Secretory process regularity monitors neuroendocrine feedback and feedforward signaling strength in humans

Synthesis of the Growth Hormone Secretion Mechanism Using Nonlinear Analysis and CAD Tools

The goal of this paper is to present a hardware realization of the feed-forward and feedback hypothalamic-pituitary growth hormone (GH) secretion mechanism based on a bio-mathematical nonlinear delay differential equation model developed by Farhy et al. (2003) and Veldhuis et al. (2001). Behavioral modeling is implemented through Verilog hardware descriptive language (HDL) to simulate the antagonistic and stimulatory interaction of growth hormone, growth hormone releasing hormone (GHRH) and somatotropin release inhibiting factor (SRIF). The model is synthesized using computer aided design (CAD) tools and is promulgated through a combinational complex programmable logic device (CPLD)/field programmable grid array (FPGA) Xilinx XSA-50 microchip. The microchip sequentially displays the decimal equivalents of the time changing hormonal concentration levels of the biomathematical model.

Primary testicular failure in Klinefelter's syndrome: the use of bivariate luteinizing hormone-testosterone reference charts

BACKGROUND

The diagnosis of androgen deficiency is based on clinical features and confirmatory low serum testosterone levels. In early primary testicular failure, a rise in serum LH levels suggests inadequate androgen action for the individual's physiological requirements despite a serum testosterone level within the normal range. The combined evaluation of serum LH and testosterone levels in the evaluation of testicular failure has not been widely advocated.

PATIENTS

Seven hundred and six healthy males and 39 patients with known primary hypogonadism due to Klinefelter's syndrome and SRY-positive 46,XX karyotypes were included in the study.

DESIGN

Testosterone, oestradiol and LH serum concentrations were measured in all individuals. Based on the 706 healthy males two-dimensional bivariate LH-testosterone reference charts were constructed.

RESULTS

Despite a median serum total and free testosterone and oestradiol levels being reduced (P < 0.001) and LH levels elevated (P < 0.001) in Klinefelter's syndrome and 46,XX-males, many subjects (69%) had total testosterone within the reference range. However, using the bivariate charts all subjects lay outside the 97.5 percentile.

CONCLUSION

Bivariate LH and testosterone charts are useful in the evaluation of men with known primary testicular failure due to sex chromosomal aneuploidy in whom evaluation based on testosterone measurement in isolation underestimates the prevalence of hypogonadism. It is, however, important to emphasize that isolated use of the bivariate evaluation should not form the basis for androgen substitution therapy. Further studies are needed in order to evaluate the use of bivariate LH and testosterone charts in the assessment of any younger man with possible primary testicular failure.

Acromegaly caused by growth hormone-releasing hormone-producing tumors: long-term observational studies in three patients

We report on three newly diagnosed patients with extracranial ectopic GHRH-associated acromegaly with long-term follow-up after surgery of the primary tumor. One patient with a pancreatic tumor and two parathyroid adenomas was the index case of a large kindred of MEN-I syndrome. The other two patients had a large bronchial carcinoid. The first patient is still in remission now almost 22 years after surgery. In the two other patients GHRH did not normalize completely after surgery and they are now treated with slow-release octreotide. IGF-I normalized in all patients. During medical treatment basal GH secretion remained (slightly) elevated and secretory regularity was decreased in 24 h blood sampling studies. We did not observe development of tachyphylaxis towards the drug or radiological evidence of (growing) metastases. We propose life-long suppressive therapy with somatostatin analogs in cases with persisting elevated serum GHRH concentrations after removal of the primary tumor. Independent parameters of residual disease are elevated basal (nonpulsatile) GH secretion and decreased GH secretory regularity.

