Complementary secretagogue pairs unmask prominent gender-related contrasts in mechanisms of growth hormone pulse renewal in young adults

Hormone and genetic study in male to female transsexual patients

BACKGROUND

Data of the literature demonstrated controversial results of a correlation between transsexualism and genetic mutations.

AIM

To evaluate the hormone and gene profile of male-female (M-F) transsexual.

SUBJECTS AND METHODS

Thirty M-F transsexuals aged 24-39. Seventeen had already undergone sex reassignment surgery, 13 were awaiting. All subjects had been undergoing estrogen and antiandrogen therapy. We studied hormones of the hypothalamus- pituitary-testicular axis, thyroid and adrenal profile, GH basal and after GHRH stimulation, IGF-I. The gene study analyzed SRY, AR, DAX1, SOX9, AZF region of the Y chromosome.

RESULTS

Pre-surgery subjects had elevated PRL, reduced testosterone and gonadotropins. Post-surgery subjects showed reduced androgens, a marked increase in LH and FSH and normal PRL. Cortisol and ACTH were similar to reference values in pre- and post-surgery patients. There was a marked increase in the baseline and post-stimulation GH values in 6 of the 13 pre-surgery patients, peaking at T15. IGF-I was similar to reference values in both groups except for one post-surgery patient, whose level was below the normal range. There were no polymorphisms in the amplified gene region for SOX9, and a single nucleotide synonimous polymorphism for DAX1. No statistically significant differences were seen in the mean of CAG repeats between controls and transsexual subjects. SRY gene was present in all subjects. Qualitative analysis of the AZFa, AZFb, and AZFc regions did not reveal any microdeletions in any subject.

CONCLUSIONS

This gender disorder does not seem to be associated with any molecular mutations of some of the main genes involved in sexual differentiation.

Somatostatin and its receptors contribute in a tissue-specific manner to the sex-dependent metabolic (fed/fasting) control of growth hormone axis in mice

Somatostatin (SST) inhibits growth hormone (GH) secretion and regulates multiple processes by signaling through its receptors sst1-5. Differential expression of SST/ssts may contribute to sex-specific GH pattern and fasting-induced GH rise. To further delineate the tissue-specific roles of SST and sst1-5 in these processes, their expression patterns were evaluated in hypothalamus, pituitary, and stomach of male and female mice under fed/fasted conditions in the presence (wild type) or absence (SST-knockout) of endogenous SST. Under fed conditions, hypothalamic/stomach SST/ssts expression did not differ between sexes, whereas male pituitary expressed more SST and sst2A/2B/3/5A/5TMD2/5TMD1 and less sst1, and male pituitary cell cultures were more responsive to SST inhibitory actions on GH release compared with females. This suggests that local pituitary SST/ssts can contribute to the sexually dimorphic pattern of GH release. Fasting (48 h) reduced stomach sst2A/B and hypothalamic SST/sst2A expression in both sexes, whereas it caused a generalized downregulation of pituitary sst subtypes in male and of sst2A only in females. Thus, fasting can reduce SST sensitivity across tissues and SST input to the pituitary, thereby jointly contributing to enhance GH release. In SST-knockout mice, lack of SST differentially altered sst subtype expression levels in both sexes, supporting an important role for SST in sex-dependent control of GH axis. Evaluation of SST, IGF-I, and glucocorticoid effects on hypothalamic and pituitary cell cultures revealed that these hormones could directly account for alterations in sst2/5 expression in the physiological states examined. Taken together, these results indicate that changes in SST output and sensitivity can contribute critically to precisely define, in a tissue-dependent manner, the sex-specific metabolic regulation of the GH axis.

Novel relationships of age, visceral adiposity, insulin-like growth factor (IGF)-I and IGF binding protein concentrations to growth hormone (GH) releasing-hormone and GH releasing-peptide efficacies in men during experimental hypogonadal clamp

BACKGROUND

Sex steroids influence GH secretion in complex ways.

HYPOTHESIS

Analyses in a low sex-steroid milieu will help unveil the effects of age and other nonsteroidal regulators on GH secretion.

CONTEXT

The study was conducted in a tertiary medical center.

SUBJECTS

The study group included 13 healthy young men and 12 healthy older men.

METHODS

We used GnRH agonist-induced down-regulation of testosterone and estradiol secretion, followed by consecutive infusion of l-arginine and GHRH or GHRP-2, to test secretagogue efficacies.

OUTCOMES

We measured basal and pulsatile GH secretion.

