of the children was 10.3+/-1.8 (6.7-13.8) y, with body mass index of 23.6+/-10.4 (11.5-47) kg/m2.

RESULTS

Leptin concentration decreased and was correlated inversely with initial leptin value (r2=-0.374, P<0.001) and decreased body mass (r2=0.338, P=0.001). Insulin sensitivity index was not significantly changed during therapy. Leptin decrease after 3 months of GH administration was correlated inversely with the increase in first phase insulin response to intravenous glucose tolerance test (IVGTT) (r2=-0.595, P<0.001). Results of long-term follow-up of treated patients demonstrated a decrease in insulin concentration after cessation of therapy. In GH-treated subjects, the glucose increase in response to glucose load appeared to be higher than in untreated subjects.

CONCLUSION

The high insulin response to glucose load seen in GH-treated subjects was appropriate to their glucose concentration and the insulin sensitivity index was unchanged relative to the pretreatment period. Increased insulin dosage in our patients did not induce an increase in leptin concentrations as had been hypothesised.

Continuation of growth hormone (GH) therapy in GH-deficient patients during transition from childhood to adulthood: impact on insulin sensitivity and substrate metabolism

The appropriate management of GH-deficient patients during transition from childhood to adulthood has not been reported in controlled trials, even though there is evidence to suggest that this phase is associated with specific problems in relation to GH sensitivity. An issue of particular interest is the impact of GH substitution on insulin sensitivity, which normally declines during puberty. We, therefore, evaluated insulin sensitivity (euglycemic glucose clamp) and substrate metabolism in 18 GH-deficient patients (6 females and 12 males; age, 20 +/- 1 yr; body mass index, 25 +/- 1 kg/m2) in a placebo-controlled, parallel study. Measurements were made at baseline, where all patients were on their regular GH replacement, after 12 months of either continued GH (0.018 +/- 0.001 mg/kg day) or placebo, and finally after 12 months of open phase GH therapy (0.016 mg/kg x day). Before study entry GH deficiency was reconfirmed by a stimulation test. During the double-blind phase, insulin sensitivity and fat mass tended to increase in the placebo group [deltaM-value (mg/kg x min), -0.7 +/- 1.1 (GH) vs. 1.3 +/- 0.8 (placebo), P = 0.18; deltaTBF (kg), 0.9 +/- 1.2 (GH) vs. 4.4 +/- 1.6 (placebo), P = 0.1]. Rates of lipid oxidation decreased [delta lipid oxidation (mg/kg x min), 0.02 +/- 0.14 (GH) vs. -0.32 +/- 0.13 (placebo), P < 0.05], whereas glucose oxidation increased in the placebo-treated group (P < 0.05). In the open phase, a decrease in insulin sensitivity was found in the former placebo group, although they lost body fat and increased fat-free mass [M-value (mg/kg x min), 5.1 +/- 0.7 (placebo) vs. 3.4 +/- 1.0 (open), P = 0.09]. In the group randomized to continued GH treatment almost all hormonal and metabolic parameters remained unchanged during the study. In conclusion, 1) discontinuation of GH therapy for 1 yr in adolescent patients induces fat accumulation without compromising insulin sensitivity; and 2) the beneficial effects of continued GH treatment on body composition in terms of decrease in fat mass and increase in fat-free mass does not fully balance the direct insulin antagonistic effects.

Acute effect of growth hormone to induce peripheral insulin resistance is independent of FFA and insulin levels in rats

To examine whether growth hormone (GH) induces peripheral insulin resistance by altering plasma free fatty acid (FFA) or insulin levels, the effects of GH infusion on insulin-stimulated glucose fluxes were studied in conscious rats under two protocols. In study 1, either saline (n = 7) or human recombinant GH (21 microg. kg(-1). h(-1); n = 8) was infused for 300 min, and insulin-stimulated glucose fluxes were estimated during the final 150-min period of hyperinsulinemic euglycemic clamps. In study 2, hyperinsulinemic euglycemic clamps were first conducted for 150 min (to raise plasma insulin and suppress FFA levels), and saline or GH (n = 7 for each) was subsequently infused for the following 300-min clamp period. In study 1, GH infusion in the basal state did not significantly alter plasma FFA or insulin levels. In contrast, GH infusion decreased insulin-stimulated glucose uptake, glycolysis, and glycogen synthesis by 32, 27, and 40%, respectively (P < 0.05). In study 2, GH infusion during hyperinsulinemic euglycemic clamps did not alter plasma FFA or insulin levels (P > 0.05). GH infusion had no effect on insulin-stimulated glucose uptake during the initial 150 min but eventually decreased insulin-stimulated glucose uptake by 37% (P < 0. 05), similar to the results in study 1. These data indicate that GH induces peripheral insulin resistance independent of plasma FFA and insulin levels. The induction of insulin resistance was preceded by suppression of glycogen synthesis, consistent with the hypothesis that metabolic impairment precedes and causes development of peripheral insulin resistance.

Behavioural rhythmicity in transgenic growth hormone mice: trade-offs, energetics, and sleep–wake cycles

Transgenic mice with extra rat growth hormone (GH) genes (TRrGH mice) are behaviourally lethargic and sleep 3.4 h/d longer than normal on standard diets. We tested the hypothesis that the doubling of the growth rate of TRrGH mice reduced the energy available for behaviour. Provision of sucrose supplements ad libitum normalized the durations of activity and sleep. Our results support a new allocative theory suggesting that sleep serves as an umbrella function for a suite of synergistic anabolic functions (e.g., growth, immunity, repair). Relegating these to the period of sleep in a secure nest allows full dedication of waking resources to niche interfacing (resource acquisition, risk avoidance and environmental stress resistance). Energy stress in TRrGH mice may arise via specific diversion of energy from waking functions via GH-induced insulin resistance. GH is normally secreted during sleep, but any causal relationship remains unresolved. We examined the circadian and ultradian behaviour of TRrGH mice to determine how a chronically elevated GH level impacts sleep. Remarkably, even the major hormonal distortion in TRrGH mice had little impact on the timing of ultradian or circadian rhythms. Increased sleeping of TRrGH mice on normal diets was due to an increased likelihood and duration of sleep at permitted times. GH did, however, appear to increase the depth of sleep.