Aging attenuates both the regularity and joint synchrony of LH and testosterone secretion in normal men: analyses via a model of graded GnRH receptor blockade

Testosterone (T) secretion declines in the aging male, albeit for unknown reasons. From an ensemble perspective, repeated incremental signaling among gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), and T is required to maintain physiological androgen availability. Pattern-regularity statistics, such as univariate approximate entropy (ApEn) and bivariate cross-ApEn, provide specific and sensitive model-free measurement of altered multi-pathway control. The present study exploits partial muting of one pathway (GnRH drive) to appraise adaptive regulation of LH and T secretion in young and aging individuals. Analyses comprised 100 paired 18-h LH and T concentration time series obtained in 25 healthy men ages 20-72 yr each administered placebo and three graded doses of a specific GnRH-receptor antagonist. Graded blockade of GnRH drive increased the individual regularity of LH and T secretion and the synchrony of LH-T feedforward and T-LH feedback in the cohort as a whole (P<0.001 for each). However, age markedly attenuated ganirelix-induced enhancement of univariate T orderliness and bivariate LH-T feedback and T-LH feedback synchrony (P <or= 0.0025). In summary, the present analyses support the thesis that aging disrupts coordinate control of T secretion, LH-T feedforward, and T-LH feedback in healthy men. Thus the experimental strategy of stepwise silencing of an agonistic pathway may have utility in dissecting the bases of altered neurohormonal linkages in other systems.

Joint synchrony of reciprocal hormonal signaling in human paradigms of both ACTH excess and cortisol depletion

The hypothalamo-pituitary-adrenal axis is a stress-adaptive neuroendocrine ensemble, in which adrenocorticotropin (ACTH) drives cortisol secretion (feedforward) and cortisol restrains ACTH outflow (feedback). Quantifying direction- and pathway-specific adjustments within this and other interlinked systems by noninvasive means remains difficult. The present study tests the hypothesis that forward and reverse cross-approximate entropy (X-ApEn), a lag-, scale-, and model-independent measure of two-signal synchrony, would allow quantifiable discrimination of feedforward (ACTH --> cortisol) and feedback (cortisol --> ACTH) control. To this end, forward X-ApEn was defined by employing serial ACTH concentrations as a template to appraise pair-wise synchrony with cortisol secretion rates and vice versa for reverse X-ApEn. Coupled hormone profiles included normal ACTH-normal cortisol, high ACTH-high cortisol, and high ACTH-low cortisol concentrations in 35 healthy subjects, 21 patients with tumoral ACTH secretion, and 9 volunteers given placebo and a steroidogenic inhibitor, respectively. We used forward and reverse X-ApEn analyses to identify marked and equivalent losses of feedforward and feedback linkages (both P < 0.001) in patients with tumoral ACTH secretion. An identical analytical strategy revealed that ACTH --> cortisol feedforward synchrony decreases (P < 0.001), whereas cortisol --> ACTH feedback synchrony increases (P < 0.001), in response to hypocortisolemia. The collective outcomes establish precedence for pathway-specific adaptations in a major neurohormonal system. Thus quantification of directionally defined joint synchrony of biologically coupled signals offers a noninvasive strategy to dissect feedforward- and feedback-selective adaptations in an interactive axis.

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.

Experimentally induced androgen depletion accentuates ethnicity-related contrasts in luteinizing hormone secretion in asian and caucasian men

The basis for ethnicity-related distinctions in gonadotropin secretion are unknown but may have important populational and physiological implications. In male contraceptive trials, exogenous testosterone and progestins suppress spermatogenesis to a greater degree in Asian than Caucasian men. In addition, iv infusion of testosterone inhibits LH release more in Asian than Caucasian volunteers. We test the converse postulate that experimental reduction of androgen-dependent negative feedback by way of the steroidogenic inhibitor combination ketoconazole/dexamethasone will unveil ethnicity-related mechanisms of regulated LH secretion in young men. LH release was monitored by sampling blood every 10 min for 24 h followed by immunoradiometric assay, model-free pulse detection, an entropy (regulatory) statistic, and cosine regression. Statistical comparisons revealed that healthy young Asian and Caucasian men maintain comparable baseline concentrations of LH, testosterone, estradiol, SHBG, and molar testosterone to SHBG ratios. In contrast, the two ethnic groups differ prominently in each of basal, pulsatile, entropic, and 24-h rhythmic LH adaptations to short-term androgen withdrawal. Therefore, we postulate that physiological nonuniformity of sex steroid-dependent negative feedback in particular may contribute to populational diversity in LH regulation.

