HormoneBayes: A novel Bayesian framework for the analysis of pulsatile hormone dynamics.

The hypothalamus is the central regulator of reproductive hormone secretion. Pulsatile secretion of gonadotropin releasing hormone (GnRH) is fundamental to physiological stimulation of the pituitary gland to release luteinizing hormone (LH) and follicle stimulating hormone (FSH). Furthermore, GnRH pulsatility is altered in common reproductive disorders such as polycystic ovary syndrome (PCOS) and hypothalamic amenorrhea (HA). LH is measured routinely in clinical practice using an automated chemiluminescent immunoassay method and is the gold standard surrogate marker of GnRH. LH can be measured at frequent intervals (e.g., 10 minutely) to assess GnRH/LH pulsatility. However, this is rarely done in clinical practice because it is resource intensive, and there is no open-access, graphical interface software for computational analysis of the LH data available to clinicians. Here we present hormoneBayes, a novel open-access Bayesian framework that can be easily applied to reliably analyze serial LH measurements to assess LH pulsatility. The framework utilizes parsimonious models to simulate hypothalamic signals that drive LH dynamics, together with state-of-the-art (sequential) Monte-Carlo methods to infer key parameters and latent hypothalamic dynamics. We show that this method provides estimates for key pulse parameters including inter-pulse interval, secretion and clearance rates and identifies LH pulses in line with the widely used deconvolution method. We show that these parameters can distinguish LH pulsatility in different clinical contexts including in reproductive health and disease in men and women (e.g., healthy men, healthy women before and after menopause, women with HA or PCOS). A further advantage of hormoneBayes is that our mathematical approach provides a quantified estimation of uncertainty. Our framework will complement methods enabling real-time in-vivo hormone monitoring and therefore has the potential to assist translation of personalized, data-driven, clinical care of patients presenting with conditions of reproductive hormone dysfunction.

Effect of Growth Hormone Secretagogue Receptor Deletion on Growth, Pulsatile Growth Hormone Secretion, and Meal Pattern in Male and Female Mice.

INTRODUCTION: While the vast majority of research investigating the role of ghrelin or its receptor, GHS-R1a, in growth, feeding, and metabolism has been conducted in male rodents, very little is known about sex differences in this system. Furthermore, the role of GHS-R1a signaling in the control of pulsatile GH secretion and its link with growth or metabolic parameters has never been characterized. METHODS: We assessed the sex-specific contribution of GHS-R1a signaling in the activity of the GH/IGF-1 axis, metabolic parameters, and feeding behavior in adolescent (5-6 weeks old) or adult (10-19 weeks old) GHS-R KO (Ghsr-/-) and WT (Ghsr+/+) male and female mice. RESULTS: Adult Ghsr-/- male and female mice displayed deficits in weight and linear growth that were correlated with reduced GH pituitary contents in males only. GHS-R1a deletion was associated with reduced meal frequency and increased meal intervals, as well as reduced hypothalamic GHRH and NPY mRNA in males, not females. In adult, GH release from Ghsr-/- mice pituitary explants ex vivo was reduced independently of the sex. However, in vivo pulsatile GH secretion decreased in adult but not adolescent Ghsr-/- females, while in males, GHS-R1a deletion was associated with reduction in pulsatile GH secretion during adolescence exclusively. In males, linear growth did not correlate with pulsatile GH secretion, but rather with ApEn, a measure that reflects irregularity of the rhythmic secretion. Fat mass, plasma leptin concentrations, or ambulatory activity did not predict differences in GH secretion. DISCUSSION/CONCLUSION: These results point to a sex-dependent dimorphic effect of GHS-R1a signaling to modulate pulsatile GH secretion and meal pattern in mice with different compensatory mechanisms occurring in the hypothalamus of adult males and females after GHS-R1a deletion. Altogether, we show that GHS-R1a signaling plays a more critical role in the regulation of pulsatile GH secretion during adolescence in males and adulthood in females.

Pregnancy, but not dietary octanoic acid supplementation, stimulates the ghrelin-pituitary growth hormone axis in mice.

