Glucose and fat metabolism in adults with growth hormone deficiency

Influence of glucocorticoids and growth hormone on insulin sensitivity in humans

The seminal concept proposed by Sir Harold Himsworth more than 75 years ago that a large number of patients with diabetes were 'insulin insensitive', now termed insulin resistance, has now expanded to include several endocrine syndromes, namely those of glucocorticoid excess, and growth hormone excess and deficiency. Synthetic glucocorticoids are increasingly used to treat a wide variety of chronic diseases, whereas the beneficial effects of recombinant growth hormone replacement therapy in children and adults with growth hormone deficiency have now been well-recognized for over 25 years. However, clinical and experimental studies have established that increased circulating levels of glucocorticoids and growth hormone can also lead to worsening of insulin resistance, glucose intolerance, overt diabetes mellitus and cardiovascular disease. Improved understanding of the physiological 24-h rhythmicity of glucocorticoid and growth hormone secretion and its influence on the dawn phenomenon and the Staub-Trauggot effect has therefore led to renewed interest in studies on the mechanisms of insulin resistance induced by exogenous administration of glucocorticoids and growth hormone in humans. In this review, we describe the physiological events that result from the presence of resistance to insulin action at the level of skeletal muscle, adipose tissue, and liver, describe the known mechanisms of glucocorticoid- and growth hormone-mediated insulin resistance, and provide an update of the contributions of glucocorticoids and growth hormone to understanding the pathophysiology of insulin resistance and its effects on several endocrine syndromes.

Hormone replacement therapy and physical function in healthy older men. Time to talk hormones?

Improving physical function and mobility in a continuously expanding elderly population emerges as a high priority of medicine today. Muscle mass, strength/power, and maximal exercise capacity are major determinants of physical function, and all decline with aging. This contributes to the incidence of frailty and disability observed in older men. Furthermore, it facilitates the accumulation of body fat and development of insulin resistance. Muscle adaptation to exercise is strongly influenced by anabolic endocrine hormones and local load-sensitive autocrine/paracrine growth factors. GH, IGF-I, and testosterone (T) are directly involved in muscle adaptation to exercise because they promote muscle protein synthesis, whereas T and locally expressed IGF-I have been reported to activate muscle stem cells. Although exercise programs improve physical function, in the long-term most older men fail to comply. The GH/IGF-I axis and T levels decline markedly with aging, whereas accumulating evidence supports their indispensable role in maintaining physical function integrity. Several studies have reported that the administration of T improves lean body mass and maximal voluntary strength in healthy older men. On the other hand, most studies have shown that administration of GH alone failed to improve muscle strength despite amelioration of the detrimental somatic changes of aging. Both GH and T are anabolic agents that promote muscle protein synthesis and hypertrophy but work through separate mechanisms, and the combined administration of GH and T, albeit in only a few studies, has resulted in greater efficacy than either hormone alone. Although it is clear that this combined approach is effective, this review concludes that further studies are needed to assess the long-term efficacy and safety of combined hormone replacement therapy in older men before the medical rationale of prescribing hormone replacement therapy for combating the sarcopenia of aging can be established.

Effect of sodium oxybate on growth hormone secretion in narcolepsy patients and healthy controls

Hypocretin deficiency causes narcolepsy and may affect neuroendocrine systems and body composition. Additionally, growth hormone (GH) alterations my influence weight in narcolepsy. Symptoms can be treated effectively with sodium oxybate (SXB; γ-hydroxybutyrate) in many patients. This study compared growth hormone secretion in patients and matched controls and established the effect of SXB administration on GH and sleep in both groups. Eight male hypocretin-deficient patients with narcolepsy and cataplexy and eight controls matched for sex, age, BMI, waist-to-hip ratio, and fat percentage were enrolled. Blood was sampled before and on the 5th day of SXB administration. SXB was taken two times 3 g/night for 5 consecutive nights. Both groups underwent 24-h blood sampling at 10-min intervals for measurement of GH concentrations. The GH concentration time series were analyzed with AutoDecon and approximate entropy (ApEn). Basal and pulsatile GH secretion, pulse regularity, and frequency, as well as ApEn values, were similar in patients and controls. Administration of SXB caused a significant increase in total 24-h GH secretion rate in narcolepsy patients, but not in controls. After SXB, slow-wave sleep (SWS) and, importantly, the cross-correlation between GH levels and SWS more than doubled in both groups. In conclusion, SXB leads to a consistent increase in nocturnal GH secretion and strengthens the temporal relation between GH secretion and SWS. These data suggest that SXB may alter somatotropic tone in addition to its consolidating effect on nighttime sleep in narcolepsy. This could explain the suggested nonsleep effects of SXB, including body weight reduction.