Age and testosterone feedback jointly control the dose-dependent actions of gonadotropin-releasing hormone in healthy men

Healthy older men manifest combined declines in testosterone concentrations, LH secretory burst mass (amount of LH released per pulse), and feedback-sensitive regularity of unknown cause. To test a unifying hypothesis of simultaneous reductions in GnRH outflow, gonadotrope responsiveness to GnRH, and androgenic negative feedback, we monitored LH secretion 1) after bolus iv injection of a 1000-fold range of randomly ordered individual doses of GnRH on separate mornings, 2) during unmodified (eugonadal) or testosterone-withdrawn (hypoandrogenemic) negative feedback, and 3) in 16 young (age, 18-35 yr) and 15 older (age, 60-85 yr) healthy men. LH secretory burst mass and pattern regularity were quantitated by intensive blood sampling, high specificity LH beta-subunit-directed immunoradiometric assay, deconvolution analysis, and approximate entropy. GnRH dose responsiveness was assessed by four-parameter nonlinear regression analysis. We demonstrated that older men exhibit 1) delayed attainment of GnRH-evoked maximal LH secretion; 2) enhanced potency of GnRH stimulation in both the feedback-intact and feedback-withdrawn states; 3) elevated gonadotrope sensitivity to GnRH, unmasked by experimental testosterone depletion; 4) comparable young adult-like GnRH efficacy, independent of testosterone feedback milieu; and 5) diminished regularity of GnRH-induced LH release evident only during unmodified androgenic feedback. We conclude that a 3-fold interaction among GnRH dose, testosterone concentration, and age governs GnRH action, and age determines both testosterone-modulated and testosterone-independent actions of GnRH.

Short-term aromatase-enzyme blockade unmasks impaired feedback adaptations in luteinizing hormone and testosterone secretion in older men

The mechanisms subserving hypoandrogenemia and relative hypogonadotropism in older men are not known. The present study tests the clinical hypothesis that aging impairs hypothalamopituitary adaptations to feedback withdrawal induced by antagonism of estrogen biosynthesis. To this end, we appraised gonadal axis responses to estrogen depletion induced by anastrozole (a potent and selective aromatase inhibitor) in nine older and 11 young men vs. placebo in 17 other older and eight young men. The study design comprised a prospectively randomized, double-blind, parallel-cohort intervention. To monitor LH release, blood was sampled every 10 min for 24 h; LH concentrations were assayed by two-site monoclonal immunoradiometric assay; pulsatile LH release quantitated by a model-free discrete peak-detection technique (Cluster); feedback-dependent orderliness of LH secretion via the approximate entropy statistic; and 24-h rhythmicity of LH concentrations by cosine analysis. At baseline, older men had comparable estradiol and testosterone but lower LH concentrations than young controls. Exposure to anastrozole reduced (24-h pooled) serum estradiol concentrations by 50% (P < 0.001) and elevated mean LH concentrations by 2.1-fold (P < 0.001) in both the young and older cohorts. However, older men failed to achieve young adult augmentation of the following: 1) total testosterone concentrations (P < 0.01) or molar testosterone to SHBG ratios (P < 0.01); 2) incremental LH pulse amplitude (P < 0.001) and LH peak area (P < 0.01); 3) mean LH pulse frequency (P = 0.0044); and 4) quantifiable irregularity (approximate entropy) of LH release patterns (P < 0.001). FSH concentrations became comparable in the two age cohorts. In summary, administration of a potent and selective aromatase antagonist reduces estradiol and elevates mean LH concentrations equivalently in young and older men. The low estrogen-feedback state in elderly men unmasks diminished incremental LH pulse amplitude and area; absence of further acceleration of LH pulse frequency; impaired regulation of the orderliness of LH release; and reduced testosterone to SHBG ratios. Thus, aging alters expected hypothalamopituitary-gonadal adaptations to short-term partial estrogen depletion in healthy men.