RESULTS

During experimental testosterone/estradiol deprivation, older (57 +/- 1.7 yr) men maintained: 1) 6.8-fold less pulsatile GH secretion (P < 0.001); and 2) 2-fold lower maximal GH responses to GHRH (P = 0.0065) and GHRP-2 (P = 0.022) than young (23 +/- 1.1 yr old) individuals. Stepwise forward-selection regression analyses identified: 1) abdominal visceral fat as a dominant negative predictor of both GHRH (R(2) = 0.49; P = 0.001) and GHRP-2 (R(2) = 0.38; P = 0.005) efficacies; and 2) fasting IGF-I concentration as a major positive correlate of GHRH (R(2) = 0.52; P < 0.001) and GHRP-2 (R(2) = 0.31; P = 0.018) efficacies. Unstimulated pulsatile GH secretion was jointly correlated with IGF-I and IGFBP-3 (P = 0.039).

CONCLUSION

Measures of body composition (abdominal visceral fat) and pulsatile GH action (IGF-I) explain up to one half of interindividual variability in the efficacies of GHRH and GHRP-2 in sex steroid-depleted men. Accordingly, normative ranges for maximal single peptide-stimulated GH secretion in short-term hypogonadal states should incorporate the influence of these determinants as well as age.

Cranially irradiated adult cancer survivors may have normal spontaneous GH secretion in the presence of discordant peak GH responses to stimulation tests (compensated GH deficiency)

CONTEXT

We have previously demonstrated that spontaneous (physiological) GH secretion was entirely normal in cranially irradiated patients who had normal individual peak GH responses to the insulin tolerance test (ITT) but reduced maximal somatotroph reserve as indicated by substantially reduced group GH responses to the GHRH + arginine stimulation test (AST). The normality of spontaneous GH secretion was attributed to a compensatory increase in hypothalamic stimulatory input within a partially damaged hypothalamic-pituitary (h-p) axis. It is unknown, however, if such compensatory stimulation can also maintain normality of GH secretion in those who fail the ITT but pass the GHRH + AST. STUDY SUBJECTS AND DESIGN: We studied 24-h spontaneous GH secretion by 20-min sampling both in the fed state (n = 11) and in the last 24 h of a 33-h fast (n = 9) in adult cancer survivors with subnormal peak GH responses to the ITT but either normal or relatively less attenuated peak GH responses to the GHRH + AST. The study was conducted 8.3 +/- 1.8 (range 2-23) years after cranial irradiation for nonpituitary brain tumours (n = 9) or leukaemia/lymphoma (n = 2) in comparison with 30 normal controls (fasting, 14).

RESULTS

Previously published diagnostic thresholds for the ITT, GHRH + AST and spontaneous GH secretion were used to characterize GH secretion. Four of the 11 patients with impaired stimulated responses to both tests showed only minor discordancies between stimulated and spontaneous GH secretion. Two of the remaining seven patients had subnormal spontaneous GH secretion. However, spontaneous GH secretion, both individually and as a group, was entirely normal in the remaining five patients who had impaired GH responses to the ITT but normal individual responses to the GHRH + AST; in these five patients, IGF-I standard deviation scores (SDS; -2.7 to -0.8) were significantly reduced to a moderate degree compared with normals.

CONCLUSIONS

In cranially irradiated adult cancer survivors, it cannot be assumed that failure to pass the ITT in isolation reflects severe GH deficiency (GHD). It appears that in some patients near-maximal compensatory overdrive of the partially damaged somatotroph axis may result in near-normal quantitative restoration of spontaneous GH secretion, thus limiting further stimulation with the ITT to the extent that impaired GH responses can be seen even before spontaneous GH secretion starts to decline in adults. However, IGF-I status continues to provide useful information about the adequacy of the compensatory process and therefore the degree of normality of GH secretion.