Central ghrelin regulates peripheral lipid metabolism in a growth hormone-independent fashion

GH plays a major role in the regulation of lipid metabolism and alterations in GH axis elicit major changes in fat distribution and mobilization. For example, in patients with GH deficiency (GHD) or in mice lacking the GH receptor, the percentage of fat is increased. In addition to the direct actions of GH on lipid metabolism, current evidence indicates that ghrelin, a stomach-derived peptide hormone with potent GH secretagogue action, increases lipogenesis in white adipose tissue (WAT) through a hypothalamic-mediated mechanism. Still, the mechanism by which GH tone modulates ghrelin actions on WAT remains unclear. Here we investigated the effect of central ghrelin administration on lipid metabolism in lipogenic tissues (liver and WAT) in the absence of GH, by using a model for the study of GHD, namely the spontaneous dwarf rat, which shows increased body fat. Our data demonstrate that central chronic ghrelin administration regulates adipose lipid metabolism, mainly in a GH-independent fashion, as a result of increased mRNA, protein expression, and activity levels of fatty acid metabolism enzymes. On the contrary, central ghrelin regulates hepatic lipogenesis de novo in a GH-independent fashion but lipid mobilization in a GH-dependent fashion because carnitine palmitoyltransferase 1 was decreased only in wild-type Lewis rats. These findings suggest the existence of a new central nervous system-based neuroendocrine circuit, regulating metabolic homeostasis of adipose tissue. Understanding the molecular mechanism underlying the interplay between GH and ghrelin and their effects on lipid metabolism will provide new strategies for the design and development of suitable drugs for the treatment of GHD, obesity, and its comorbidities.

Insulin sensitivity is impaired in adults with varying degrees of GH deficiency

BACKGROUND

Severe GH deficiency (GHD) is arbitrarily defined by a peak GH (pGH) response to provocative tests of less than 3 microg/l. The impact of lesser degrees of GHD (pGH of 3-7 microg/l) in the adult is less well defined. Hypopituitary adults with severe GHD are insulin resistant as defined by the short insulin tolerance test (ITT), homeostatic model assessment (HOMA) or the hyperinsulinaemic euglycaemic clamp. Whether insulin action is impaired in the presence of lesser degrees of GHD (GH insufficiency, GHI) is not known.

PATIENTS AND METHODS

We studied 30 patients with GHD (median pGH 0.5 microg/l; 0.3-1.3 microg/l), 24 with GHI (median pGH 4.0 microg/l; 3.6-4.9 microg/l), and 30 age-, sex- and body mass index (BMI)-matched controls. A short ITT was performed in a subset of 20 patients from each group and the rate constant for glucose disappearance (K(ITT)) calculated.

RESULTS

Both GHD and GHI hypopituitary adults were found to be insulin resistant from the K(ITT) values compared with the control group [analysis of variance (anova) on ranks, -0.0210 vs. -0.0223 vs. -0.0261, P < 0.05]. No difference in K(ITT) values was detected between the GHD and GHI groups. Calculation of insulin resistance using the HOMA did not detect a significant difference between groups (anova on ranks, 2.04 vs. 1.54 vs. 1.34, P = 0.40). No significant difference in the fasting glucose, insulin or IGFBP-I levels between groups was detected. Insulin resistance estimated from the K(ITT) correlated with the serum IGF-I (r = -0.30, P = 0.01), and pGH level to the ITT (r = -0.31, P = 0.03). K(ITT) also correlated with total body percentage fat mass (r = -0.28, P = 0.03). Multivariate analysis found K(ITT) to be dependent on percentage fat mass, gender and age (r = 0.53, P = 0.0002). Insulin resistance increased in concert with percentage fat mass, age and male gender.

CONCLUSIONS

Although not detected under basal conditions, hypopituitary adults with both severe GHD and GHI are insulin resistant under conditions of insulin stimulation. This finding may be explained by the concomitant adverse changes in body composition observed in both these states of varying degrees of GHD. Insulin resistance is associated with a number of adverse cardiovascular risk factors that may place patients with GHI at risk of premature cardiovascular disease.

