Elements in the pathophysiology of diminished growth hormone (GH) secretion in aging humans

Impact of sarcopenia in trauma and surgical patient population: A literature review

Sarcopenia refers to the progressive and generalised loss of skeletal muscle mass and strength with a risk of adverse outcomes such as physical disability, poor quality of life and mortality. The present review explored the impact, diagnosis, prevention and management of sarcopenia in surgical and trauma patients. Recent evidence suggests that there are many factors contributing to its development other than age. Pathophysiology of sarcopenia is complex which makes its diagnosis difficult and there is no universal approach. It is a predictor of poor outcomes including post-operative complications, length of hospital stay and mortality in trauma and surgery patients. Sedentary lifestyle, age-dependent hormone and cytokine imbalance, decreased protein synthesis and regeneration, and motor unit remodelling are some of the main risk factors for sarcopenia. Dual energy X-ray absorptiometry, bio-electrical impedance analysis and computed tomography are frequently utilized for its diagnosis. Management of sarcopenia involves appropriate management of comorbidities, interventions to facilitate physical activities, nutrition interventions and pharmacotherapy.

Acromegaly and aging: a comparative cross-sectional study

OBJECTIVE

Cognitive and functional geriatric assessment may change in acromegaly. Herein we aimed to determine at which points geriatric assessment of the cases with acromegaly differs from that of general elderly population.

DESIGN

In this comparative cross-sectional study, a total of 30 cases with acromegaly (controlled n = 14, uncontrolled n = 16) and 30 gender and body-mass index-matched cases without acromegaly (control group, CG) above 60 years old were included. Cognitive functions were evaluated on the basis of the mini-mental state exam (MMSE). Affective status was determined using the geriatric depression scale. Activities of daily living (ADL) were ranked according to the Barthel index while instrumental activities of daily living (IADL) were graded on the basis of the Lawton scale. Nutritional status was evaluated using the mini-nutritional assessment (MNA). Body composition was measured through bioimpedance analysis. Functional mobility was determined using the Timed Up and Go test (TUG) and muscle strength with the handgrip strength test.

RESULTS

Scores on the MMSE were significantly lower in the elderly cases with acromegaly than in the cases without acromegaly (p < 0.001). Dementia was more frequent in the acromegaly group than in the CG (p = 0.04). Total MNA scores were significantly lower in cases with acromegaly than in the CG (p = 0.006). More subjects in the acromegaly group (33%) were at greater risk of malnutrition than in the CG (3%) (p = 0.003). There was greater moderate functional impairment based on Barthel ADL in the acromegaly group than in the CG (p = 0.04).

CONCLUSION

Acromegaly may impair cognitive functions, functional mobility and instrumental daily living activities in the geriatric population. With acromegaly, the risk of malnutrition may also increase.

[Cross-sectional and longitudinal associations of physical performance and skeletal muscle mass with cognition and cognitive decline among community-dwelling older Japanese]

AIM

This study had two objectives: to confirm the cross-sectional associations between the physical performance scale (PPS), skeletal muscle mass (SMM) and sarcopenia and cognition, and to examine whether PPS, SMM and sarcopenia are independent risk markers of cognitive decline (CD).

METHODS

Among 805 adults 65 years of age or older who participated in a baseline survey (2008-2011), 649 were reevaluated for their level of cognition at least once until 2012, and PPS, SMM, sarcopenia and cognition were assessed. CD was defined as a decrease of at least 0.5 points per year on the MMSE.

RESULTS

After controlling for sex, age and years of education, PPS, SMM and sarcopenia were associated with cognition, respectively. During a median follow-up of 3.0 years, 201 adults (31.0%) had CD. After controlling for important confounders, PPS showed a significant association with CD [OR=0.75 (95%CI: 0.65-0.87)], whereas SMM showed no significant association with CD. As compared with the people who had normal PPS and SMM, people who had low PPS and normal SMM had 2.10 times higher risk for CD.

CONCLUSIONS

We confirmed the cross-sectional associations between physical performance, SMM and sarcopenia and cognition in a general population of older adults. Although people had normal SMM, low physical performance was found to be a risk marker for subsequent CD.

Basal growth hormone concentration increased following a weight loss focused dietary intervention in older overweight and obese women

OBJECTIVES

Growth and sex steroid hormones decrease with aging and obesity. The effect of dietary weight loss and exercise training lifestyle interventions was examined on hormones as well as determining their relationships with physical function in older obese and overweight adults.

DESIGN

Individuals were randomized into one of four 18 month interventions: Healthy Lifestyle (HL), Exercise, Diet, and Exercise-Diet.

SETTING

Clinical research setting with facility based exercise and nutrition education and behavior classrooms.

PARTICIPANTS

Older (≥60 yrs) overweight and obese (BMI≥28 kg/m(2)) adults with knee osteoarthritis (n=309) were recruited for the study.

INTERVENTION

Weight loss goal for Diet groups was ≥5%. Exercise groups trained (mostly walking and resistance training) 3 days/week for 60 min/session.

MEASUREMENTS

Body weight, growth hormone (GH), corticosterone, sex-hormone binding globulin (SHBG), testosterone, and dehydroepiandrosterone (DHEA) were measured at baseline, 6, and 18 months. Physical function was determined through performance task (6-min walking distance) and self-reported questionnaires (Western Ontario McMaster University Osteoarthritis Index-WOMAC) at similar time points.

RESULTS

Diet, Exercise, and Exercise-Diet groups lost 4.9%, 3.5%, and 6.2% of their weight at 18 months, respectively. There was a significant diet treatment effect on GH levels in women as higher concentrations of this hormone were apparent following dietary weight loss intervention (p=0.01). No other hormones were affected by either diet or exercise treatments in men or women. A significant inverse correlation between baseline 6-minute walking distance and SHBG (r=-0.33) was found in men.

CONCLUSION

The increase in basal GH levels from the diet treatment in women suggests that this lifestyle behavior intervention may mitigate the age- and obesity-related decreases in growth hormone levels, to help preserve muscle mass, strength, and physical function in older adults.