Insulin-like growth factor-I in man enhances lipid mobilization and oxidation induced by a growth hormone pulse

Growth hormone (GH) secretion is suppressed during insulin-like growth factor-I (IGF-I) administration. The aim of the study was to examine whether IGF-I alters the metabolic response to a GH pulse. Seven healthy male subjects (age 27 +/- 4 years, BMI 21.8 +/- 1.7 kg/m2) were treated with NaCl 0.9% (saline) or IGF-I (8 micrograms.kg-1.h-1) for 5 days by continuous subcutaneous infusion in a randomized, crossover fashion while receiving an isocaloric diet (30 kcal.kg-1.day-1). On the third treatment day an intravenous bolus of 0.5 U GH was administered. Forearm muscle metabolism was examined by measuring arterialized and deep venous blood samples, forearm blood flow by occlusion plethysmography and substrate oxidation by indirect calorimetry. IGF-I treatment significantly reduced insulin concentrations by 80% (p < 0.02) and C-peptide levels by 78% (p < 0.02), as assessed by area under the curve. Non-esterified fatty acid (NEFA), glycerol and 3-OH-butyrate levels were elevated and alanine concentration decreased. Forearm blood flow rose from 2.10 +/- 0.43 (saline) to 2.79 +/- 0.37 ml.100ml-1. min-1 (IGF-I) (p < 0.02). GH-pulse: 10 h after i.v. GH injection serum GH peaked at 40.9 +/- 7.4 ng/ml. GH did not influence circulating levels of total IGF-I, C-peptide, insulin or glucose, but caused a further increase in NEFA, glycerol and 3-OH-butyrate levels, indicating enhanced lipolysis and ketogenesis. This effect of GH was much more pronounced during IGF-I: NEFA rose from 702 +/- 267 (saline) and 885 +/- 236 (IGF-I) to 963 +/- 215 (saline) (p < 0.05) and 1815 +/- 586 mumol/l (IGF-I) (p < 0.02), respectively; after 5 h, 3-OH-butyrate rose from 242 +/- 234 (saline) and 340 +/- 280 (IGF-I) to 678 +/- 638 (saline) (p < 0.02) and 1115 +/- 578 mumol/l (IGF-I) (p < 0.02) respectively. After injection of GH, forearm uptake of 3-OH-butyrate was markedly elevated only in the subjects treated with IGF-I: from 44 +/- 195 to 300 +/- 370 after 20 min (p < 0.03) and to 287 +/- 91 nmol.100 ml-1. min-1 after 120 min (p < 0.02). In conclusion, the lipolytic and ketogenic response to GH was grossly enhanced during IGF-I treatment, and utilization of ketone bodies by skeletal muscle was increased.

Persistence of counter-regulatory abnormalities in insulin-dependent diabetes mellitus after pancreas transplantation

Conventional insulin therapy does not correct the counter-regulatory abnormalities of insulin-dependent diabetes mellitus. Pancreas transplantation is an alternative therapy that restores the endogenous insulin secretion in diabetes. In this study, the effects of segmental pancreas transplantation on counter-regulation to mild hypoglycaemia were evaluated. Glucose kinetics and the counter-regulatory hormonal responses were assessed in eight insulin-dependent diabetics with end-stage renal failure who had received pancreas and kidney transplantation 1 year previously, seven diabetic uraemic subjects (candidates for combined transplantation), five patients with chronic uveitis on immunosuppressive therapy comparable to pancreas recipients and 10 normal subjects. Insulin (0.3 mU kg-1 min-1) was infused for 2 h to induce mild hypoglycaemia (plasma glucose 3.2-3.5 mmol l-1) and exogenous glucose was infused as required to prevent any glucose decrease below 3.1 mmol l-1. After transplantation, two of eight recipients had hypoglycaemic episodes reported in their medical records. During the study, hepatic glucose production was rapidly suppressed in the controls and in the patients on immunosuppression (-80 +/- 7 and -54 +/- 7%, P < 0.001 vs. basal), and rebounded to the baseline values within 1 h (-3 +/- 1 and -6 +/- 2%, P = NS vs. basal). The transplant recipients had similar suppression in the first hour (-88 +/- 8%, P < 0.001 vs. basal), but the suppression persisted in the second hour (-69 +/- 11%, P < 0.001 vs. basal) indicating a lack of glucose counter-regulatory response.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the effects of growth hormone and insulin-like growth factor I on substrate oxidation and on insulin sensitivity in growth hormone-deficient humans

Insulin-like growth factor-I (IGF-I) is considered to be the mediator of the growth-promoting effects of growth hormone (GH). The metabolic effects of these two hormones, however, are different. Whereas GH treatment leads to elevated insulin and glucose levels, reduced insulin sensitivity, and impaired glucose tolerance, IGF-I treatment leads to reduced insulin and GH levels and enhanced insulin sensitivity. IGF-I may, therefore, not only be the mediator of the growth-promoting effects of GH but also a modulator of the effects of GH on insulin action and glucose metabolism. To study the influence of GH and IGF-I on substrate metabolism and insulin sensitivity (assessed by euglycemic, hyperinsulinemic clamping combined with indirect calorimetry and glucose tracer infusion), we have treated eight GH-deficient adults with GH (2 IU/m2 daily subcutaneously [s.c.]), IGF-I (10 micrograms/kg.h s.c.), or both hormones together for 7 d, respectively, and compared the effects of these treatment regimens with a control phase. Our findings suggest that (a) both GH and IGF-I promote lipolysis and lipid oxidation, albeit by different mechanisms; (b) treatment with either hormone is followed by enhanced energy expenditure and reduced protein oxidation; and (c) IGF-I reverses the insulin resistance induced by GH.