Growth hormone secretion in primary adrenal Cushing's syndrome is disorderly and inversely correlated with body mass index

To evaluate the impact on the somatotropic axis of endogenous cortisol excess in the absence of primary pituitary disease, we investigated spontaneous 24-h growth hormone (GH) secretion in 12 adult patients with ACTH-independent hypercortisolism. Plasma GH concentration profiles (10-min samples) were analyzed by deconvolution to reconstruct secretion and approximate entropy to quantitate orderliness of the release process. Comparisons were made with a body mass index (BMI)-, age-, and gender-matched control group and an age- and gender-matched lean control group. GH secretion rates did not differ from BMI-matched controls but were twofold lower compared with lean subjects, mainly due to a 2.5-fold attenuation of the mean secretory burst mass (P = 0.001). In hypercortisolemic patients, GH secretion was negatively correlated with BMI (R = -0.55, P = 0.005) but not cortisol secretion. Total serum IGF-I concentrations were similar in the three groups. Approximate entropy (ApEn) was increased in patients with Cushing's syndrome compared with both control groups (vs. BMI-matched, P = 0.04; vs. lean, P = 0.001), denoting more irregular GH secretion patterns. ApEn in patients correlated directly with cortisol secretion (R = 0.77, P = 0.003). Synchrony between cortisol and GH concentration series was analyzed by cross-correlation, cross-ApEn, and copulsatility analyses. Patients showed loss of pattern synchrony compared with BMI-matched controls, but copulsatility was unchanged. We conclude that hyposomatotropism in primary adrenal hypercortisolism is only partly explained (approximately 30%) by increased body weight and that increased GH secretory irregularity and loss of synchrony suggest altered coordinate regulation of GH release.

Enhanced circadian ACTH release in obese premenopausal women: reversal by short-term acipimox treatment

Several studies suggest that the hypothalamo-pituitary-adrenal (HPA) axis is exceedingly active in obese individuals. Experimental studies show that circulating free fatty acids (FFAs) promote the secretory activity of the HPA axis and that human obesity is associated with high circulating FFAs. We hypothesized that HPA axis activity is enhanced and that lowering of circulating FFAs by acipimox would reduce spontaneous secretion of the HPA hormonal ensemble in obese humans. To evaluate these hypotheses, diurnal ACTH and cortisol secretion was studied in 11 obese and 9 lean premenopausal women (body mass index: obese 33.5 +/- 0.9 vs. lean 21.2 +/- 0.6 kg/m(2), P < 0.001) in the early follicular stage of their menstrual cycle. Obese women were randomly assigned to treatment with either acipimox (inhibitor of lipolysis, 250 mg orally four times daily) or placebo in a double-blind crossover design, starting one day before admission until the end of the blood-sampling period. Blood samples were taken during 24 h with a sampling interval of 10 min for assessment of plasma ACTH and cortisol concentrations. ACTH and cortisol secretion rates were estimated by multiparameter deconvolution analysis. Daily ACTH secretion was substantially higher in obese than in lean women (7,950 +/- 1,212 vs. 2,808 +/- 329 ng/24 h, P = 0.002), whereas cortisol was not altered (obese 36,362 +/- 5,639 vs. lean 37,187 +/- 4,239 nmol/24 h, P = 0.912). Acipimox significantly reduced ACTH secretion in the obese subjects (acipimox 5,850 +/- 769 ng/24 h, P = 0.039 vs. placebo), whereas cortisol release did not change (acipimox 33,542 +/- 3,436 nmol/24 h, P = 0.484 vs. placebo). In conclusion, spontaneous ACTH secretion is enhanced in obese premenopausal women, whereas cortisol production is normal. Reduction of circulating FFA concentrations by acipimox blunts ACTH release in obese women, which suggests that FFAs are involved in the pathophysiology of this neuroendocrine anomaly.