Circulating growth hormone (GH) concentrations increase during pregnancy in mice and remain pituitary-derived. Whether abundance or activation of the GH secretagogue ghrelin increase during pregnancy, or in response to dietary octanoic acid supplementation, is unclear. We therefore measured circulating GH profiles in late pregnant C57BL/6J mice and in aged-matched non-pregnant females fed with standard laboratory chow supplemented with 5% octanoic or palmitic (control) acid (n = 4-13/group). Serum total and acyl-ghrelin concentrations, stomach and placenta ghrelin mRNA and protein expression, Pcsk1 (encoding prohormone convertase 1/3) and Mboat4 (membrane bound O-acyl transferase 4) mRNA were determined at zeitgeber (ZT) 13 and ZT23. Total and basal GH secretion were higher in late pregnant than non-pregnant mice (P < 0.001), regardless of diet. At ZT13, serum concentrations of total ghrelin (P = 0.004), but not acyl-ghrelin, and the density of ghrelin-positive cells in the gastric antrum (P = 0.019) were higher, and gastric Mboat4 and Pcsk1 mRNA expression were lower in pregnant than non-pregnant mice at ZT23. In the placenta, ghrelin protein was localised mostly to labyrinthine trophoblast cells. Serum acyl-, but not total, ghrelin was lower at mid-pregnancy than in non-pregnant mice, but not different at early or late pregnancy. In conclusion, dietary supplementation with 5% octanoic acid did not increase activation of ghrelin in female mice. Our results further suggest that increases in maternal GH secretion throughout murine pregnancy are not due to circulating acyl-ghrelin acting at the pituitary. Nevertheless, time-dependent increased circulating total ghrelin could potentially increase ghrelin action in tissues that express the acylating enzyme and receptor.

Aromatized Estrogens Amplify Nocturnal Growth Hormone Secretion in Testosterone-Replaced Older Hypogonadal Men.

Context: Testosterone (T) increases GH secretion in older men with a relative lack of T, in hypogonadal men of all ages, and in patients undergoing sex reassignment. The role of estradiol (E2) in men is less well defined. Objective: To assess the contribution of aromatization of T to spontaneous nocturnal and stimulated GH secretion. Participants: Four groups of healthy older men (N = 74, age range 57 to 77 years) were studied. The gonadotropic axis was clamped with the gonadotropin-releasing hormone antagonist degarelix. Three groups received T and one group placebo addback. Two T-replaced groups were treated with anastrozole (an aromatase inhibitor) and either placebo or E2 addback. Main Outcome Measures: Ten-minute GH concentration profiles were quantified by deconvolution analysis, after overnight (2200 to 0800 hours) sampling, and after combined IV injection of GHRH (0.3 µg/kg) and GHRH-2 (0.3 µg/kg) and withdrawal of a 2-hour somatostatin infusion (1 µg/kg/h). Results: E2 addback during aromatase inhibition increased basal (P = 0.046), pulsatile (P = 0.020), and total (P = 0.018) GH secretion by 60% to 70%. E2 did not potentiate GH secretory stimuli. Logarithmically transformed pulsatile GH secretion correlated strongly and positively with concurrent E2 concentrations overall (P = 0.028) and under anastrozole treatment (P = 0.005). Conclusion: E2 administration in older men transdermally stimulates overnight pulsatile GH secretion. The exact site of E2 action cannot be ascertained from these experiments but may include hypothalamic loci involved in GH regulation, especially because GH secretagogue effects on somatotrope pituitary cells were not affected.

Pulsatility of Hypothalamo-Pituitary Hormones: A Challenge in Quantification.

Neuroendocrine systems control many of the most fundamental physiological processes, e.g., reproduction, growth, adaptations to stress, and metabolism. Each such system involves the hypothalamus, the pituitary, and a specific target gland or organ. In the quantification of the interactions among these components, biostatistical modeling has played an important role. In the present article, five key challenges to an understanding of the interactions of these systems are illustrated and discussed critically.

Actions of NPY, and its Y1 and Y2 receptors on pulsatile growth hormone secretion during the fed and fasted state.