Insulin-like growth factor I stimulates lipid oxidation, reduces protein oxidation, and enhances insulin sensitivity in humans

To elucidate the effects of insulin-like growth factor I (IGF-I) on fuel oxidation and insulin sensitivity, eight healthy subjects were treated with saline and recombinant human (IGF-I (10 micrograms/kg.h) during 5 d in a crossover, randomized fashion, while receiving an isocaloric diet (30 kcal/kg.d) throughout the study period. On the third and fourth treatment days, respectively, an L-arginine stimulation test and an intravenous glucose tolerance test were performed. A euglycemic, hyperinsulinemic clamp combined with indirect calorimetry and a glucose tracer infusion were performed on the fifth treatment day. IGF-I treatment led to reduced fasting and stimulated (glucose and/or L-arginine) insulin and growth hormone secretion. Basal and stimulated glucagon secretion remained unchanged. Intravenous glucose tolerance was unaltered despite reduced insulin secretion. Resting energy expenditure and lipid oxidation were both elevated, while protein oxidation was reduced, and glucose turnover rates were unaltered on the fifth treatment day with IGF-I as compared to the control period. Enhanced lipolysis was reflected by elevated circulating free fatty acids. Moreover, insulin-stimulated oxidative and nonoxidative glucose disposal (i.e., insulin sensitivity) were enhanced during IGF-I treatment. Thus, IGF-I treatment leads to marked changes in lipid and protein oxidation, whereas, at the dose used, carbohydrate metabolism remains unaltered in the face of reduced insulin levels and enhanced insulin sensitivity.

Effects of treatment with recombinant human growth hormone on insulin sensitivity and glucose metabolism in adults with growth hormone deficiency

In a double-blind, cross-over, placebo-controlled trial, the effect of 26 weeks of replacement therapy with recombinant human growth hormone (rhGH) on insulin sensitivity and glucose metabolism in nine patients with adult-onset growth hormone deficiency was studied with a euglycemic clamp. Glucose production and utilization were studied with D-(3-3H)-glucose infusions. Comparisons were made with placebo treatment for 6 and 26 weeks, respectively. GH therapy for 6 weeks increased fasting plasma concentrations of glucose and insulin. However, after 26 weeks of GH treatment, no significant changes in glucose or insulin concentrations were recorded. GH treatment induced a marked change in insulin action evident after 6 weeks of therapy as shown by lower glucose infusion rates (GIRs) during the clamp compared with placebo treatment (2.6 +/- 0.4 v 4.1 +/- 0.7 mg.kg-1.min-1). This change in insulin action was due to a decreased insulin effect on glucose utilization. After 26 weeks of GH therapy, there was no significant difference in GIRs. During placebo treatment, insulin sensitivity and insulin, glucose, and nonesterified fatty acid (NEFA) concentrations were unchanged compared with concentrations measured before the study. Thus GH replacement therapy induces a change in insulin action in GH-deficient individuals. Whether this change represents a decrease in insulin action (ie, insulin resistance) or a restoration of action to normal is presently unclear, since a healthy control group was not included in the study. During long-term treatment, the present study suggests that the change in insulin action can be reversed, probably secondarily to changes in body composition.

Growth hormone dose regimens in adult GH deficiency: effects on biochemical growth markers and metabolic parameters

OBJECTIVE

We examined the effects of different doses of GH on insulin-like growth factor I (IGF-I), IGF binding protein 3 (IGFBP-3), body composition, energy expenditure, and various metabolites in GH deficient adults, in order to approach a metabolically appropriate GH dosage in young GH deficient adults.

DESIGN

Ten GH deficient patients (age 21-43) were studied after 4 weeks without GH followed by three consecutive 4-week periods, where the patients received in a fixed order GH 1, 2 and 4 IU/m2 s.c. per day. At the end of each period the patients were hospitalized for a 24-hour examination.

RESULTS

Mean 24-hour levels of GH (mIU/l) were 2.7 +/- 0.3 (0 GH), 7.2 +/- 0.9 (1), 10.8 +/- 1.5 (2) and 18.9 +/- 2.7 (4 IU/m2) (mean +/- SEM) (P < 0.01). Likewise, IGF-I levels increased dose dependently from 61 +/- 21 to 206 +/- 65, 260 +/- 70 and 468 +/- 171 micrograms/l (P < 0.05); serum IGF-I in an age and sex matched control group was 248 +/- 25 micrograms/l. Corresponding serum IGFBP-3 levels also increased from 1860 +/- 239 to 3261 +/- 379, 3762 +/- 434 and 4384 +/- 652 micrograms/l (P = 0.01) respectively. Significant increases in diurnal serum insulin levels after 4 IU/m2 were recorded, whereas plasma glucose levels remained unchanged. Lipid intermediates increased dose independently during GH administration. GH caused a significant increase in resting energy expenditure, whereas the respiratory exchange ratio was unaltered. Fat mass was increased without GH therapy and decreased during the study. Four patients made complaints during 4 IU/m2 GH administration, probably related to GH induced fluid retention.