Hypopituitarism following radiotherapy

Deficiencies in anterior pituitary hormones secretion ranging from subtle to complete occur following radiation damage to the hypothalamic-pituitary (h-p) axis, the severity and frequency of which correlate with the total radiation dose delivered to the h-p axis and the length of follow up. Selective radiosensitivity of the neuroendocrine axes, with the GH axis being the most vulnerable, accounts for the high frequency of GH deficiency, which usually occurs in isolation following irradiation of the h-p axis with doses less than 30 Gy. With higher radiation doses (30-50 Gy), however, the frequency of GH insufficiency substantially increases and can be as high as 50-100%. Compensatory hyperstimulation of a partially damaged h-p axis may restore normality of spontaneous GH secretion in the context of reduced but normal stimulated responses; at its extreme, endogenous hyperstimulation may limit further stimulation by insulin-induced hypoglycaemia resulting in subnormal GH responses despite normality of spontaneous GH secretion in adults. In children, failure of the hyperstimulated partially damaged h-p axis to meet the increased demands for GH during growth and puberty may explain what has previously been described as radiation-induced GH neurosecretory dysfunction and, unlike in adults, the ITT remains the gold standard for assessing h-p functional reserve. Thyroid-stimulating hormone (TSH) and ACTH deficiency occur after intensive irradiation only (>50 Gy) with a long-term cumulative frequency of 3-6%. Abnormalities in gonadotrophin secretion are dose-dependent; precocious puberty can occur after radiation dose less than 30 Gy in girls only, and in both sexes equally with a radiation dose of 30-50 Gy. Gonadotrophin deficiency occurs infrequently and is usually a long-term complication following a minimum radiation dose of 30 Gy. Hyperprolactinemia, due to hypothalamic damage leading to reduced dopamine release, has been described in both sexes and all ages but is mostly seen in young women after intensive irradiation and is usually subclinical. A much higher incidence of gonadotrophin, ACTH and TSH deficiencies (30-60% after 10 years) occur after more intensive irradiation (>60 Gy) used for nasopharyngeal carcinomas and tumors of the skull base, and following conventional irradiation (30-50 Gy) for pituitary tumors. The frequency of hypopituitarism following stereotactic radiotherapy for pituitary tumors is mostly seen after long-term follow up and is similar to that following conventional irradiation. Radiation-induced anterior pituitary hormone deficiencies are irreversible and progressive. Regular testing is mandatory to ensure timely diagnosis and early hormone replacement therapy.

Cranial irradiation and growth hormone neurosecretory dysfunction: a critical appraisal

CONTEXT

It has been suggested that radiation-induced GH neurosecretory dysfunction exists in children; however, the pathophysiology is poorly understood, and it is unknown if such a phenomenon exists in adult life.

STUDY SUBJECTS

Twenty-four-hour spontaneous GH secretion was studied by 20-min sampling both in the fed state (n = 16; six women) and the last 24 h of 33-h fast (n = 10; three women) in adult cancer survivors of normal GH status defined by two GH provocative tests, 13.1 +/- 1.6 (range, 3-28) yr after cranial irradiation (18-40 Gy) for nonpituitary brain tumors (n = 12) or leukemia (n = 4) in comparison with 30 (nine women) age- and body mass index-matched normal controls (fasting, 11 men and three women).

RESULTS

Using previously published diagnostic thresholds, all patients had stimulated peak GH responses in the normal range to both the insulin tolerance test and the combined GHRH plus arginine stimulation test, as well as normal individual mean profile GH levels during the fed and fasting states. However, gender-specific comparisons revealed marked reduction (by 40%) in the overall peak GH responses to both provocative tests but similar GH secretory profiles; no differences were seen in the pulsatile attributes of GH secretion (cluster analysis) or the profile absolute and mean GH levels in the fed state or when the hypothalamic-pituitary axis was stimulated by fasting.

CONCLUSIONS

Radiation-induced GH neurosecretory dysfunction either does not exist or is a very rare phenomenon in irradiated adult cancer survivors. The normality of physiological GH secretion in the context of reduced maximum somatotroph reserve suggests compensatory overdrive of the partially damaged somatotroph axis and constitutes a relative argument against somatotroph dysfunction being explained purely by hypothalamic damage with secondary atrophy due to GHRH deficiency. It is therefore possible that radiation in doses less than 40 Gy causes dual damage to both the pituitary and the hypothalamus.

Model-projected mechanistic bases for sex differences in growth hormone regulation in humans

Models of physiological systems facilitate rational experimental design, inference, and prediction. A recent construct of regulated growth hormone (GH) secretion interlinks the actions of GH-releasing hormone (GHRH), somatostatin (SRIF), and GH secretagogues (GHS) with GH feedback in the rat (Farhy LS, Veldhuis JD. Am J Physiol Regul Integr Comp Physiol 288: R1649–R1663, 2005). In contrast, no comparable formalism exists to explicate GH dynamics in any other species. The present analyses explore whether a unifying model structure can represent species- and sex-defined distinctions in the human and rodent. The consensus principle that GHRH and GHS synergize in vivo but not in vitro was explicable by assuming that GHS 1) evokes GHRH release from the brain, 2) opposes inhibition by SRIF both in the hypothalamus and on the pituitary gland, and 3) stimulates pituitary GH release directly and additively with GHRH. The gender-selective principle that GH pulses are larger and more irregular in women than men was conferrable by way of 4) higher GHRH potency and 5) greater GHS efficacy. The overall construct predicts GHRH/GHS synergy in the human only in the presence of SRIF when the brain-pituitary nexus is intact, larger and more irregular GH pulses in women, and observed gender differences in feedback by GH and the single and paired actions of GHRH, GHS, and SRIF. The proposed model platform should enhance the framing and interpretation of novel clinical hypotheses and create a basis for interspecies generalization of GH-axis regulation.