Effects of long-term continuous positive airway pressure on body composition and IGF1

OBJECTIVE

To investigate the long-term effects of nasal continuous positive airway pressure (CPAP) ventilation in patients with obstructive sleep apnea syndrome (OSAS) on body composition (BC) and IGF1.

DESIGN

Observational study.

SUBJECTS

Seventy-eight (11 females and 67 males) OSAS patients who were compliant with CPAP (age 51+/-1.1 years) participated in the study. We assessed body mass index (BMI), total body mass (TBM), total body fat (TBF; kg) and lean body mass (LBM; kg), abdominal subcutaneous (SC) and visceral (V) fat (cm(2)), and waist circumference (WC; cm) by magnetic resonance imaging, and IGF1 (ng/ml) before and after 7.8+/-1.3 months of CPAP use of an average of 5.9+/-1.2 h.

RESULTS

Women had a higher BMI, WC; TBM, TBF, and more SC fat. Men had a higher LBM and more V fat. CPAP increased WC (+2.8+/-9.6 cm, P=0.02) and LBM (2.2+/-0.5 kg, P=0.006), but not IGF1. In men, CPAP increased BMI (0.5+/-0.2 kg/m(2), P=0.02), WC (1.7+/-6.9 cm, P=0.002), TBM (1.7+/-0.4 kg, P=0.0001), LBM (1.5+/-0.4 kg, P=0.0003), SC fat (12.9+/-5.1 cm(2), P=0.02), and IGF1 (13.6+/-4.2 ng/ml, P=0.002). Compliance with CPAP increased LBM in men aged <60 years, but not in those aged >60 years, and IGF1 increased in men aged 40-60 years only.

CONCLUSIONS

Long-term CPAP increased LBM in both sexes and IGF1 in men, while fat mass remained unchanged, suggesting a sexually dimorphic response of IGF1 to CPAP. The role of the GH axis activity and age to this response is unclear. The metabolic consequences of changes in LBM are still to be determined. Future studies on the effects of CPAP on BC should include LBM as an outcome.

The somatotropic axis and aging: mechanisms and persistent questions about practical implications

Reduced somatotropic (GH/IGF-1) signaling delays aging and extends longevity in laboratory mice. However, it is unclear whether the physiological decrease of GH and IGF-1 levels with age represents a symptom of declining neuroendocrine function, a cause of age-related alterations in body composition and functionality or a protective mechanism against age-associated disease. Although available clinical evidence does not support the use of recombinant GH as an anti-aging therapy, many studies suggest the potential utility of GH and GH secretagogues in the treatment of sarcopenia and frailty.

Role of the GH/IGF-1 axis in lifespan and healthspan: lessons from animal models

Animal models are fundamentally important in our quest to understand the genetic, epigenetic, and environmental factors that contribute to human aging. In comparison to humans, relatively short-lived mammals are useful models as they allow for rapid assessment of both genetic manipulation and environmental intervention as related to longevity. These models also allow for the study of clinically relevant pathologies as a function of aging. Data associated with more distant species offers additional insight and critical consideration of the basic physiological processes and molecular mechanisms that influence lifespan. Consistently, two interventions, caloric restriction and repression of the growth hormone (GH)/insulin-like growth factor-1/insulin axis, have been shown to increase lifespan in both invertebrates and vertebrate animal model systems. Caloric restriction (CR) is a nutrition intervention that robustly extends lifespan whether it is started early or later in life. Likewise, genes involved in the GH/IGF-1 signaling pathways can lengthen lifespan in vertebrates and invertebrates, implying evolutionary conservation of the molecular mechanisms. Specifically, insulin and insulin-like growth factor-1 (IGF-1)-like signaling and its downstream intracellular signaling molecules have been shown to be associated with lifespan in fruit flies and nematodes. More recently, mammalian models with reduced growth hormone (GH) and/or IGF-1 signaling have also been shown to have extended lifespans as compared to control siblings. Importantly, this research has also shown that these genetic alterations can keep the animals healthy and disease-free for longer periods and can alleviate specific age-related pathologies similar to what is observed for CR individuals. Thus, these mutations may not only extend lifespan but may also improve healthspan, the general health and quality of life of an organism as it ages. In this review, we will provide an overview of how the manipulation of the GH/IGF axis influences lifespan, highlight the invertebrate and vertebrate animal models with altered lifespan due to modifications to the GH/IGF-1 signaling cascade or homologous pathways, and discuss the basic phenotypic characteristics and healthspan of these models.

Plasmid growth hormone releasing hormone therapy in healthy and laminitis-afflicted horses-evaluation and pilot study

BACKGROUND

In vivo electroporation dramatically improves the potency of plasmid-mediated therapies, including in large animal models. Laminitis and arthritis are common and debilitating diseases in the horse, as well as humans.

METHODS

The effects of growth hormone releasing hormone (GHRH) on healthy horses and on horses with laminitis that were followed for 6 months after a single intramuscular injection and electroporation of 2.5 mg of an optimized myogenic GHRH-expressing plasmid were examined.

RESULTS

In the first study on six healthy horses, we observed a significant increase in body mass by day 180 compared to baseline (P < 0.003), and an increase in erythrocyte production (hematocrit, red blood cells, hemoglobin, P = 0.03). IGF-I levels were increased by 7% by day 120 (P = 0.02). A pilot study was performed on two horses with chronic laminitis, a vascular condition often associated with arthritis, with two horses with similar clinical disease serving as non-treated controls. Treated horses experienced an increase in weight compared to control horses that received standard care (P = 0.007). By 6 months post-treatment, treated subjects were rated pasture sound. Physical and radiographic evaluation demonstrated significant improvement with reduced inflammation and decreased lameness.

CONCLUSIONS

These results demonstrate that a plasmid therapy delivered by electroporation can potentially be used to treat chronic conditions in horses, and possibly other very large mammals. While further studies are needed, overall this proof-of-concept work presents encouraging data for studying gene therapeutic treatments for Raynaud's syndrome and arthritis in humans.