CONCLUSION

Based primarily on IGF-I and IGFBP-3 levels our data suggest that a GH replacement dose in young GH deficient adults in the order of 1-2 IU/m2 per day is adequate. This is a relatively low dose as compared to dose regimens in children and adolescents.

Growth hormone acutely stimulates skeletal muscle but not whole-body protein synthesis in humans

In a previous study, a 6-hour local infusion of growth hormone (GH) into the brachial artery of normal subjects stimulated net muscle protein anabolism by augmenting skeletal muscle protein synthesis. In the present study, we examined whether systemically infused GH affects forearm and whole-body protein metabolism. Normal volunteers aged 18 to 24 years (n = 8) were given an 8-hour systemic infusion of 3H-phenylalanine and 14C-leucine. Between 90 and 120 minutes of tracer infusion, basal samples for determination of forearm and whole-body amino acid kinetics were taken. GH was then infused at 0.06 micrograms/kg/min, increasing GH concentration from 2.4 +/- 0.3 to 32 +/- 3 ng/mL. Systemic insulin-like growth factor 1 (IGF-1) level increased from 224 +/- 20 to 262 +/- 21 ng/mL (basal v 6-hour, P < .01). By 6 hours, GH suppressed forearm phenylalanine and leucine net release (each P < .05) by increasing 3H-phenylalanine (66%, P < .05) and 14C-leucine (13%, P < .05) extraction or disposal (Rd). Whole-body leucine rate of appearance ([Ra] an index of whole-body proteolysis) and nonoxidative leucine Rd (whole-body protein synthesis) did not change over the course of the GH infusion, whereas oxidative leucine Rd decreased (20%, P < .03). Acute stimulation of muscle but not whole-body protein synthesis by systemically infused GH suggests that muscle protein is acutely and specifically regulated by GH.

Metabolic status and growth hormone secretion in man

The metabolic state influences GH secretion and, in turn, GH secretion influences overall body metabolism. While these are distinct physiological entities, separation of these interrelated phenomena should be considered artificial since the overall result is an organism which retains functional capacity in the setting of a variety of circumstances.

Effect of prolonged hyperglycemia on growth hormone levels and insulin sensitivity in insulin-dependent diabetes mellitus

The aim of the present study was to characterize the effect of a hyperglycemic period (44 hours) on the levels of insulin-antagonistic hormones and insulin sensitivity in seven subjects with well-controlled insulin-dependent diabetes mellitus (IDDM). Hyperglycemia (approximately 15 mmol.L-1) was induced by a glucose infusion while the degree of insulinization was similar to that of the period with near normoglycemia (approximately 6.9 mmol.L-1). Insulin sensitivity was measured with hyperinsulinemic euglycemic clamps performed 4 hours before and after the periods of normoglycemia (control) and hyperglycemia. D-[3-3H]glucose was infused in the second clamp in each study to evaluate glucose production and utilization. Since growth hormone (GH) levels frequently are elevated during poor diabetic control, diurnal GH secretion was measured in blood samples continuously drawn for 24 hours during the euglycemic and hyperglycemic periods. Levels of epinephrine, norepinephrine, cortisol, and nonesterified free fatty acids (NEFA) were similar during the control and hyperglycemic periods and during the clamps. GH levels were also similar, but an abnormal diurnal secretion pattern was present with increased numbers of daytime peaks. Hyperglycemia did not reduce GH secretion in IDDM. Hyperglycemia for 44 hours induced insulin resistance (32% reduction of glucose infusion rate, P < .02). In the control study, a 21% reduction (P = .064, NS) of the glucose disposal rate (Rd) was seen, suggesting that the hospitalization period per se may also reduce insulin sensitivity. In conclusion, a period of hyperglycemia leads to insulin resistance in IDDM patients. This insulin resistance cannot be attributable to increased levels of insulin-antagonistic hormones, although an abnormal secretion pattern for GH was found.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of the anabolic effects of growth hormone and insulin-like growth factor I by use of both agents simultaneously

The use of growth hormone (GH) as an anabolic agent is limited by its tendency to cause hyperglycemia and by its inability to reverse nitrogen wasting in some catabolic conditions. In a previous study comparing the anabolic actions of GH and IGF-I (insulin-like growth factor I), we observed that intravenous infusions of IGF-I (12 micrograms/kg ideal body wt [IBW]/h) attenuated nitrogen wasting to a degree comparable to GH given subcutaneously at a standard dose of 0.05 mg/kg IBW per d. IGF-I, however, had a tendency to cause hypoglycemia. In the present study, we treated seven calorically restricted (20 kcal/kg IBW per d) normal volunteers with a combination of GH and IGF-I (using the same doses as in the previous study) and compared its effects on anabolism and carbohydrate metabolism to treatment with IGF-I alone. The GH/IGF-I combination caused significantly greater nitrogen retention (262 +/- 43 mmol/d, mean +/- SD) compared to IGF-I alone (108 +/- 29 mmol/d; P < 0.001). GH/IGF-I treatment resulted in substantial urinary potassium conservation (34 +/- 3 mmol/d, mean +/- SE; P < 0.001), suggesting that most protein accretion occurred in muscle and connective tissue. GH attenuated the hypoglycemia induced by IGF-I as indicated by fewer hypoglycemic episodes and higher capillary blood glucose concentrations on GH/IGF-I (4.3 +/- 1.0 mmol/liter, mean +/- SD) compared to IGF-I alone (3.8 +/- 0.8 mmol/liter; P < 0.001). IGF-I caused a marked decline in C-peptide (1,165 +/- 341 pmol/liter; mean +/- SD) compared to the GH/IGF-I combination (2,280 +/- 612 pmol/liter; P < 0.001), suggesting maintenance of normal carbohydrate metabolism with the latter regimen. GH/IGF-I produced higher serum IGF-I concentrations (1,854 +/- 708 micrograms/liter; mean +/- SD) compared to IGF-I only treatment (1,092 +/- 503 micrograms/liter; P < 0.001). This observation was associated with increased concentrations of IGF binding protein 3 and acid-labile subunit on GH/IGF-I treatment and decreased concentrations on IGF-I alone. These results suggest that the combination of GH and IGF-I treatment is substantially more anabolic than either IGF-I or GH alone. GH/IGF-I treatment also attenuates the hypoglycemia caused by IGF-I alone. GH/IGF-I treatment could have important applications in diseases associated with catabolism.