The hypothalamic NPY system plays an important role in regulating food intake and energy expenditure. Different biological actions of NPY are assigned to NPY receptor subtypes. Recent studies demonstrated a close relationship between food intake and growth hormone (GH) secretion; however, the mechanism through which endogenous NPY modulates GH release remains unknown. Moreover, conclusive evidence demonstrating a role for NPY and Y-receptors in regulating the endogenous pulsatile release of GH does not exist. We used genetically modified mice (germline Npy, Y1, and Y2 receptor knock-out mice) to assess pulsatile GH secretion under both fed and fasting conditions. Deletion of NPY did not impact fed GH release; however, it reversed the fasting-induced suppression of pulsatile GH secretion. The recovery of GH secretion was associated with a reduction in hypothalamic somatotropin release inhibiting factor (Srif; somatostatin) mRNA expression. Moreover, observations revealed a differential role for Y1 and Y2 receptors, wherein the postsynaptic Y1 receptor suppresses GH secretion in fasting. In contrast, the presynaptic Y2 receptor maintains normal GH output under long-term ad libitum-fed conditions. These data demonstrate an integrated neural circuit that modulates GH release relative to food intake, and provide essential information to address the differential roles of Y1 and Y2 receptors in regulating the release of GH under fed and fasting states.

Estradiol regulates GH-releasing peptide's interactions with GH-releasing hormone and somatostatin in postmenopausal women.

OBJECTIVE: Estrogen stimulates pulsatile secretion of GH, via mechanisms that are largely unknown. An untested hypothesis is that estradiol (E2) drives GH secretion by amplifying interactions among GH-releasing hormone (GHRH), somatostatin (SS), and GH-releasing peptide (GHRP). DESIGN: The design comprised double-blind randomized prospective administration of transdermal E2 vs placebo to healthy postmenopausal women (n=24) followed by pulsatile GHRH or SS infusions for 13 h overnight with or without continuous GHRP2 stimulation. METHODS: End points were mean concentrations, deconvolved secretion, and approximate entropy (ApEn; a regularity measure) of GH. RESULTS: By generalized ANOVA models, it was observed that E2 vs placebo supplementation: i) augmented mean (13-h) GH concentrations (P=0.023), GHRH-induced pulsatile GH secretion over the first 3 h (P=0.0085) and pulsatile GH secretion over the next 10 h (P=0.054); ii) increased GHRP-modulated (P=0.022) and SS-modulated (P<0.001) GH ApEn; and iii) did not amplify GHRH/GHRP synergy during pulsatile GH secretion. By linear regression, E2 concentrations were found to be positively correlated with GH secretion during GHRP2 infusion (P=0.022), whereas BMI was found to be negatively correlated with GH secretion during GHRH (P=0.006) and combined GHRH/GHRP (P=0.015) stimulation. E2 and BMI jointly determined triple (combined l-arginine, GHRH, and GHRP2) stimulation of GH secretion after saline (R²=0.44 and P=0.003) and pulsatile GHRH (R²=0.39 and P=0.013) infusions. CONCLUSION: In summary, in postmenopausal women, E2 supplementation augments the amount (mass) and alters the pattern (regularity) of GH secretion via interactions among GHRH, SS, GHRP, and BMI. These outcomes introduce a more complex model of E2 supplementation in coordinating GH secretion in aging women.

Differential pulsatile secretagogue control of GH secretion in healthy men.