Cellular and molecular mechanisms underlying age-related skeletal muscle wasting and weakness

Some of the most serious consequences of ageing are its effects on skeletal muscle. The term 'sarcopenia' describes the slow but progressive loss of muscle mass with advancing age and is characterised by a deterioration of muscle quantity and quality leading to a gradual slowing of movement and a decline in strength. The loss of muscle mass and strength is thought to be attributed to the progressive atrophy and loss of individual muscle fibres associated with the loss of motor units, and a concomitant reduction in muscle 'quality' due to the infiltration of fat and other non-contractile material. These age-related changes in skeletal muscle can be largely attributed to the complex interaction of factors affecting neuromuscular transmission, muscle architecture, fibre composition, excitation-contraction coupling, and metabolism. Given the magnitude of the growing public health problems associated with sarcopenia, there is considerable interest in the development and evaluation of therapeutic strategies to attenuate, prevent, or ultimately reverse age-related muscle wasting and weakness. The aim is to review our current understanding of some of the cellular and molecular mechanisms responsible for age-related changes in skeletal muscle.

Insights into a role of GH secretagogues in reversing the age-related decline in the GH/IGF-I axis

Growth hormone (GH) secretion and serum insulin-like growth factor-I (IGF-I) decline with aging. This study addresses the role played by the hypothalamic regulators in the aging GH decline and investigates the mechanisms through which growth hormone secretagogues (GHS) activate GH secretion in the aging rats. Two groups of male Wistar rats were studied: young-adult (3 mo) and old (24 mo). Hypothalamic growth hormone-releasing hormone (GHRH) mRNA and immunoreactive (IR) GHRH dramatically decreased (P < 0.01 and P < 0.001) in the old rats, as did median eminence IR-GHRH. Decreases of hypothalamic IR-somatostatin (SS; P < 0.001) and SS mRNA (P < 0.01), and median eminence IR-SS were found in old rats as were GHS receptor and IGF-I mRNA (P < 0.01 and P < 0.05). Hypothalamic IGF-I receptor mRNA and protein were unmodified. Both young and old pituitary cells, cultured alone or cocultured with fetal hypothalamic cells, responded to ghrelin. Only in the presence of fetal hypothalamic cells did ghrelin elevate the age-related decrease of GH secretion to within normal adult range. In old rats, growth hormone-releasing peptide-6 returned the levels of GH and IGF-I secretion and liver IGF-I mRNA, and partially restored the lower pituitary IR-GH and GH mRNA levels to those of young untreated rats. These results suggest that the aging GH decline may result from decreased GHRH function rather than from increased SS action. The reduction of hypothalamic GHS-R gene expression might impair the action of ghrelin on GH release. The role of IGF-I is not altered. The aging GH/IGF-I axis decline could be rejuvenated by GHS treatment.

Pituitary alterations involved in the decline of growth hormone gene expression in the pituitary of aging rats

Growth hormone (GH) declines during aging. This study investigates whether pituitary constitutive alterations may be involved in the GH decline. Two groups of male Wistar rats were studied (young: 3-month-old; old: 24-month-old). The old rats showed lower pituitary GH messenger RNA (mRNA) levels, immunoreactive rat (IR)-GH content, and GH secretion with no difference in pituitary Pit-1 and cAMP-response element-binding protein (CREB) expression. Pituitary GH releasing hormone receptor (GHRH-R), GH secretagogue receptor (GHS-R), sstr2, and sstr5 mRNA levels were significantly reduced in old rats. The percentage of GH immunoreactive cells was similar in both groups. In vitro, pituitary IR-GH response to GHRH, forskolin (FK), ghrelin, and insulin-like growth factor I (IGF-I) was similar when compared with respective basal secretion and somatostatin-diminished GHRH- and ghrelin-induced IR-GH release in both groups. These results indicate that, as somatotrope function is maintained in aging, the changes observed in GH gene expression and secretion could be reversed by GHS.

Benefits of physical training in fibromyalgia and related syndromes

OBJECTIVE

To review the published information on physical training for fibromyalgia (FM) and related syndromes.

METHODS

A search of Medline literature (via Ovid and PubMed) with the following keywords: FM, chronic fatigue syndrome, therapy, rehabilitation, aerobic, exercise, and cognitive behavioral therapy. The reference lists of articles were examined for additional related articles.

RESULTS

Several studies investigated the benefits of graded exercise therapy for patients with FM or related syndromes. Although some systematic reviews have not established an unequivocal benefit of physical training, most authors report a benefit for patients with chronic pain or fatigue. Ideally, such a therapy should be a part of multidisciplinary program. Muscular rehabilitation is reserved for preventing the deconditioning syndrome often reported in patients and the vicious cycle of pain, avoidance and inactivity behaviors, or even kinesiophobia, deconditioning, incapacity and psychological distress.

CONCLUSION

This review emphasizes the relevance of graded physical training for treating FM and related syndromes. The development of rehabilitation centers, with experts able to propose a relevant therapy to patients with chronic pain or fatigue, should help alleviate this public health problem.

Growth Hormone Responses to Low-Dose Physostigmine in Elderly vs. Young Women and Men

BACKGROUND

Growth hormone (GH) secretion is a sensitive measure of CNS cholinergic neurotransmission, and GH decreases considerably with age. Cholinesterase inhibitors, which increase acetylcholine concentrations, have been used in elderly subjects to investigate the neuroendocrine effects of aging and Alzheimer's disease. However, there have been only a few studies of a potential sex difference in GH responses to cholinesterase inhibitors in elderly subjects, with mixed results.

OBJECTIVE

We therefore administered low-dose physostigmine (PHYSO), a cholinesterase inhibitor, to normal, non-hormone-replaced, elderly women and men, to ascertain a potential sex difference in GH response. We hypothesized: (1) elderly women and men would have similar hormone responses, because of relatively low circulating estrogen in the women, and (2) the elderly women would have significantly lower baseline GH and GH responses to cholinergic challenge than the young women we studied previously.

METHODS

Normal elderly women and men > or =65 years of age meeting stringent inclusion and exclusion criteria were studied on three test days, 4-7 days apart, by serial blood sampling for several hours for baseline GH, followed by administration of low-dose PHYSO (first and third days) or saline (second day) at 18:00 h. Frequent blood sampling was continued for several hours. Plasma GH and hypothalamo-pituitary-adrenal cortical hormones were measured in each sample.