Effect of growth hormone and resistance exercise on muscle growth in young men

The purpose of this study was to determine whether growth hormone (GH) administration enhances the muscle anabolism associated with heavy-resistance exercise. Sixteen men (21-34 yr) were assigned randomly to a resistance training plus GH group (n = 7) or to a resistance training plus placebo group (n = 9). For 12 wk, both groups trained all major muscle groups in an identical fashion while receiving 40 micrograms recombinant human GH.kg-1.day-1 or placebo. Fat-free mass (FFM) and total body water increased (P less than 0.05) in both groups but more (P less than 0.01) in the GH recipients. Whole body protein synthesis rate increased more (P less than 0.03), and whole body protein balance was greater (P = 0.01) in the GH-treated group, but quadriceps muscle protein synthesis rate, torso and limb circumferences, and muscle strength did not increase more in the GH-treated group. In the young men studied, resistance exercise with or without GH resulted in similar increments in muscle size, strength, and muscle protein synthesis, indicating that 1) the larger increase in FFM with GH treatment was probably due to an increase in lean tissue other than skeletal muscle and 2) resistance training supplemented with GH did not further enhance muscle anabolism and function.

Characterization of the insulin-antagonistic effect of growth hormone in man

The insulin-antagonistic effect of growth hormone was characterized by infusing the hormone at three different infusion rates (6, 12 or 24 mU.kg-1.min-1) for one h in 11 healthy subjects. The insulin effect was measured with the euglycaemic clamp technique combined with D-(3-3H)-glucose infusion to evaluate glucose production and utilization. A control study with NaCl (154 mmol.l-1) infusion was also performed. The insulin levels during the clamps were similar in all studies (36 +/- 0.2 mU.l-1). Peak growth hormone levels were reached at 60 min (growth hormone 6 mU.kg-1.h-1: 31 +/- 5; growth hormone 12 mU.kg-1.h-1: 52 +/- 4 and growth hormone 24 mU.kg-1.h-1; 102 +/- 8 mU.l-1). The insulin-antagonistic effect of growth hormone started after approximately 2 h, was maximal after 4-5 h (approximately 39% inhibition of glucose infusion rate between control and growth hormone 24 mU.kg-1.h-1) and lasted for 6-7 h after peak levels. The resistance was due to a less pronounced insulin effect both to inhibit glucose production and to stimulate glucose utilization. Growth hormone infusion of 12 mU.kg-1.h-1 induced a similar insulin-antagonistic effect as the higher infusion rate whereas 6 mU.kg-1.h-1 induced a smaller response with a duration of 1 h between 3-4 h after peak levels of growth hormone. The present study demonstrates that growth hormone levels similar to those frequently seen in Type 1 (insulin-dependent) diabetic patients during poor metabolic control or hypoglycaemia, have pronounced insulin-antagonistic effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of growth hormone on fuel utilization and muscle glycogen synthase activity in normal humans

To examine the insulin antagonistic effects of growth hormone (GH), seven healthy subjects underwent, in random order, two 5-h euglycemic clamp studies with moderate hyperinsulinemia. A GH infusion (45 ng.kg-1.min-1) was given throughout one of the studies. GH inhibited the insulin-stimulated glucose disposal by 27% from 4.4 +/- 0.7 to 3.3 +/- 0.4 mg.kg-1.min-1 (P less than 0.02) and raised the nonprotein energy expenditures (NPEE) from 18.7 +/- 0.5 to 20.5 +/- 0.3 kcal.kg-1.24 h-1 (P less than 0.03). Lipid oxidation contributed 71.7 +/- 5.6% of NPEE during the GH infusion as compared with 48.7 +/- 5.2% during the control clamp (P less than 0.02). In skeletal muscle biopsies, insulin binding to wheat germ agglutinin-purified insulin receptors and insulin receptor kinase activity were unaffected by GH infusion. Glycogen synthase activation by insulin was inhibited by 41% during the GH clamp (fractional velocity 14.1 +/- 2.5 vs. 8.3 +/- 1.4%, P less than 0.03). In conclusion, GH 1) increases energy expenditures and inhibits glucose oxidation in favor of an increased lipid oxidation, and 2) inhibits insulin-mediated activation of the glycogen synthase in skeletal muscle biopsies by a mechanism distal to insulin receptor binding and kinase activity.

Short time effects of growth hormone on glucose metabolism and insulin and glucagon secretion in normal man

The present study was undertaken in order to evaluate the acute metabolic and hormonal effects of human growth hormone in healthy subjects. Glucose turnover, plasma glucose, FFA, insulin, C-peptide, glucagon, and somatostatin concentrations were determined in the fasting state after a bolus injection of placebo or growth hormone in quantities producing increases in plasma growth hormone levels within the normal physiological range. We found that growth hormone administration resulted in negligible changes in plasma glucose, no significant changes in appearance or disappearance rates of glucose, a moderate increase in FFA and a moderate fall in plasma insulin, C-peptide and glucagon concentrations, while plasma somatostatin levels were unchanged. These findings suggest that rapid changes in plasma growth hormone concentrations, corresponding to the fluctuations seen during normal daily life, may play a role in the short time regulation of blood glucose concentration through an inhibition of insulin and glucagon secretion.