Inter-relationships between the secretory dynamics of thyrotrophin-releasing hormone, thyrotrophin and prolactin in periovulatory mares: effect of hypothyroidism

We used our nonsurgical technique for collecting pituitary venous blood to relate the dynamics of thyrotrophin-releasing hormone (TRH) secretion to the secretion patterns of both prolactin and thyrotrophin in periovulatory mares, either euthyroid (n = 5) or made hypothyroid by treatment with propyl-thiouracil (n = 5). Pituitary venous blood was collected continuously and divided into 1-min aliquots for 4 h. To test the effect of dopamine on the relationship between secretion patterns, sulpiride, a selective D2 receptor antagonist, was given i.m. after 2 h of sampling. Thorough testing of the model and blood collection procedure revealed no sites of TRH loss. Hypothyroidism increased the mean secretion rates of TRH (P = 0.04) and thyrotrophin (P < 0.0001) but not prolactin. Sulpiride increased prolactin secretion rates in hypothyroid (P < 0.0001) and control (P = 0.007) mares, but did not alter TRH or thyrotrophin secretion rates. In both groups of mares, all three hormones were secreted episodically but not rhythmically. In both groups, the secretion pattern of TRH was almost always significantly related to that of thyrotrophin, as assessed by cross correlation and cross approximate entropy (ApEn) analysis. However, the degree of linear correlation was weak, with only 14% (hypothyroid) or 8% (controls) of the variation in thyrotrophin secretion rates attributable to TRH. Prolactin and TRH secretion patterns before sulpiride were coupled on cross ApEn analysis in both groups, and the minute-to-minute secretion rates of the two hormones were correlated in four hypothyroid and three euthyroid mares. Overall, the small, but significant, degree of association between TRH and prolactin was similar to that between TRH and thyrotrophin. In hypothyroid mares, sulpiride increased (P = 0.02) the synchrony between TRH and prolactin patterns. We conclude that in horses: (i) little TRH degradation occurs during passage through the pituitary or in blood after 1 h at 37 degrees C; (ii) TRH is not the major factor controlling minute-to-minute fluctuations in either thyrotrophin or prolactin; and (iii) reducing two strongly inhibitory inputs (i.e. dopamine and thyroid hormones) may magnify the stimulatory effect of TRH on prolactin secretion.

Process irregularity of cortisol and adrenocorticotropin secretion in men with major depressive disorder

Although evidence suggests that major depressive disorder (MDD) is associated with hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis, research on basal HPA axis hormone levels in MDD patients has been inconclusive. Definitive characterization of basal cortisol and adrenocorticotropin (ACTH) secretion may be important for understanding the pathophysiology of this disorder. In recent years, a new approach to the analysis of basal hormone secretion has been developed involving the approximate entropy (ApEn) statistic, which represents the degree of disorderliness or serial irregularity in a time series of hormone levels. ApEn has been shown to reflect the degree of coordination in integrated network systems and has provided new insights into the pathophysiology of a number of endocrine conditions. In the study reported here, 15 medication-free men with MDD and 15 healthy control men were admitted to a General Clinical Research Center and had blood sampled for cortisol and ACTH determinations every hour over a 24-h period. The cortisol and ACTH time series were characterized with a cosinor analysis and with analysis of ApEn. Depressed patients and control subjects did not differ significantly on any parameter derived from the cosinor analysis or on several other standard indices of basal hormone secretion. However, the depressed men had significantly increased cortisol ApEn and significantly decreased ACTH ApEn compared with the healthy subjects. The ApEn findings suggest a loss of regulatory control over cortisol secretion, and possibly increased cortisol feedback on the pituitary in the depressed patients. Together, these results are most consistent with a primary abnormality of the adrenal gland and suggest that further investigation of adrenal gland physiology may be informative for the pathophysiology of depression.