RESULTS

PHYSO administration produced no side effects in about half the elderly subjects and mild side effects in the other half, with no significant female-male differences and no significant relationship between the presence or absence of side effects and GH response. PHYSO significantly increased GH compared to saline, to a similar degree in the elderly women and men. The elderly women had a significantly greater GH response to PHYSO than did the young women, whereas GH responses were similar in the elderly and young men.

CONCLUSIONS

These results indicate similar GH responses to low-dose PHYSO in elderly women compared to elderly men, and a significantly greater GH response in elderly women compared to young women. A likely mechanism is increased sensitivity of central cholinergic systems that inhibit somatostatin and/or enhance GHRH release from the hypothalamus.

Effects of aging on muscle fibre type and size

Aging has been associated with a loss of muscle mass that is referred to as 'sarcopenia'. This decrease in muscle tissue begins around the age of 50 years, but becomes more dramatic beyond the 60th year of life. Loss of muscle mass among the aged directly results in diminished muscle function. Decreased strength and power contribute to the high incidence of accidental falls observed among the elderly and can compromise quality of life. Moreover, sarcopenia has been linked to several chronic afflictions that are common among the aged, including osteoporosis, insulin resistance and arthritis. Loss of muscle fibre number is the principal cause of sarcopenia, although fibre atrophy--particularly among type II fibres--is also involved. Several physiological mechanisms have been implicated in the development of sarcopenia. Denervation results in the loss of motor units and thus, muscle fibres. A decrease in the production of anabolic hormones such as testosterone, growth hormone and insulin-like growth factor-1 impairs the capacity of skeletal muscle to incorporate amino acids and synthesise proteins. An increase in the release of catabolic agents, specifically interleukin-6, amplifies the rate of muscle wasting among the elderly. Given the demographic trends evident in most western societies, i.e. increased number of those considered aged, management interventions for sarcopenia must become a major goal of the healthcare profession.

Metabolism, pharmacokinetics, tissue distribution, and excretion of [14C]CP-424391 in rats

CP-424391, 2-amino-N-[3aR-benzyl-2-methyl-3-oxo-2,3,3a,4,6,7-hexahydro-pyrazolo[4,3-c]pyridin-5-yl)-1R-benzyloxymethyl-2-oxoethyl]-isobutyramide, is an orally active growth hormone secretagogue currently being developed. In this study, we investigated the metabolic fate and disposition of radiolabeled CP-424391 in rats. Following 15 mg/kg single oral administration to Sprague-Dawley rats, 91% of the radiolabeled dose was recovered. Feces was the major route of excretion: 77% of the dose recovered in feces of the female rat and 84% in the male. Excretion in the urine was 15% in the female rat compared with 7% in the male. Both fecal and urinary metabolic profiles were consistent in both genders. The metabolic pathways of CP-424391 were oxidation at the benzyl group of the O-benzylserine moiety, N-demethylation of pyrazolidine, and/or O-debenzylation. In circulation, CP-424391 was absorbed within the first hour to an average apparent C(max) of 1.44 microg/ml. CP-424391 accounts for about 40% of radioactivity area under the plasma concentration-time curve and C(max) in circulation. The plasma terminal elimination half-life of CP-424391 was 2.4 h and for total radioactivity was 2.8 h. The radioactivity was widely distributed in all tissues except for the central nervous system. [(14)C]CP-424391 radioactivity was eliminated from most tissues by 9 h with the exception of liver, skin, and uvea. By 168 h, [(14)C]CP-424391 radioactivity remained localized only in the uvea.

Alterations in stomach ghrelin production and in ghrelin-induced growth hormone secretion in the aged rat

Ghrelin is a recently discovered stomach hormone that stimulates growth hormone (GH) secretion, food intake, adiposity, and growth. In this report, we present new and previously published data from our laboratory showing that stomach ghrelin production and secretion are increased, and that GH release in response to exogenous ghrelin is enhanced in the aged rat. Together, these data suggest that the aging associated decline in GH secretion is not due to a reduction in stomach ghrelin secretion or a stimulatory action on GH release.

Sustained elevation of pulsatile growth hormone (GH) secretion and insulin-like growth factor I (IGF-I), IGF-binding protein-3 (IGFBP-3), and IGFBP-5 concentrations during 30-day continuous subcutaneous infusion of GH-releasing peptide-2 in older men and women

We test the interlinked hypotheses that in healthy older adults: 1). i.v. injection of GH-releasing peptide-2 (GHRP-2) and GHRH synergizes more in aging women than men; 2). sc infusion of both GHRP-2 (1 microg/kg.h = 1) and GHRH (1, 3, or 10) for 24 h augments GH secretion more than either agonist alone; and 3). continuous sc delivery of GHRP-2 (1) for 30 d stimulates daily GH secretion and IGF-I, IGF-binding protein-3 (IGFBP-3), and IGFBP-5. Acute two-peptide synergy was 3-fold greater in young (n = 16) than older volunteers (n = 17; P < 0.025) and was 2.3-fold higher in elderly women than men (P < 0.025). The 24-h infusion of GHRP-2 (1) combined with GHRH (3 or 10) in men and with GHRH (10) in women drove GH secretion more than GHRH alone (P <or= 0.024). In the entire cohort (n = 11), GHRP-2/GHRH (1/10) stimulated GH secretion more than either GHRP-2 (1; P = 0.021) or GHRH (10; P = 0.012). The 30-d delivery of GHRP-2 (1; n = 17 subjects): 1). stimulated pulsatile, rhythmic, and entropic GH secretion by more than 3-fold on d 1 and more than 1.8-fold on d 14 and 30 (each P < 0.001 vs. saline); 2). elevated IGF-I to a stable plateau on d 1, 14, and 30 (P < 0.025 vs. baseline); and 3). increased IGFBP-3 (P < 0.01) and IGFBP-5 (P < 0.025) on d 14 and/or 30. Safety screening tests remained normal. In summary, in healthy elderly women and men: 1). acute synergy of GHRP-2 and GHRH is greater in the female; 2). 24-h combined GHRP-2 and GHRH drive is more effective than either agonist alone; and 3). 30-d stimulation with GHRP-2 sustains a physiologically activated somatotropic axis. We conclude that age, gender, stimulus duration, and secretagogue combination determine acute, intermediate, and extended responses of the somatotropic axis in the older adult.