Effects of a growth hormone pulse on total and forearm substrate fluxes in humans

Under physiological circumstances growth hormone (GH) is secreted in bursts after the onset of sleep and a few hours postprandially. Because most relevant studies have employed constant or repeated infusion of high doses of GH, the possible metabolic effects of such bursts are largely unknown. We have studied seven healthy, male subjects for 7 h after an intravenous bolus of 1) 140 micrograms GH and 2) saline. When injected, serum GH rose to a peak of 21 +/- 3 micrograms/l 10 min after injection. GH caused 1) a rapid, sustained 55% decrease in forearm glucose uptake (P less than 0.05) followed by increases toward control values, 2) a delayed 5 mg/100 ml decrease in plasma glucose (P less than 0.05), and 3) significant 60-250% increases (P less than 0.05) in all measured lipid intermediates (nonesterified fatty acids, 3-hydroxybutyrate, and glycerol) 120-160 min after administration followed by decreases to below control values (P less than 0.05). GH did not influence circulating levels of insulin, C-peptide, glucagon, or insulin-like growth factor I (IGF-I), or isotopically determined glucose turnover. Physiological bursts of GH secretion appear to have acute insulin antagonistic effects with maximal effect on lipolysis after 2 h. These effects are reversed after 4 h. Therefore, GH could play a key role in regulation of diurnal rhythms of substrate levels and fuel utilization in humans.

Somatostatin analogues in diabetes mellitus

Growth hormone (GH) has long been considered to have importance in diabetes. With poor control in Type 1 diabetes GH levels are high and may aggravate poor metabolic control. Pharmacological suppression of GH release at this stage might reverse the metabolic changes, with the possible added benefit of lower plasma insulin concentrations. Diabetic patients with life-long GH deficiency rarely develop retinopathy, while pituitary ablation in patients with retinopathy often leads to improvement. Growth hormone release inhibiting factor, somatostatin, has a short plasma half-life, and multiple effects on the endocrine system and on the gastrointestinal tract, making it unsuitable for clinical use as a GH suppressant. Long-acting analogues have a long half-life, but remain non-specific in their effects. In Type 2 diabetes the analogue Octreotide suppresses insulin and glucagon release, leaving glucose levels either unchanged or somewhat elevated. Gastrointestinal side-effects have been common, but may diminish with long-term treatment. In Type 1 diabetes insulin requirement is decreased by Octreotide, but as in Type 2 diabetes GH suppression has been observed consistently only when the drug was given at bed-time. The decrease in insulin requirement may reflect suppression of glucagon release and/or gut effects. Amelioration of the 'dawn phenomenon' has not proved possible, and hypoglycaemia has proved a particular problem with Octreotide given subcutaneously at night. The lack of effective GH suppression (particularly in patients with proliferative retinopathy), lack of specificity, and the gut and hypoglycaemic side-effects, argue strongly against a clinical role for the current somatostatin analogues in diabetes mellitus.

Regulation of fatty acid and carbohydrate metabolism by insulin, growth hormone and tri-iodothyronine in hepatocyte cultures from normal and hypophysectomized rats

The interactions of insulin, growth hormone (somatotropin) and tri-iodothyronine (T3) in the long-term (24 h) regulation of fatty acid and carbohydrate metabolism were studied in hepatocyte primary cultures isolated from normal or hypophysectomized Sprague-Dawley rats. Hepatocytes from hypophysectomized rats had similar rates of palmitate metabolism, but lower rates of ketogenesis, than hepatocytes from normal rats. They also had a lower endogenous triacylglycerol content and lower activities of NADP-linked dehydrogenases than did cells from normal rats. The inhibitions of ketogenesis and gluconeogenesis by insulin were more marked in hepatocytes from hypophysectomized than from normal rats. Insulin caused a 7-10-fold increase in cellular glycogen in hepatocytes from hypophysectomized rats, compared with a 2-3-fold increase in cells from normal rats, and it increased cellular triacylglycerol by 65% in cells from hypophysectomized rats, compared with 11% in cells from normal rats. In hepatocytes from hypophysectomized rats, growth hormone and T3 increased ketogenesis both separately and in combination (12% and 23% respectively; P less than 0.05), whereas in hepatocytes from normal rats only the combination of growth hormone and T3 caused a significant increase in ketogenesis. In cells from hypophysectomized rats, T3 and growth hormone had different effects on carbohydrate metabolism: T3, but not growth hormone, potentiated the anti-gluconeogenic and glycogenic effects of insulin. It is concluded that hypophysectomy increases the responsiveness of hepatocytes to insulin, growth hormone and T3, and that growth hormone and T3 regulate fatty acid and carbohydrate metabolism by different mechanisms.

Effects of growth hormone on insulin sensitivity and forearm metabolism in normal man

To elucidate the short-term actions of growth hormone on insulin sensitivity and forearm metabolism, we have studied six normal male subjects receiving a 6-h hyperinsulinaemic euglycemic clamp with and without a concomitant 4-h growth hormone infusion. When infused, serum growth hormone rose to 25 +/- 4 mU/l and during administration of insulin serum insulin increased by 11 +/- 1 mU/l. During euglycemic clamp, administration of growth hormone decreased forearm glucose uptake after 180 min and onward (240 min 0.216 +/- 0.031 vs 0.530 +/- 0.090 mg/100 ml/min, p less than 0.05). Glucose infusion rate (240 min 2.83 +/- 0.24 vs 4.35 +/- 0.28 mg.kg-1.min-1, p less than 0.05) and glucose disposal rate (240 min 3.57 +/- 0.17 vs 4.00 +/- 0.15 mg.kg-1.min-1, p less than 0.05) also decreased. Growth hormone persistently increased hepatic glucose production after 120 min. After 210 min, all circulating lipid intermediates increased slightly. The decrease in forearm glucose uptake and glucose infusion rate and the increase in hepatic glucose production was observed before there was any detectable increase in circulating levels and forearm uptake of lipid intermediates. These data suggest that growth hormone induces insensitivity to insulin in liver, muscle and fat after 120, 180 and 210 min respectively. The early effects of growth hormone on glucose metabolism seems independent of changes in the rate of lipolysis.