A rare case of adulthood-onset growth hormone deficiency presenting as sporadic, symptomatic hypoglycemia

Symptomatic hypoglycemia is described in children with severe GH deficiency (GHD), but is rare in adults with GHD. We describe the case of a 62- yr-old man, referred for recurrent hypoglycemic events. He reported a previous head trauma at the age of 20 yr and a diagnosis of reactive hypoglycemia at the age of 50 yr. In the last months, during a period of job-related stress, the hypoglycemic episodes became more frequent and severe (glucose <2.2 mmol/l), finally requiring hospitalization. On admission, the patient was in good general health, with normal renal and hepatic function. During hospitalization, no hypoglycemic episodes were recorded, also during a 72-h fasting test. Biochemical data and abdominal computed tomography (CT) excluded insulinoma. A tumor-induced hypoglycemia was ruled out. The 4-h oral glucose tolerance test (OGTT) showed an impaired glucose tolerance with a tendency toward asymptomatic hypoglycemia. Hormonal study disclosed low levels of GH (0.2 ng/ml) and IGF-I (51 ng/ml); the response of GH to GHRH plus arginine confirmed a severe GHD (GH peak 2.7 ng/ml). Other pituitary and counterregulation hormones were within the normal range and magnetic resonance imaging (MRI) of the pituitary gland was normal. Replacement therapy with a low dose of rhGH induced an increase of IGF-I up to low-normal values, accompanied by lasting regression of hypoglycemic events. In conclusion, hypoglycemia was the main clinical symptom of isolated adult onset GHD, in the present case. The possible pathogenesis of isolated adult onset GHD and the association of GHD with conditions predisposing to hypoglycemia are considered and discussed.

Effects of GH replacement on metabolism and physical performance in GH deficient adults

GH deficiency (GHD) in adults is associated with abnormalities in body composition, metabolic derangements, sub-optimal physical performance, high incidence of adverse cardiovascular risk factors and poor quality of life. GHD adults are insulin resistant and have reduced hepatic glycogen stores, reduced insulin stimulated glucose utilization and reduced glycogen synthesis in muscle. GH replacement results in either no change or slight reduction in insulin sensitivity. Hence, it is important to monitor for the development of glucose intolerance in patients on long-term GH replacement. GHD is associated with a lipid profile known to predispose to premature atherosclerosis and cardiovascular disease, i.e. increased total and LDL cholesterol, decreased HDL cholesterol, increased small dense LDL particles and increased triglycerides. LDL-cholesterol abnormalities appear to improve with GH replacement even if maintained within physiological dose range; the greatest improvement occurs in those subjects with higher baseline total and LDL cholesterol values and in female patients with adult onset GHD compared with male patients with childhood onset GHD. In contrast, hypertriglyceridaemia is not corrected by GH replacement. The majority of the reports suggest GH replacement increases Lipoprotein-a levels. Long-term observation will be required to determine whether GH replacement reduces cardiovascular morbidity and mortality in GHD adults. The reduced muscle mass and strength associated with GHD has been shown to improve after GH replacement. GH treatment also improves maximal and sub-maximal exercise performance in GHD adults. The effects on protein metabolism, energy expenditure and thyroid metabolism in GHD adults are also critical.

The role of the growth hormone-insulin-like growth factor axis in glucose homeostasis

Homeostatic mechanisms normally maintain the plasma glucose concentration within narrow limits despite major fluctuations in supply and demand. There is increasing evidence that the growth hormone (GH)-insulin-like growth factor (IGF) axis may play an important role in glucose metabolism. GH has potent effects on intermediary metabolism, some of which antagonize the actions of insulin. In contrast, IGF-I has insulin-like actions, which are, in the case of glucose metabolism, opposite to those of GH. There is often deranged glucose metabolism in situations where GH is deficient or in excess. The clinical administration of GH or IGF-I results in altered glucose metabolism and changes in insulin resistance. Despite these observations, the precise role of GH and IGF-I and their interactions with insulin in controlling normal glucose homeostasis are unknown. In diabetes, GH secretion is abnormally increased as a result of reduced portal insulin resulting in impaired hepatic IGF-I generation. Evidence suggests that this may contribute to the development of diabetic microvascular complications. IGF-I 'replacement' in diabetes is under investigation and new methods of delivering IGF-I as a complex with IGFBP-3 offer exciting new prospects.