Sex-steroid modulation of growth hormone (GH) secretory control: three-peptide ensemble regulation under dual feedback restraint by GH and IGF-I

Technical, genetic, and clinical developments have unveiled a burgeoning array of novel effectors of GH secretion. The present appraisal of central neuroregulatory components of the somatotropic axis highlights a simplifying concept of ensemble control by the final common peptides, GH-releasing hormone (GHRH), GH-releasing peptide(s) (GHRP, ghrelin), and somatostatin. These potent signals act individually, antagonistically, and synergistically to direct pulsatile GH secretion. GHRH, GHRP/ghrelin, and somatostatin further adapt to autonegative feedback by GH and IGF-I. Estradiol modulates the impact of each of the primary peptidyl inputs; viz.: (i) enhances submaximally effective feedforward by discrete pulses of (injected) recombinant human GHRH-1,44-amide (as defined by increased agonistic potency and pituitary sensitivity); (ii) potentiates the submaximally stimulatory effects of GHRP-2, a hexapeptidyl mimetic of ghrelin; (iii) blunts dose-dependent inhibition of fasting GH secretion by somatostatin- 14; and (iv) relieves rhGH-enforced negative feedback on GHRP-2 (but not on basal, exercise, or GHRH)-stimulated GH secretion. The foregoing estrogenic activities collectively augment GH secretory burst mass by amplifying feedforward (via both GHRH and GHRP) and attenuating feedback (imposed by somatostatin and GH). Whether testosterone fully mimics the foregoing mechanistic actions of estradiol is not known. In conclusion, the present conceptual platform of tri-peptide-directed feedforward and GH/IGF-I-mediated feedback should aid in unraveling some of the complex regulatory dynamics targeted by sex-steroid hormones.

Human GH pulsatility: an ensemble property regulated by age and gender

Age and gender impact the full repertoire of neurohormone systems, including most prominently the somatotropic, gonadotropic and lactotropic axes. For example, daily GH production is approximately 2-fold higher in young women than men and varies by 20-fold by sexual developmental status and age. Deconvolution estimates of 24-h GH secretion rates exceed 1200 microg/m2 in adolescents and fall below 60 microg/m2 in aged individuals. The present overview highlights plausible factors driving such lifetime variations in GH availability, i.e., estrogen, aromatizable androgen, hypothalamic peptides and negative feedback by GH and IGF-I. In view of the daunting complexity of potential neuromodulatory signals, we underline the utility of conceptualizing a simplified three-peptide regulatory ensemble of GHRH, GHRP (ghrelin) and somatostatin. The foregoing signals act as individual and conjoint mediators of adaptive GH control. Regulation is enforced at 3-fold complementary time scales, which embrace pulsatile (burst-like), entropic (orderly) and 24-h rhythmic (nycthemeral) modes of GH release. This unifying platform offers a convergent perspective of multivalent control of GH outflow.

Androgen replacement therapy in the hypogonadal ageing man

In men, gonadal function is affected in a slow, progressive way as part of the normal ageing process. Recently, however, significant interest has developed on the importance of this condition, which is variously known as male climacteric, andropause or, more appropriately, androgen decline in the ageing male (ADAM). The term andropause is biologically wrong and clinically inappropriate but, it adequately conveys the concept of emotional and physical changes that, although related to ageing in general, are also associated with significant hormonal alterations. The inappropriateness of the term is based on the fact that in women, the reproductive cycle invariably ends with ovarian failure. In men, this process is not universal and when it occurs it is normally subtle in its clinical manifestations. This has led to a tendency to ignore the syndrome as an unavoidable and untreatable result of the ageing process. For the sake of simplicity and directness, this review will use the terms ADAM and andropause to denote the global hormonal alterations associated with ageing.

Effects of dietary fats on bone health in advanced age

Evidence is accumulating that dietary lipids play an important role in bone health. Most of the data supporting the effects of lipids on bones have been collected in young adult and/or developing animals. Based upon this work, mechanisms have been proposed to explain how lipids act to enhance or inhibit bone resorption and deposition. Little work, however, has been done in older models. Since osteoporosis primarily afflicts the elderly, such work is needed in order to determine if mechanisms relevant to the young differ in advanced age, and to develop effective interventions for this especially vulnerable segment of the population. This article reviews evidence that dietary lipids are important to bone health in older individuals, and describes possible mechanisms that may be of particular relevance to the elderly. Specifically, studies supporting the influence of dietary lipids on calcium excretion, growth hormone secretion, fatty acid metabolism, and osteoblast formation are reviewed.

The insulin-like growth factor-I generation test: peripheral responsiveness to growth hormone is not decreased with ageing

OBJECTIVE

Ageing is accompanied by a reduction in GH secretion, and a decrease in circulating IGF-I. Few data are available on whether the responsiveness of IGF-I to GH stimulation changes with age.

SUBJECTS AND METHODS

Therefore we carried out multiple IGF-I generation tests in 26 healthy volunteers (16 male) of normal body mass index (BMI); nine aged 20-40 years, six aged 41-60 years, and 11 aged > 61 years. Each subject received three single doses of GH: 0.8, 2.0 and 21 IU in random order at least 4 weeks apart. Serum samples were taken 0, 18, 24, 48, 72 and 120 h following each dose of GH.

RESULTS

Basal serum levels of IGF-I (P < 0.0001) and IGFBP-3 (P < 0.01) declined with age, but serum acid-labile subunit (ALS) levels did not (P = 0.2). Peak IGF-I levels (P < 0.01 for 0.8 IU and P < 0.05 for the 2 IU dose) and area under curve (AUC) IGF-I (P < 0.01 for the 0.8 IU and 2.0 IU doses of GH and P < 0.05 for the 21 IU dose) after GH administration continued to demonstrate a significant trend towards lower values with increasing age. However, the increment in IGF-I, IGFBP-3 and ALS in response to GH did not decline with age. Indeed, the increment in IGF-I after 2 IU of GH, judged by the increase from basal to peak levels, increased with advancing age (P = 0.05), and a positive relationship was seen between the increment in the area under the IGF-I curve following the 21 IU dose of GH and age (P < 0.02).