Changes in endogenous insulin secretion during childhood as expressed by plasma and urinary C-peptide

Basal fasting values of plasma C-peptide (CP), plasma insulin and 24 h urine CP were determined in 224 normal non-obese subjects of both sexes ranging in age from 1 to 20 years. Analysis of the results by age, pubertal rating, sex and bone age (BA) during childhood showed that mean +/- SD plasma CP levels in both sexes rose from 0.07 +/- 0.08 pmol/ml at the age of 1-2 years to 0.21 +/- 0.11 pmol/ml at 8-10 years. Mean +/- SD plasma insulin levels in both sexes rose from 3.2 +/- 4.3 microU/ml at the age of 1-2 years to 5.9 +/- 4.5 microU/ml at 8-10 years. Mean +/- SD urine CP levels rose from 6.5 +/- 2.8 pmol/mg creatinine per 24 h at the age of 2-8 years to 7.7 +/- 3.5 pmol/mg creatinine per 24 h at 8-11 years in both sexes. During puberty, plasma and urine CP and plasma insulin levels rose further to peak at pubertal stage P3, the values in females being higher (CP = 0.32 +/- 0.06 pmol/ml) than those in males (CP = 0.22 +/- 0.06 pmol/ml) (P less than 0.005). Plasma insulin levels in females were 13.2 +/- 6.9 microU/ml and 6.4 +/- 3.1 microU/ml in males (P less than 0.05). Urine CP levels were 14.5 +/- 5.7 pmol/mg creatinine per 24 h and 10.8 +/- 5.4 pmol/mg creatinine per 24 h in females and males respectively (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Glucagon, not insulin, may play a secondary role in defense against hypoglycemia during exercise

The sympathochromaffin system, probably sympathetic neural norepinephrine, plays a primary role in the prevention of hypoglycemia during exercise in humans. Our previous data indicated that changes in pancreatic islet hormones are not normally critical but decrements in insulin, increments in glucagon, or both become critical when catecholamine actions are blocked pharmacologically. To distinguish between the role of insulin and that of glucagon in this secondary line of defense against hypoglycemia during exercise in humans, glucoregulation during moderate exercise (approximately 55% of maximum O2 consumption over 60 min) was studied in people who could not decrease insulin but could increase glucagon, i.e., patients with insulin-dependent diabetes mellitus (IDDM). While receiving constant intravenous infusions of regular insulin, in individualized doses shown to result in stable plasma glucose concentrations of approximately 95 mg/dl before exercise, patients with IDDM were studied under two conditions: 1) a control study (n = 13) and 2) an adrenergic blockade study (propranolol infusion, n = 8). In the control study, mean plasma glucose concentrations did not change (from 95 +/- 2 to 100 +/- 11 mg/dl) during exercise despite constant plasma free insulin levels. In the adrenergic blockade study plasma glucose declined (from 96 +/- 2 to 74 +/- 7 mg/dl, P less than 0.01) but stabilized; hypoglycemia did not occur. Exercise-associated increments in plasma glucagon were comparable in the two studies. These data confirm that decrements in insulin are not critical to the prevention of hypoglycemia during moderate exercise in humans and indicate that compensation for deficient catecholamine action does not require decrements in insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired glucose tolerance as a disorder of insulin action. Longitudinal and cross-sectional studies in Pima Indians

Impaired glucose tolerance often presages the development of non-insulin-dependent diabetes mellitus. We have studied insulin action and secretion in 24 Pima Indians before and after the development of impaired glucose tolerance and in 254 other subjects representing the whole spectrum of glucose tolerance, including subjects with overt non-insulin-dependent diabetes. The transition from normal to impaired glucose tolerance was associated with a decrease in glucose uptake during hyperinsulinemia, from 0.018 to 0.016 mmol per minute (from 3.3 to 2.8 mg per kilogram of fat-free body mass per minute) (P less than 0.0003). Mean plasma insulin concentrations increased during an oral glucose-tolerance test, from 1200 to 1770 pmol per liter (from 167 to 247 microU per milliliter). In 151 subjects with normal glucose tolerance, the insulin concentration measured during an oral glucose-tolerance test correlated with the plasma glucose concentration (r = 0.48, P less than or equal to 0.0001). This relation was used to predict an insulin concentration of 1550 pmol per liter (216 microU per milliliter) in subjects with impaired glucose tolerance (actual value, 1590 pmol per liter [222 microU per milliliter]; P not significant), suggesting that these subjects had normal secretion of insulin. In contrast, plasma insulin concentrations in the diabetics decreased as glucose concentrations increased (r = -0.75, P less than or equal to 0.0001), suggesting deficient secretion of insulin. This relative insulin deficiency first appears at the lower end of the second (diabetic) mode seen in population frequency distributions of plasma glucose concentrations. Our data show that impaired glucose tolerance in our study population is primarily due to impaired insulin action. In patients with non-insulin-dependent diabetes mellitus, by contrast, impaired insulin action and insulin secretory failure are both present.