Avaliação do Metabolismo Glicídico e da Gordura Visceral em Adultos Deficientes de Hormônio de Crescimento

A síndrome da deficiência de hormônio de crescimento (DGH) no adulto e o conseqüente aumento no risco cardiovascular têm sido bastante estudados nos últimos anos. Com o objetivo de avaliar as alterações na composição corporal e a presença de resistência à insulina em adultos com DGH, estudamos 27 pacientes comparados a 27 indivíduos saudáveis pareados por idade, sexo e índice de massa corporal, através de tomografia computadorizada de abdômen para medida da gordura visceral e teste de tolerância oral à glicose (TTOG), com curva de glicose e insulina e estimativa da resistência à insulina pelo Homeostasis Model Assessment (HOMA). Observamos, nos pacientes, aumento do tecido adiposo visceral (p= 0,008) sem aumento da freqüência de alterações no TTOG. As glicemias e insulinemias basais e após 2 horas de sobrecarga oral de glicose e as áreas sob as curvas de glicose e insulina foram semelhantes ao grupo controle (p> 0,05). Não houve diferença na sensibilidade à insulina pelo método HOMA (p= 0,989). Houve correlação positiva significativa da medida de gordura visceral nos pacientes com as dosagens de glicemia (r= 0,583; p= 0,001) e insulina (r= 0,728; p= 0,001) após sobrecarga de glicose e as áreas sob a curvas de glicose (r= 0,403; p= 0,040) e insulina (r= 0,713; p= 0,001), porém sem correlação significativa nos controles (r< 0,40; p> 0,05). Em conclusão, não houve alteração significativa no metabolismo glicídico, apesar do aumento da adiposidade visceral observada em adultos DGH.

Effects of GH on protein metabolism during dietary restriction in man

The metabolic response to dietary restriction involves a series of hormonal and metabolic adaptations leading to protein conservation. An increase in the serum level of growth hormone (GH) during fasting has been well substantiated. GH has potent protein anabolic actions, as evidenced by a significant decrease in lean body mass and muscle mass in chronic GH deficiency, and vice versa in patients with acromegaly. The present review outlines current knowledge about the role of GH in the metabolic response to fasting, with particular reference to the effects on protein metabolism. Physiological bursts of GH secretion seem to be of seminal importance for the regulation of protein conservation during fasting. Apart from the possible direct effects of GH on protein dynamics, a number of additional anabolic agents, such as insulin, insulin-like growth factor-I, and free fatty acids (FFAs), are activated. Taken together the effects of GH on protein metabolism seem to include both stimulation of protein synthesis and inhibition of breakdown, depending on the nature of GH administration, which tissues are being studied, and on the physiological conditions of the subjects.

Individualized low-dose growth hormone (GH) treatment in GH-deficient adults with childhood-onset disease: metabolic effects during fasting and hypoglycemia

Growth hormone (GH) has insulin-antagonistic effects, and GH secretion is augmented during fasting and hypoglycemia. In the present study, 10 patients aged 21 to 28 years with childhood-onset GH deficiency (GHD) were studied during a 24-hour fast and a hypoglycemic glucose clamp before and after 9 months of GH replacement. During the 24-hour fast, blood glucose, serum insulin, and serum free fatty acid (FFA) levels were measured. In the hypoglycemic clamp, the counterregulatory hormones (plasma catecholamines, serum glucagon, and serum cortisol), serum insulin-like growth factor (IGF) binding protein-1 (IGFBP-1), serum FFA, and glucose uptake were measured. The GH dose was adjusted to the response of serum IGF-I, and the median GH dose was 0.14 IU/kg/wk (range, 0.08 to 0.19). At the end of the study, serum IGF-I levels were normalized in all but one patient, in whom serum IGF-I was above the normal range. Nine months of GH treatment did not cause any significant changes in the blood glucose level, insulin to glucose ratio, or serum FFA level during the 24-hour fast, and none of the patients experienced hypoglycemia either before or after GH treatment. However, GH therapy resulted in increased insulin resistance during hypoglycemia, without changes in the counterregulatory hormonal responses, serum IGFBP-1, or serum FFA.

Growth hormone deficiency in adults: a review

For more than 35 years, growth hormone (GH) has been used to promote linear growth in GH-deficient children. Previously, GH replacement in adults was limited to the supply of human pituitary-derived GH. In addition, until recently, GH replacement was not deemed clinically indicated. With the introduction of recombinant human prion-free GH, replacement therapy in GH-deficient adults has become feasible, and its use has burgeoned. In this review, recent studies on GH therapy in healthy and GH-deficient adults are evaluated to provide a rational basis for the widened scope of its clinical application.