CONCLUSION

These data illustrate that although activity of the GH/IGF-I axis declines with age, peripheral responsiveness to GH is not attenuated. This suggests that a decrease in GH responsiveness does not contribute to the age-related fall in circulating GH-dependent peptides. Thus, for those embarking on trials of GH therapy or GH secretagogues in the elderly, the capacity to generate IGF-I will not limit potential efficacy. Furthermore, the dose of GH replacement required for patients with organic GH deficiency is likely to be lower in the elderly compared with young adults.

Is there a relationship between fat-free soft tissue mass and low cognitive function? Results from a study of 7,105 women

OBJECTIVES

To test the hypothesis that low fat-free soft tissue mass and cognitive impairment are independently associated.

DESIGN

Cross-sectional study.

SETTING

Five geographic areas of France.

PARTICIPANTS

Seven thousand one hundred five community-dwelling women aged 75 and older recruited from electoral rolls between 1992 and 1994.

MEASUREMENTS

Fat-free soft tissue mass, body fat mass, and bone mineral density were measured using dual-energy x-ray absorptiometry. Study participants were assessed for cognitive impairment using the Short Portable Mental Status Questionnaire and divided into two groups according to their scores. Logistic regression models were used to calculate multivariate-adjusted differences in body composition between two groups of subjects according to their cognitive function.

RESULTS

After adjustment for confounders, compared with women in the highest quartile of fat-free soft tissue mass, women in the lowest quartile had an odds ratio of 1.43 (95% confidence interval (CI) = 1.07-1.91) for cognitive impairment. Low fat mass was also associated with lower cognitive function, with an odds ratio of 1.35 (95% CI = 1.01-1.79) for the lower quartile of fat mass compared with the highest quartile. There was no association between cognitive impairment and bone mineral density.

CONCLUSIONS

This finding supports the hypothesis that low muscle mass is associated with cognitive impairment in older women. These two components represent major causes of frailty and functional decline in older people and could have some common mechanisms. Nevertheless, these results do not predict the causal variable.

Adult growth hormone deficiency in patients with fibromyalgia

Adult growth hormone (GH) deficiency is a well-described clinical syndrome with many features reminiscent of fibromyalgia. There is evidence that GH deficiency as defined in terms of a low insulin-like growth factor-1 (IGF-1) level occurs in approximately 30% of patients with fibromyalgia and is probably the cause of some morbidity. It seems most likely that impaired GH secretion in fibromyalgia is related to a physiologic dysregulation of the hypothalamic-pituitary-adrenal axis (HPA) with a resulting increase in hypothalamic somatostatin tone. It is postulated that impaired GH secretion is secondary to chronic physical and psychological stressors. It appears that impaired GH secretion is more common than clinically significant GH deficiency with low IGF-1 levels. The severe GH deficiency that occurs in a subset of patients with fibromyalgia is of clinical relevance because it is a treatable disorder with demonstrated benefits to patients.

Evaluation and management of endocrine dysfunction in fibromyalgia

Fibromyalgia-like symptoms such as muscle pain and tenderness, exhaustion, reduced exercise capacity, and cold intolerance, resemble symptoms associated with endocrine dysfunction like hypothyroidism, and adrenal or growth hormone insufficiency. To investigate the potential of management of endocrine abnormalities for relieve of symptoms of patients with fibromyalgia, we reviewed experimental and clinical studies of endocrine functioning and endocrine treatment. Serum GH, androgen, and 24-hour urinary cortisol levels of patients with fibromyalgia tend to be in the lower part of the normal range, while serum levels of thyroid hormone, female sex hormones, prolactin, and melatonin are normal. With exception of GH, these conclusions are based on studies in small samples. With respect to dynamic responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis, the dexamethasone suppression test and stimulation with ACTH show normal results, while patients show marked ACTH hypersecretion in response to severe acute stressors, perhaps indicative of chronic CRH hyposecretion. This finding and slightly altered responsiveness of growth hormone, thyroid hormone, and prolactin in pharmacologic stimulation tests suggest a central rather than peripheral origin of endocrine deviations. Because hormone level deviations were not severe, occurred in subgroups of patients only, and few controlled clinical trials were performed, there is--unless future research shows otherwise--little support for hormone supplementation as a general therapy in the common patient with fibromyalgia. In patients with clinically overt hormone deficiency, hormonal supplementation is an option. In patients with hormone levels that are in the lower part of the normal range, interventions aimed at pain, fatigue, sleep or mood disturbance, and physical deconditioning may indirectly improve endocrine functioning.

GI side-effects of a possible therapeutic GRF analogue in monkeys are likely due to VIP receptor agonist activity

Growth hormone (GH) is used or is being evaluated for efficacy in treatment of short stature, aspects of aging, cardiac disorders, Crohn's disease, and short bowel syndrome. Therefore, we synthesized several stable growth hormone-releasing factor (GRF) analogues that could be therapeutically useful. One potent analog, [D-Ala(2),Aib(8, 18,)Ala(9, 15, 16, 22, 24-26,)Gab(27)]hGRF(1-27)NH(2) (GRF-6), with prolonged infusion caused severe diarrhea in monkeys; however, it had no side-effects in rats. Because GRF has similarity to VIP/PACAP and VIPomas cause diarrhea, this study investigated the ability of this and other GRF analogues to interact with the VIP/PACAP receptors. Rat VPAC(1)-R (rVPAC(1)-R), human VPAC(1)-R (hVPAC(1)-R), rVPAC(2)-R and hVPAC(2)-R stably transfected CHO and PANC 1 cells were made and T47D breast cancer cells containing native human VPAC(1)-R and AR4-2J cells containing PAC(1)-R were used. hGRF(1-29)NH(2) had low affinity for both rVPAC(1)-R and rVPAC(2)-R while VIP had a high affinity for both receptors. GRF-6 had a low affinity for both rVPAC(1)-R and rVPAC(2)-R and very low affinity for the rPAC(1)-R. VIP had a high affinity, whereas hGRF(1-29)NH(2) had a low affinity for both hVPAC(1)-R and hVPAC(2)-R. In contrast GRF-6, while having a low affinity for hVPAC(2)-R, had relatively higher affinity for the hVPAC(1)-R. In guinea pig pancreatic acini, all GRF analogues were full agonists at the VPAC(1)-R causing enzyme secretion. These results demonstrate that in contrast to native hGRF(1-29)NH(2,) GRF-6 has a relatively high affinity for the human VPAC(1)-R but not for the human VPAC(2)-R, rat VPAC(1)-R, rat VPAC(2)-R or rat PAC(1)-R. These results suggest that the substituted GRF analog, GRF-6, likely causes the diarrheal side-effects in monkeys by interacting with the VPAC(1)-R. Furthermore, they demonstrate significant species differences can exist for possible therapeutic peptide agonists of the VIP/PACAP/GRF receptor family and that it is essential that receptor affinity assessments be performed in human cells or from a closely related species.