Physiological role of growth hormone in adult life

Some historical background on the physiological effects of pituitary gland extract and later of purified hGH in experimental animals is given, and the regulatory role of hGH in the human adult is reviewed. The effects of hGH on carbohydrate, protein and lipid metabolism are complex and closely linked to the action of other hormones, particularly insulin.

Effect of insulin on growth hormone-induced metabolic derangements in diabetes

To investigate the mechanism of growth hormone-induced hyperglycemia in diabetes, two studies were done in insulin-dependent diabetic patients receiving intensive insulin therapy with the insulin pump. First, the metabolic response to a standard breakfast following a subcutaneous insulin bolus was examined before and after 20 hourly boluses of intravenous growth hormone in eight patients. Despite unchanged insulin therapy, growth hormone administration produced a marked rise in fasting glucose concentrations (197 +/- 21 v 96 +/- 11 mg/dL), as well as increases in fasting levels of free fatty acids and branched chain amino acids. Nevertheless, postprandial blood glucose increments were only slightly greater after growth hormone (36 +/- 14 v 20 +/- 12 mgdL). Moreover, the increased levels of other insulin-sensitive fuels induced by growth hormone fell to normal following the meal. In a second study, six patients received a low-dose insulin clamp (designed to reproduce the mean postprandial concentrations of glucose and insulin observed in the meal study) before and after growth hormone administration. Despite endogenous glucose overproduction after growth hormone, modest elevations in free insulin (40 to 50 microU/mL) were sufficient to suppress glucose production to an extent comparable to the control day (from 2.8 +/- 0.2 to 0.6 +/- 0.3 mg/kg min after growth hormone v 1.6 +/- 0.1 to 0.4 +/- 0.2 mg/kg min on the control day). However, the normal stimulation of glucose uptake by insulin was abolished by growth hormone. We conclude that in diabetic patients growth hormone-stimulated hepatic glucose overproduction (and the increases in other insulin-sensitive fuels) can be relatively easily overcome with extra insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretory patterns of growth hormone during basal periods in man

Human growth hormone (hGH) concentrations in plasma often fall to levels not detectable by RIA. These so-called basal levels prevail during the greater part of the day. Since hGH is involved in the homeostasis of several metabolic processes, it is important to examine its exact plasma concentration and secretory pattern during basal periods. We used an immunoadsorbent technique to extract hGH from large plasma samples to precisely measure basal hGH concentrations and their variation with time. Blood samples (20 mL) were drawn from 12 normal subjects in the fasted and rested state every 15 minutes over a three-hour period. Plasma hGH levels varied over three orders of magnitude (range, 34 to 60,000 pg/mL). During basal periods, episodes of secretory pulses, of moderate sustained secretion, and of complete secretory inactivity occurred. Women had significantly higher overall hGH levels as well as basal hGH levels than men, but no significant sex difference in the pulse frequency during basal periods could be detected in the limited time allotted for study. No convincing relationship was noted between variations in plasma glucose and the secretory pattern of hGH, or vice versa. We conclude that hGH is secreted in an episodic fashion during basal periods. Conceptually, basal and stimulated hGH secretion may be viewed as extremes of a continuous spectrum of pituitary activity, basal hGH levels are lower than heretofore appreciated, the known tendency of women to higher hGH levels is also evident in the basal range, and oscillations in plasma glucose do not affect the microsecretory pattern of hGH, nor are endogenous hGH pulses followed by acute changes in glycemia.

Development of a salmon growth hormone radioimmunoassay

This study describes the development of a growth hormone (GH) radioimmunoassay (RIA) using chum salmon (Oncorhynchus keta) GH and an antiserum raised against this preparation. The assay does not cross-react with salmon prolactin and is valid for the genera Salmo and Oncorhynchus. Hypophysectomy of coho salmon (O. kisutch) reduced plasma immunoreactivity to nondetectable levels in seven of eight individuals. Handling stress had no effect upon GH levels in the rainbow trout (Salmo gairdneri) whereas starvation (3 weeks) induced a ninefold increase in plasma immunoreactivity. Plasma GH levels in trout were positively correlated, following a lag phase of 1 week, with the weekly changes in growth rate displayed by this species.

Pathogenesis of the dawn phenomenon in patients with insulin-dependent diabetes mellitus. Accelerated glucose production and impaired glucose utilization due to nocturnal surges in growth hormone secretion

The early-morning increase in insulin requirements of patients with insulin-dependent diabetes mellitus (IDDM) has been referred to as the "dawn phenomenon." To determine the roles of growth hormone levels and sympathoadrenal activity in this phenomenon, we studied six subjects with IDDM on four occasions during a constant overnight infusion of insulin. In control experiments (infusion of insulin alone), plasma glucose increased from 98 +/- 5 mg per deciliter at midnight to 225 +/- 36 at 8:00 a.m. (P less than 0.001), glucose production increased by 65 per cent (P less than 0.001), and glucose clearance decreased by 50 per cent (P less than 0.001). When nocturnal surges in growth hormone secretion were prevented by infusion of somatostatin plus replacement glucagon, neither plasma glucose levels nor glucose production increased significantly, and glucose clearance did not decrease. When nocturnal surges in growth hormone secretion were simulated by hourly intravenous injections of growth hormone (15 to 100 micrograms) during infusion of somatostatin and glucagon, plasma glucose levels and glucose production increased and glucose clearance decreased to values observed in control experiments. During combined alpha- and beta-adrenergic blockade (phentolamine and propranolol), values for plasma glucose, glucose production, and glucose utilization were not significantly different from those in control experiments. Increases in plasma glucose were significantly correlated with peak plasma growth hormone concentrations (r = 0.58, P less than 0.01). We conclude that nocturnal surges in growth hormone secretion are primarily responsible for the dawn phenomenon in patients with IDDM.