Pulsatile growth hormone (GH) secretion putatively reflects integrated regulation by GH-releasing hormone (GHRH), somatostatin (SST), and GH-releasing peptide (GHRP). GHRH and SST secretion is itself pulsatile. However, how GHRH and SST pulses act along with GHRP to jointly determine pulsatile GH secretion is unclear. Moreover, how testosterone (T) modulates such interactions is unknown. These queries were assessed in a prospectively randomized, placebo-controlled double-blind cohort comprising 26 healthy older men randomized to testosterone (T) vs. placebo supplementation. Pulses of GHRH, SST, or saline were infused intravenously at 90-min intervals for 13 h, along with either continuous saline or ghrelin analog (GHRP-2). The train of pulses was followed by a triple stimulus (combined l-arginine, GHRH, and GHRP-2) to estimate near-maximal GH secretion over a final 3 h. Testosterone vs. placebo supplementation doubled pulsatile GH secretion during GHRH pulses combined with continuous saline (GHRH/saline) (P < 0.01). Pulsatile GH secretion correlated positively with T concentrations (270-1,170 ng/dl) in the 26 men during saline pulses/saline (P = 0.015, R(2) = 0.24), GHRH pulses/saline (P = 0.020, R(2) = 0.22), and combined GHRH pulses/GHRP-2 (P = 0.016, R(2) = 0.25) infusions. Basal nonpulsatile GH secretion correlated with T during saline pulses/GHRP-2 drive (P = 0.020, R(2) = 0.16). By regression analysis, pulsatile GH secretion varied negatively with body mass index (BMI) during saline/GHRP-2 infusion (P = 0.001, R(2) = 0.36), as well as after the triple stimulus preceded by GHRH/GHRP-2 (P = 0.013, R(2) = 0.23). Mean (10-h) GH concentrations under GHRP-2 were predicted jointly by estradiol (positively) and BMI (negatively) (P < 0.001, R(2) = 0.520). These data indicate that estradiol, T, and BMI control pulsatile secretagogue-specific GH-regulatory mechanisms in older men.

A super-agonist of growth hormone-releasing hormone causes rapid improvement of nutritional status in patients with chronic kidney disease.

Chronic kidney disease is frequently associated with protein-energy wasting related to chronic inflammation and a resistance to anabolic hormones such as insulin and growth hormone (GH). In this study, we determined whether a new GH-releasing hormone super-agonist (AKL-0707) improved the anabolism and nutritional status of nondialyzed patients with stage 4-5 chronic kidney disease randomized to twice daily injections of the super-agonist or placebo. After 28 days, this treatment significantly increased 24-h GH secretion by almost 400%, without altering the frequency or rhythmicity of secretory bursts or fractional pulsatile GH release, and doubled the serum insulin-like growth factor-1 level. There was a significant change in the Subjective Global Assessment from 'mildly to moderately malnourished' to 'well-nourished' in 6 of 9 patients receiving AKL-0707 but in none of 10 placebo-treated patients. By dual-energy X-ray absorptiometry, both the mean fat-free mass and the body mineral content increased, but fat mass decreased, all significantly. In the AKL-0707-treated group, both serum urea and normalized protein equivalent of nitrogen appearance significantly decreased with no change in dietary protein intake, indicating a protein anabolic effect of treatment. Thus, our study shows that stimulation of endogenous GH secretion by AKL-0707 overcomes uremic catabolism of patients with advanced chronic kidney disease.

Hypocortisolemic clamp unmasks jointly feedforward- and feedback-dependent control of overnight ACTH secretion.

BACKGROUND: ACTH secretion is under hypothalamic stimulatory (feedforward) and adrenal inhibitory (feedback) control. HYPOTHESIS: Assessment of overnight ACTH secretion during a hypocortisolemic clamp will permit the estimation of changing feedforward and feedback. SUBJECTS: Seven healthy men. INTERVENTIONS: An oral dose of placebo (PLAC), metyrapone (METY, 3 g), or ketoconazole (KTCZ, 1.2 g) was given at midnight (MN) to block glucocorticoid synthesis. Plasma ACTH was sampled every 10 min (MN to 0800 h). ANALYSIS: Variable-waveform deconvolution analysis of ACTH secretion and approximate entropy (ApEn) analysis of pattern regularity. RESULTS: Compared with PLAC, administration of METY and KTCZ reduced morning cortisol concentrations by >or=77 and 54% respectively (P<0.001). Hypocortisolemia elevated pulsatile ACTH secretion by 8.2- (METY) and 5.3-fold (KTCZ; both P<0.001). Basal ACTH secretion rose by 3.4-fold under METY-induced cortisol depletion (P=0.020). ACTH secretory-burst shape and half-life were stable. ApEn of ACTH release declined overnight (P=0.021) and with the drug (P=0.001), denoting enhanced feedforward coordination. CONCLUSION: The combined data predict overnight amplification and coordination of hypothalamic feedforward drive onto ACTH release. Therefore, disruption of either mechanism might contribute to clinical pathophysiology, such as late-day elevations of cortisol output in fasting, alcoholism, depression, or aging.