Circadian and sleep disturbances in the elderly

The incidence of disturbed sleep is strongly increased in healthy and demented elderly. Age-related alterations in the circadian timing system appear to contribute strongly to these problems. With increasing age, a lack of input to the suprachiasmatic nucleus (SCN), the biological clock of the brain, may accelerate de-activation of neurons involved in the generation of 24-h rhythm or output of this rhythm. This process appears to be reversible, since supplementation of stimuli that impinge on the SCN can re-activate these neurons and ameliorate disturbances in the sleep-wake rhythm.

Circadian rhythms and sleep in human aging

This issue of Chronobiology International is dedicated to the age-related changes in circadian rhythms as they occur in humans. It seems timely to give an overview of the knowledge and hypotheses on these changes now that we enter a century in which the number and percentage of elderly in the population will be unprecedented. Although we should take care not to follow the current tendency to think of old age as a disease--ignoring the fine aspects of being old--there is definitely an age-related increase in the risk of a number of conditions that are at least uncomfortable. Circadian rhythms have been attributed adaptive values that usually go unnoticed, but can surface painfully clear when derangements occur. Alterations in the regulation of circadian rhythms are thought to contribute to the symptoms of a number of conditions for which the risk is increased in old age (e.g., sleep disturbances, dementia, and depression). A multidisciplinary approach to investigate the mechanisms of age-related changes in circadian regulation eventually may result in treatment strategies that will improve the quality of life of the growing number of elderly. Although diverse topics are addressed in this issue, the possible mechanisms by which a deranged circadian timing system may be involved in sleep disturbances receives the most attention. This seems appropriate in view of the numerous studies that have addressed this relation in the last decade and also because of the high frequency and strong impact of sleep disturbances in the elderly. This introduction to the special issue first briefly addresses the impact of disturbed sleep in the elderly to show that the development of therapeutic methods other than the currently available pharmacological treatments should be given high priority. I believe that chronobiological insights may play an important role in the development of rational therapeutical methods.

Andropause: a misnomer for a true clinical entity

PURPOSE

A progressive decrease in androgen production is common in males after middle age. The resulting clinical picture has been erroneously named male menopause or andropause. A more appropriate designation is androgen decline in the aging male (ADAM). The syndrome is characterized by alterations in the physical and intellectual domains that correlate with and can be corrected by manipulation of the androgen milieu. We review the epidemiological aspects of aging and endocrinological manifestations of ADAM, and provide recommendations for treatment and monitoring of these patients.

MATERIALS AND METHODS

We performed MEDLINE, Pubmed, Current Contents and Pharmaceutical Abstracts searches of relevant peer reviewed publications on andropause, male climacteric, adult hypogonadism and aging. In addition, conference proceedings were researched to provide a more complete review of the literature. Information was scrutinized and collated, and contributory data were reviewed and summarized.

RESULTS

ADAM is a clinical entity characterized biochemically by a decrease not only in serum androgen, but also in other hormones, such as growth hormone, melatonin and dehydroepiandrosterone. Clinical manifestations include fatigue, depression, decreased libido, erectile dysfunction, and alterations in mood and cognition.

CONCLUSIONS

The onset of ADAM is unpredictable and its manifestations are subtle and variable, which has led to a paucity of interest in its diagnosis and treatment. Urological practice commonly includes a large proportion of men older than 50 years. Therefore, it is important for urologists to recognize the manifestations of and be familiar with evaluations necessary to document ADAM as well as its treatment and monitoring.

A paradoxical gender dissociation within the growth hormone/insulin-like growth factor I axis during protracted critical illness

Female gender appears to protect against adverse outcome from prolonged critical illness, a condition characterized by blunted and disorderly GH secretion and impaired anabolism. As a sexual dimorphism in the GH secretory pattern of healthy humans and rodents determines gender differences in metabolism, we here compared GH secretion and responsiveness to GH secretagogues in male and female protracted critically ill patients. GH secretion was quantified by deconvolution analysis and approximate entropy estimates of 9-h nocturnal time series in 9 male and 9 female patients matched for age (mean +/- SD, 67+/-11 and 67+/-15 yr), body mass index, severity and duration of illness, feeding, and medication. Serum concentrations of PRL, TSH, cortisol, and sex steroids were measured concomitantly. Serum levels of GH-binding protein, insulin-like growth factor I (IGF-I), IGF-binding proteins (IGFBPs), and PRL were compared with those of 50 male and 50 female community-living control subjects matched for age and body mass index. In a second study, GH responses to GHRH (1 microg/kg), GH-releasing peptide-2 (GHRP-2; 1 microg/ kg) and GHRH plus GHRP-2 (1 and 1 microg/kg) were examined in comparable, carefully matched male (n = 15) and female (n = 15) patients. Despite identical mean serum GH concentrations, total GH output, GH half-life, and number of GH pulses, critically ill men paradoxically presented with less pulsatile (mean +/- SD pulsatile GH fraction, 39+/-14% vs. 67+/-20%; P = 0.002) and more disorderly (approximate entropy, 0.946+/-0.113 vs. 0.805+/-0.147; P = 0.02) GH secretion than women. Serum IGF-I, IGFBP-3, and acid-labile subunit (ALS) levels were low in patients compared with controls, with male patients revealing lower IGF-I (P = 0.01) and ALS (P = 0.005) concentrations than female patients. Correspondingly, circulating IGF-I and ALS levels correlated positively with pulsatile (but not with nonpulsatile) GH secretion. Circulating levels of GH-binding protein and IGFBP-1, -2, and -6 were higher in patients than controls, without a detectable gender difference. In female patients, PRL levels were 3-fold higher, and TSH and cortisol tended to be higher than levels in males. In both genders, estrogen levels were more than 3-fold higher than normal, and testosterone (2.25+/-1.94 vs. 0.97+/-0.39 nmol/L; P = 0.03) and dehydroepiandrosterone sulfate concentrations were low. In male patients, low testosterone levels were related to reduced GH pulse amplitude (r = 0.91; P = 0.0008). GH responses to GHRH were relatively low and equal in critically ill men and women (7.3+/-9.4 vs. 7.8+/-4.1 microg/L; P = 0.99). GH responses to GHRP-2 in women (93+/-38 microg/L) were supranormal and higher (P<0.0001) than those in men (28+/-16 microg/L). Combining GHRH with GHRP-2 nullified this gender difference (77+/-58 in men vs. 120+/-69 microg/L in women; P = 0.4). In conclusion, a paradoxical gender dissociation within the GH/ IGF-I axis is evident in protracted critical illness, with men showing greater loss of pulsatility and regularity within the GH secretory pattern than women (despite indistinguishable total GH output) and concomitantly lower IGF-I and ALS levels. Less endogenous GHRH action in severely ill men compared with women, possibly due to profound hypoandrogenism, accompanying loss of the putative endogenous GHRP-like ligand action with prolonged stress in both genders may explain these novel findings.