Regulation of growth hormone action by gonadal steroids

Sex steroids modulate growth hormone (GH) secretion and action. Estrogen attenuates GH action in a dose- and route-dependent manner by inhibiting GH-regulated endocrine function of the liver. Testosterone amplifies the metabolic action of GH while exhibiting similar but independent effects of its own. The strong modulatory effect of gonadal steroids on GH responsiveness provides insights into the biologic basis of sexual dimorphism in growth, development, and body composition and practical information for the clinical endocrinologist in the treatment of hypopituitary patients.

Modulation of growth hormone action by sex steroids

Growth hormone (GH) is a major regulator of growth, somatic development and body composition. Sex steroids can act centrally by regulating GH secretion and peripherally modulating GH responsiveness. This review addresses data of potential clinical relevance on how sex steroids modulate GH secretion and action, aiming to increase the understanding of sex steroid/GH interactions and leading to improved management of patients. Sex steroids regulate GH secretion directly as well as indirectly through IGF-I modulation. Testosterone stimulates GH secretion centrally, an effect dependent on prior aromatization to oestrogen. Oestrogen stimulates GH secretion indirectly by reducing IGF-I feedback inhibition. Whether oestrogen stimulates GH secretion centrally in females is unresolved. Gonadal steroids modify the metabolic effects of GH. Testosterone amplifies GH stimulation of IGF-I, sodium retention, substrate metabolism and protein anabolism while exhibiting similar but independent actions of its own. Oestrogen attenuates GH action by inhibiting GH-regulated endocrine function of the liver. This is a concentration-dependent phenomenon that arises invariably from oral administration of therapeutic doses of oestrogen, an effect that can be avoided by using a parenteral route. This strong modulatory effect of gonadal steroids on GH responsiveness provides insights into the biological basis of sexual dimorphism in growth, development and body composition and practical information for the clinical endocrinologist. It calls for an appraisal of the diagnostic criteria for GH deficiency of GH stimulation tests, which currently are based on arbitrary cut-offs that do not take into account the shifting baseline from the changing gonadal steroid milieu. In the management of GH deficiency in the hypopituitary female, oestrogen should be administered by a nonoral route. In hypopituitary men, androgens should be replaced concurrently to maximize the benefits of GH. In the general population, the metabolic consequences of long-term treatment of women with oral oestrogen compounds, including selective oestrogen receptor modulators, are largely unknown and warrant study.

Testosterone supplementation in healthy older men drives GH and IGF-I secretion without potentiating peptidyl secretagogue efficacy

OBJECTIVE

Testosterone supplementation increases GH and IGF-I concentrations in healthy older men via unknown mechanisms. We examine the hypotheses that (i) testosterone amplifies stimulation of GH secretion by GH-releasing peptide (GHRP)-2 or GH-releasing hormone (GHRH) infused with l-arginine to limit somatostatin outflow (i.e. upregulates each agonistic pathway), (ii) testosterone augments the effect of both peptidyl secretagogues infused together (i.e. reduces opposition by hypothalamic somatostatin) and (iii) abdominal visceral fat (AVF) mass is a negative determinant of specific secretagogue-stimulated GH secretion.

DESIGN

Randomized double-blind crossover design of placebo versus testosterone administration in healthy older men.

METHODS

Deconvolution analysis was used to estimate basal GH secretion and the mass (integral) and waveform (time-shape) of GH secretory bursts.

RESULTS

Statistical contrasts revealed that administration of testosterone compared with placebo in seven men aged 60-77 years increased fasting concentrations of GH (P < 0.01) and IGF-I (P = 0.003), and basal (P < 0.005) and pulsatile (P < 0.01) GH secretion. Testosterone did not alter the absolute value or rank order of secretagogue efficacy: l-arginine/GHRP-2 (23-fold effect over saline) = GHRH/GHRP-2 (20-fold) > l-arginine/GHRH (7.5-fold). Waveform reconstruction indicated that each stimulus pair accelerated initial GH secretion within a burst (P < 0.01). Regression analysis disclosed a significant inverse association between GH secretory-burst mass and computer tomography-estimated AVF following stimulation with l-arginine/GHRH after testosterone supplementation (R(2) = 0.54, P = 0.015).

CONCLUSION

Supraphysiological testosterone concentrations augment GH and IGF-I production in the elderly male without altering maximal somatotrope responses to single and combined GHRH and GHRP-2 drive, thus predicting multifactorial mechanisms of testosterone upregulation.