Somatotropic and gonadotropic axes linkages in infancy, childhood, and the puberty-adult transition.

Integrative neuroendocrine control of the gonadotropic and somatotropic axes in childhood, puberty, and young adulthood proceeds via multiple convergent and divergent pathways in the human and experimental animal. Emerging ensemble concepts are required to embody independent, parallel, and interacting mechanisms that subserve physiological adaptations and pathological disruption of reproduction and growth. Significant advances in systems biology will be needed to address these challenges.

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.

Graded inhibition of pulsatile luteinizing hormone secretion by a selective gonadotropin-releasing hormone (GnRH)-receptor antagonist in healthy men: evidence that age attenuates hypothalamic GnRH outflow.

Healthy older men manifest concomitant hypoandrogenemia and attenuation of LH pulse size. Because exogenous GnRH remains effective, a plausible intuition is that aging reduces hypothalamic GnRH secretion, thus mediating relative hypogonadotropic hypogonadism. To assess the impact of age on central GnRH outflow indirectly, we quantitated graded suppression of pulsatile LH secretion by saline and escalating doses of a potent and selective GnRH-receptor antagonist, ganirelix, in 18 healthy men ages 23-72 yr. The rationale is that ganirelix should reduce the amplitude of LH pulses in proportion to both drug concentration and endogenous GnRH feedforward. To this end, blood was sampled every 10 min for 2 h before and 16 h after sc administration of saline or ganirelix and for 3 additional hours after iv injection of a fixed dose of GnRH (100 ng/kg); concentrations of LH and ganirelix were measured by immunochemiluminometry and RIA, respectively; and pulsatile LH secretion was quantitated by a deconvolution procedure. Log-linear regression analysis was used to estimate the sensitivity of pulsatile LH secretion to inhibition by a unit increase in serum ganirelix concentrations in each subject. Statistical analyses revealed that increasing age markedly attenuated the capability of ganirelix to decrease LH pulse size (viz., r = -0.648; P = 0.004). In contrast, age did not modify the competitive interaction between injected GnRH and ganirelix. These joint outcomes support the clinical hypothesis that age diminishes hypothalamic GnRH outflow without impairing GnRH action in healthy men.

Gender modulates sequential suppression and recovery of pulsatile growth hormone secretion by physiological feedback signals in young adults.

The basic mechanisms that drive the renewal of GH pulses in the human are not understood. Recent ensemble models predict that pulse regeneration requires quenching of an ongoing GH pulse by somatostatin outflow and evocation of a new burst by rebound GHRH release. We reasoned that related principles might explain why women consistently maintain higher-amplitude GH secretory bursts than men. Accordingly, the present study tests the hypothesis that gender modulates the successive dynamics of GH feedback and escape in the morning fasting, when GH pulses are larger in women. To this end, we infused single iv pulses of recombinant human (rh) GH (0, 1, and 3 microg/kg) in eight young men and six women on separate randomly ordered mornings fasting and quantitated serial inhibition and recovery of GH secretion by frequent sampling, immunochemiluminometry, a deconvolution procedure, and regularity analysis. Statistical contrasts revealed gender-comparable peak concentrations and kinetics of rhGH. However, women differed from men by way of: (1) 3.5- and 4.0-fold less feedback suppression of GH secretory-burst mass; (2) more irregular patterns of GH release during negative feedback; and (3) 12-and 14-fold greater postnadir rebound-like GH secretion after rhGH pulses. Mechanistic analyses based on a minimal feedback construct predicted that women generate higher endogenous secretagogue stimulation per unit somatostatin outflow than men. In summary, negative feedback induced by near-physiological GH pulses unmasks prominent gender-related contrasts in hypothalamo-pituitary autoregulation in young adults. A frugal but sufficient explanation of the ensemble outcomes is that women sustain greater hypothalamo-pituitary agonist input than men.