The growth hormone axis and cognition: empirical results and integrated theory derived from giant transgenic mice

Sleep is required for the consolidation of memory for complex tasks, and elements of the growth-hormone (GH) axis may regulate sleep. The GH axis also up-regulates protein synthesis, which is required for memory consolidation. Transgenic rat GH mice (TRGHM) express plasma GH at levels 100-300 times normal and sleep 3.4 h longer (30%) than their normal siblings. Consequently, we hypothesized that they might show superior ability to learn a complex task (8-choice radial maze); 47% of the TRGHM learned the task before any normal mice. All 17 TRGHM learned the task, but 33% of the 18 normal mice learned little. TRGHM learned the task significantly faster than normal mice (p < 0.05) and made half as many errors in doing so, even when the normal nonlearners were excluded from the analysis. Whereas normal mice expressed a linear learning curve, TRGHM showed exponentially declining error rates. The contribution of the GH axis to cognition is conspicuously sparse in literature syntheses of knowledge concerning neuroendocrine mechanisms of learning and memory. This paper synthesizes the crucial role of major components of the GH axis in brain functioning into a holistic framework, integrating learning, sleep, free radicals, aging, and neurodegenerative diseases. TRGHM show both enhanced learning in youth and accelerated aging. Thus, they may provide a powerful new probe for use in gaining an understanding of aspects of central nervous system functioning, which is highly relevant to human health.

Myogenic expression of an injectable protease-resistant growth hormone-releasing hormone augments long-term growth in pigs

Ectopic expression of a new serum protease-resistant porcine growth hormone-releasing hormone, directed by an injectable muscle-specific synthetic promoter plasmid vector (pSP-HV-GHRH), elicits growth in pigs. A single 10 mg intramuscular injection of pSP-HV-GHRH DNA followed by electroporation in three-week-old piglets elevated serum GHRH levels by twofold to fourfold, enhanced growth hormone secretion, and increased serum insulin-like growth factor-I by threefold to sixfold over control pigs. After 65 days the average body weight of the pigs injected with pSP-HV-GHRH was approximately 37% greater than the placebo-injected controls and resulted in a significant reduction in serum urea concentration, indicating a decrease in amino acid catabolism. Evaluation of body composition indicated a uniform increase in mass, with no organomegaly or associated pathology.

Serum Insulin-like Growth Factors and Insulin-like Growth Factor-binding Proteins: Clinical Implications

The last decade has been characterized by a major investigative thrust into the physiology of two unique but ubiquitous peptides, insulin-like growth factor (IGF)-I and IGF-II. The regulatory systems that control the tissue bioactivity of the IGFs have been delineated, and subcellular signaling mechanisms have been clarified. Clearly, both tissue and circulating growth factor concentrations are important in defining the relationship between IGF-I and cell activity. Bone, liver, and circulatory IGF-I have received the most attention by investigators, in part because of the ease of measurement and the interaction with disease states such as osteoporosis. More recently, attention has focused on the role IGF-I plays in neoplastic transformation and growth. Two large prospective observational studies have demonstrated greater risk for prostate and breast cancer associated with high circulating concentrations of IGF-I. Animal models and in vitro studies confirm that there is a close, albeit complex, interaction between IGF-I signaling and bone turnover. This report will focus on: (a) IGF physiology, including IGF ligands, binding proteins, and proteases; (b) the relationship between IGF-I and bone mass in respect to risk for osteoporosis; (c) the heritable regulation of the IGF-I phenotype; and (d) the association between serum IGF-I and cancer risk. The IGFs remain a major area for basic and clinical investigations; future studies may define both diagnostic and therapeutic roles for these peptides or their related proteins in several disease states.

Peripheral injection of growth hormone stimulates protein intake in aged male and female Lou rats

It is well established that growth hormone (GH) induces growth rate and food efficiency and stimulates protein accretion in young mammals. Senescence is characterized by metabolic and hormonal disorders, particularly a decrease in protein turnover, which could be correlated to a decrease in GH and insulin-like growth factor I (IGF-I) secretion. We have shown that body weight, protein intake, and IGF-I plasma levels are greatly decreased with aging in Lou/C rats, particularly in males. In order to specify the GH effect on protein intake during aging, males and females (6, 19, and 24 mo) placed on a self-selection regimen were injected daily with a physiological dose of human GH (0.023 mg/rat sc). No GH effect on caloric intake was observed. Nevertheless, GH treatment stimulated body weight in older rats. It also increased protein intake in females and older males (19-24 mo). This stimulating effect was positively correlated with the degree of weight loss in senescent rats, suggesting that the decrease in protein intake observed with aging could be a marker of senescence.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.