Octreotide represses secretory-burst mass and nonpulsatile secretion but does not restore event frequency or orderly GH secretion in acromegaly

Octreotide is a potent somatostatin analog that inhibits growth hormone (GH) release and restricts somatotrope cell growth. The long-acting octreotide formulation Sandostatin LAR is effective clinically in approximately 60% of patients with acromegaly. Tumoral GH secretion in this disorder is characterized by increases in pulse amplitude and frequency, nonpulsatile (basal) release, and irregularity. Whether sustained blockade by octreotide can restore physiological secretion patterns in this setting is unknown. To address this question, we studied seven patients with GH-secreting tumors during chronic receptor agonism. Responses were monitored by sampling blood at 10-min intervals for 24 h, followed by analyses of secretion and regularity by multiparameter deconvolution and approximate entropy (ApEn). The somatostatin agonist suppressed GH secretory-burst mass, nonpulsatile (basal) GH release, and pulsatile secretion, thereby decreasing total GH secretion by 86% (range 70-96%). ApEn decreased from 1.203 +/- 0.129 to 0.804 +/- 0.141 (P = 0.032), denoting greater regularity. None of GH pulse frequency, basal GH secretion rates, or ApEn normalized. In summary, chronic somatostatin agonism is able to repress amplitude-dependent measures of excessive GH secretion in acromegaly. Presumptive tumoral autonomy is inferred by continued elevations of event frequency, overall pattern disruption (irregularity), and nonsuppressible basal GH secretion.

Acipimox enhances spontaneous growth hormone secretion in obese women

We hypothesized that a high circulating free fatty acid (FFA) concentration is involved in the pathogenesis of hyposomatotropism associated with obesity. To evaluate this hypothesis, 10 healthy premenopausal women (body mass index 33.8 +/- 1.0 kg/m(2)) were studied in the follicular phase of their menstrual cycle at two occasions with a time interval of at least 8 wk, where body weight remained stable. Subjects were randomly assigned to treatment with either acipimox (an inhibitor of lipolysis, 250 mg orally 4 times daily) or placebo in a double-blind crossover design, starting 1 day before admission until the end of the blood sampling period. Blood samples were taken during 24 h with a sampling interval of 10 min for assessment of growth hormone (GH) concentrations, and GH secretion was estimated by deconvolution analysis. Identical methodology was used to study GH secretion in a historical control group of age-matched normal weight women. GH secretion was clearly blunted in obese women (total daily release 66 +/- 10 vs. lean controls: 201 +/- 23 mU x l(Vd)(-1) x 24 h(-1), P = 0.005, where l(Vd) is lite of distribution volume). Acipimox considerably enhanced total (113 +/- 50 vs. 66 +/- 10 mU x l(Vd)(-1) x 24 h(-1), P = 0.02) and pulsatile GH secretion (109 +/- 49 vs. 62 +/- 30 mU x l(Vd)(-1) x 24 h(-1), P = 0.02), but GH output remained lower compared with lean controls. Further analysis did not show any relationship between the effects of acipimox on GH secretion and regional body fat distribution. In conclusion, acipimox unleashes spontaneous GH secretion in obese women. It specifically enhances GH secretory burst mass. This might mean that lowering of systemic FFA concentrations by acipimox modulates neuroendocrine mechanisms that orchestrate the activity of the somatotropic ensemble.

Joint pituitary-hypothalamic and intrahypothalamic autofeedback construct of pulsatile growth hormone secretion

Growth hormone (GH) secretion is vividly pulsatile in all mammalian species studied. In a simplified model, self-renewable GH pulsatility can be reproduced by assuming individual, reversible, time-delayed, and threshold-sensitive hypothalamic outflow of GH-releasing hormone (GHRH) and GH release-inhibiting hormone (somatostatin; SRIF). However, this basic concept fails to explicate an array of new experimental observations. Accordingly, here we formulate and implement a novel fourfold ensemble construct, wherein 1) systemic GH pulses stimulate long-latency, concentration-dependent secretion of periventricular-nuclear SRIF, thereby initially quenching and then releasing multiphasic GH volleys (recurrent every 3-3.5 h); 2) SRIF delivered to the anterior pituitary gland competitively antagonizes exocytotic release, but not synthesis, of GH during intervolley intervals; 3) arcuate-nucleus GHRH pulses drive the synthesis and accumulation of GH in saturable somatotrope stores; and 4) a purely intrahypothalamic mechanism sustains high-frequency GH pulses (intervals of 30-60 min) within a volley, assuming short-latency reciprocal coupling between GHRH and SRIF neurons (stimulatory direction) and SRIF and GHRH neurons (inhibitory direction). This two-oscillator formulation explicates (but does not prove) 1) the GHRH-sensitizing action of prior SRIF exposure; 2) a three-site (intrahypothalamic, hypothalamo-pituitary, and somatotrope GH store dependent) mechanism driving rebound-like GH secretion after SRIF withdrawal in the male; 3) an obligatory role for pituitary GH stores in representing rebound GH release in the female; 4) greater irregularity of SRIF than GH release profiles; and 5) a basis for the paradoxical GH-inhibiting action of centrally delivered GHRH.