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

The present study examines the thesis that pulsatile GH secretion is controlled simultaneously by three principal signals; viz., GHRH, GH-releasing peptide (GHRP, ghrelin), and somatostatin (SS). According to this ensemble notion, no single regulatory peptide acts alone or can be interpreted in isolation. Therefore, to investigate gender-specific control of pulsatile GH secretion, we designed dual-effector stimulation paradigms in eight young men and six women as follows: 1) L-arginine/GHRH (to clamp low SS and high GHRH input); 2) L-arginine/GHRP-2 (to clamp low SS and high GHRP drive); 3) GHRH/GHRP-2 (to clamp high GHRH and high GHRP feedforward); vs. 4) saline (unclamped). Statistical comparisons revealed that: 1) fasting pulsatile GH secretion was 7.6-fold higher in women than men (P < 0.001); 2) L-arginine/GHRH and L-arginine/GHRP-2 evoked, respectively, 4.6- and 2.2-fold greater burst-like GH release in women than men (P < 0.001 and P = 0.015); and 3) GHRH/GHRP-2 elicited comparable GH secretion by gender. In the combined cohorts, estradiol concentrations positively predicted responses to L-arginine/GHRP-2 (r2= 0.49, P = 0.005), whereas testosterone negatively predicted those to L-arginine/GHRH (r2= 0.56, P = 0.002). Based upon a simplified biomathematical model of three-peptide control, the current outcomes suggest that women maintain greater GHRH potency, GHRP efficacy, and opposing SS outflow than men. This inference upholds recent clinical precedence and yields valid predictions of sex differences in self-renewable GH pulsatility.

Sustained growth hormone (GH) and insulin-like growth factor I responses to prolonged high-dose twice-daily GH-releasing hormone stimulation in middle-aged and older men.

Postulated mechanisms underlying the relative hyposomato-tropism of aging include reduced hypothalamic drive by GHRH. To test this notion, we administered 1 mg (n = 11) vs. 4 mg (n = 11) recombinant human GHRH-1,44-amide s.c. twice daily for 3 months in a double-blind, parallel-cohort design to 22 healthy men (ages, 53-68 yr). After 3 months, GHRH elevated: overnight GH concentrations from 0.71 +/- 0.19 to 1.74 +/- 0.39 microg/liter (P < 0.001; 1 mg) and from 0.80 +/- 0.15 to 5.12 +/- 0.40 microg/liter (P < 0.001; 4 mg) and IGF-I concentrations from 117 +/- 14 to 234 +/- 20 microg/liter (P = 0.007; 1 mg) and from 147 +/- 13 to 286 +/- 22 microg/liter (P < 0.001; 4 mg). Only the higher GHRH dose also increased total body water (tritium space; P = 0.024) and fat-free mass (dual-energy x-ray absorptiometry; P = 0.021), and reduced total abdominal adiposity (computed axial tomography scan; P = 0.042). Both supplementation schedules shortened the time required to walk 30 m and ascend four flights of stairs (P < 0.025 each). Lower extremity strength, aerobic capacity, and bone mineral density did not change. Local injection site reactions were common. We conclude that sc administration of a large dose of GHRH (4 mg) twice daily for 3 months elevates GH and IGF-I concentrations, increases total body water and fat-free mass, reduces total abdominal adiposity, and enhances certain performance measures in healthy aging men but causes local skin reactions.

Estradiol supplementation modulates growth hormone (GH) secretory-burst waveform and recombinant human insulin-like growth factor-I-enforced suppression of endogenously driven GH release in postmenopausal women.

The present study tests the mechanistic postulate that estrogen confers resistance to negative feedback by systemic IGF-I. To this end, eight postmenopausal women received a constant iv infusion of recombinant human (rh)IGF-I (10 micro g/kg.h x 6 h) and saline in randomized order on the 10th day of supplementation with oral estradiol (E(2)) and placebo (Pl). GH secretion was quantitated by 10-min blood sampling, immunochemiluminometry assay, and deconvolution analysis. Administration of E(2) compared with Pl followed by saline infusion: 1) stimulated pulsatile GH secretion ( micro g/liter.6 h), viz., 12 +/- 3.3 (Pl) and 18 +/- 4.6 (E(2)) (mean +/- SEM, paired comparison, P < 0.05); 2) halved the time latency (min) to achieve peak GH secretion after GHRH injection, 24 +/- 2.2 (Pl) and 12 +/- 2.1 (E(2)) (P < 0.01); and 3) did not alter the mass of GH secreted ( micro g/liter) in response to a maximally effective dose of GHRH, 30 +/- 7.2 (Pl) and 37 +/- 11 (E(2)). Exposure to E(2) compared with Pl followed by rhIGF-I infusion: 1) accelerated the rate of decline of GH concentrations by 3.3-fold, viz., absolute slope ( micro g/liter.1000 min), 3.8 (range, 2.5-5.0) (Pl) and 12 (range, 10-14) (E(2)) (P < 0.001); 2) augmented the algebraic decrement in GH concentrations ( micro g/liter) enforced by rhIGF-I infusion, 0.73 +/- 0.21 (Pl) and 1.6 +/- 0.25 (E(2)) (P < 0.01); 3) halved the time delay (min) to peak GHRH-induced GH secretion, 20 +/- 1.2 (Pl) vs. 10 +/- 1.3 (E(2)) min (P < 0.01). In contradistinction, E(2) did not alter: 1) the capability of rhIGF-I to suppress GHRH-stimulated GH secretory burst mass significantly, viz., by 50 +/- 8% (Pl) and 52 +/- 14% (E(2)) (P < 0.05 each vs. saline); 2) the hourly rate of rise of infused (total) IGF-I concentrations; and 3) total and ultrafiltratably free IGF-I concentrations ( micro g/liter) attained at the end of the two rhIGF-I infusions. In summary, compared with Pl, E(2) supplementation in postmenopausal women: 1) amplifies endogenously driven GH secretory-burst mass; 2) initiates rapid onset of GHRH-stimulated GH release; and 3) potentiates IGF-I-dependent suppression of unstimulated GH concentrations. Based upon companion modeling data, we postulate that E(2) facilitates the upstroke and IGF-I enforces the downstroke of high-amplitude GH secretory bursts in estrogen-replete individuals.

Estradiol supplementation in postmenopausal women doubles rebound-like release of growth hormone (GH) triggered by sequential infusion and withdrawal of somatostatin: evidence that estrogen facilitates endogenous GH-releasing hormone drive.

We postulated that short-term estradiol replacement in postmenopausal women may act, in part, by facilitating endogenous GHRH release or action. A prediction of this hypothesis is that estradiol repletion should enhance postsomatostatin rebound GH secretion, which appears to be driven by hypothalamic outflow of GHRH. To this end, we administered placebo and estradiol to eight healthy estrogen-withdrawn postmenopausal volunteers in a prospectively randomized, patient-blinded, within-subject crossover design for a total of 36 d. Rebound release of GH was assessed between d 7 and 36 of intervention on separate randomly ordered mornings after continuous iv infusion of saline or somatostatin (9 micro g/kg.h for 3 h). Secretion was quantitated by frequent (10-min) blood sampling for 7 h, GH chemiluminescence assay, and deconvolution analysis. Compared with placebo, estradiol replacement: 1) stimulated spontaneous pulsatile GH secretion by 3.5-fold (95% confidence interval, 2.1- to 5.6-fold) (P < 0.001); and 2) amplified the mass of GH secreted in response to abrupt somatostatin withdrawal by 2.1-fold (95% confidence interval, 1.3- to 3.4-fold) (P = 0.003). Estrogenic augmentation of rebound-like GH secretion was specific, because the pharmacological effects of exogenous GHRH (1 micro g/kg) and GH-releasing peptide-2 (1 micro g/kg, a synthetic ghrelin analog) were not affected. In summary, short-term supplementation with estradiol in postmenopausal individuals doubles the mass of rebound-like GH secretion induced by abrupt somatostatin withdrawal without modifying stimulation by a pharmacological dose of GHRH or GH-releasing peptide-2. Accordingly, we hypothesize that estradiol stimulates pulsatile GH secretion, at least in part, by enhancing the release and/or action of hypothalamic GHRH.

Estradiol supplementation enhances submaximal feed-forward drive of growth hormone (GH) secretion by recombinant human GH-releasing hormone-1,44-amide in a putatively somatostatin-withdrawn milieu.

To test the clinical hypothesis that an estrogen-enriched milieu enhances GHRH action, we administered placebo (Pl) and estradiol-17 beta (E(2)) orally for 23 d to six postmenopausal women in a prospectively randomized, double-masked, within-subject crossover design with 6 wk intervening. The GHRH stimulation protocol entailed consecutive i.v. infusion of L-arginine and a single i.v. pulse of saline or one of five randomly ordered doses of recombinant human GHRH-1,44-amide (0.03, 0.1, 0.3, 1.0, or 3.0 microg/kg) in a total of 12 separate morning, fasting sessions. GH secretion was monitored by sampling blood every 10 min for 6 h; chemiluminescence assay of GH concentrations; deconvolution analysis of stimulated GH release; and nonlinear dose-response reconstruction. Supplementation with E(2), compared with Pl: 1) increased (mean +/- SEM) E(2) concentrations from 18 +/- 3 (Pl) to 164 +/- 12 pg/ml (to convert to picomoles per liter, multiply by 3.57) (P < 0.001); 2) decreased IGF-I concentrations from 181 +/- 14 to 120 +/- 11 microg/liter (P < 0.01); 3) elevated mean GH concentrations from 0.27 +/- 0.06 to 0.59 +/- 0.08 microg/liter (P = 0.014); 4) potentiated GH secretion stimulated by L-arginine alone by 1.43-fold (P = 0.012); 5) reduced the ED(50) of GHRH from 0.27 +/- 0.02 to 0.13 +/- 0.01 microg/kg (P < 0.01), denoting enhanced GHRH potency; and 6) heightened the maximal slope of the dose-response function from 1.1 +/- 0.1 to 1.4 +/- 0.05 [( microg/liter) ( microg/kg)(-1)] (P < 0.05), signifying augmented pituitary sensitivity. The foregoing facilitative mechanisms were specific because E(2) replacement did alter maximal L-arginine/GHRH-induced GH secretion, indicating unchanged secretagogue efficacy. In conclusion, inasmuch as E(2) also attenuates inhibition of GH secretion by infused somatostatin and potentiates stimulation of GH secretion by GH-releasing peptide-2, we postulate that estrogenic steroids drive pulsatile GH production in part via mechanisms that include all three of GHRH, somatostatin, and putatively GH-releasing peptide/ghrelin signaling.

Neuroendocrine adaptations in renal disease.

Chronic renal failure (CRF) disrupts the time-dependent secretion of multiple hormones. The present review focuses on altered pulsatile release of peptide hormones. CRF is marked by impaired tissue actions, disorderly release patterns, and relative [growth hormone (GH)] or absolute [luteinizing hormone (LH)] deficiency of secretion. At the hypothalamo-pituitary level, experimental evidence suggests that CRF reduces the synthesis and/or release of the cognate hypothalamic releasing factors, GHRH and LHRH, and enforces excessive inhibition by somatostatin. Parathyroid hormone (PTH) and insulin are secreted in both basal and pulsatile modes, wherein the latter is putatively coordinated by autonomic innervation. Amplitude and frequency-dependent adaptations of PTH and insulin outflow fail in CRF, as assessed under steady-state conditions and during metabolic drive (i.e., calcium for PTH and glucose for insulin). A common feature in CRF is a diminished mass of hormone released per burst, due in principle to attenuation of feedforward signals and/or accentuation of (unknown) feedback signals. Damping of neuronal control and/or prolonged network response times may contribute to aberrant pulse frequency, disproportionate basal (nonpulsatile) hormone release, and consistent erosion of secretory process regularity in the uremic state. The homeostatic consequences of distorted secretory dynamics, tissue resistance, impaired hormone clearance, and altered mean agonist concentrations are evident in certain therapeutic interventions, such as GH supplementation in CRF.