Control of LH secretory-burst frequency and interpulse-interval regularity in women

Hypothalamic neurons generate discrete bursts of gonadotropin-releasing hormone (GnRH) and thereby pulses of luteinizing hormone (LH) at randomly timed intervals centered on a probabilistic mean frequency. We tested the hypothesis that physiological mechanisms govern not only the number but also the stochastic dispersion of the GnRH/LH pulse-renewal process in humans; for example, in young women in the early (EF) and late (LF) follicular and midluteal (ML) phases of the menstrual cycle (n = 18) and in postmenopausal individuals (PM, n = 16). To this end, we quantify stochastic interpulse variability by way of the order-independent, two-parameter Weibull renewal process (Keenan DM and Veldhuis J. Am J Physiol Regul Integr Comp Physiol 281: R1917-R1924, 2001) and the sequence-specific, model-free approximate-entropy statistic (ApEn) (Pincus SM. Proc Natl Acad Sci USA 88: 2297-2301, 1991). Statistical testing unveiled 1) reduced probabilistic mean LH secretory-burst frequency (lower lambda of the Weibull distribution) in ML compared with each of EF, LF, and PM (P < 0.001); 2) quantifiably more regular LH interburst-interval sets (elevated gamma of the Weibull density) in PM than in each of EF, LF, and ML (P < 0.01); 3) uniquely prolonged latency to maximal LH secretion within individual secretory bursts in ML (P < 0.01); and 4) comparably mean random, sequential LH interburst-interval and mass values (normalized ApEn) among the distinct hormonal milieus. From these data, we postulate that sex steroids and age determine daily LH secretory-burst number, quantifiable pulse-renewal variability, and secretory-waveform evolution.

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.

Physiological control of pituitary hormone secretory-burst mass, frequency, and waveform: a statistical formulation and analysis

The present study investigates the time-varying control of pituitary hormone secretion over the day and night (D/N). To this end, we implemented an analytical platform designed to reconstruct simultaneously 1) basal (nonpulsatile) secretion, 2) single or dual secretory-burst waveforms, 3) random effects on burst amplitude, 4) stochastic pulse-renewal properties, 5) biexponential elimination kinetics, and 6) experimental uncertainty. The statistical solution is conditioned on a priori pulse-onset times, which are estimated in the first stage. Primary data composed of thyrotropin (TSH) concentrations were monitored over 24 h in 27 healthy adults. According to statistical criteria, 21/27 profiles favored a dual compared with single secretory-burst waveform. An objectively defined waveform change point (D/N boundary) emerged at 2046 (+/-23 min), after which 1) the mass of TSH released per burst increases by 2.1-fold (P < 0.001), 2) TSH secretory-burst frequency rises by 1.2-fold (P < 0.001), 3) the latency to maximal TSH secretion within a burst decreases by 67% (P < 0.001), 4) variability in secretory-burst shape diminishes by 50% (P < 0.001), and 5) basal TSH secretion declines by 17% (P < 0.002). In contrast, the regularity of successive burst times and the slow-phase half-life are stable. In conclusion, nycthemeral mechanisms govern TSH secretory-burst mass, frequency, waveform, and variability but not evidently TSH elimination kinetics or the pulse-timing process. Further studies will be required to assess the generality of the foregoing distinctive control mechanisms in other hypothalamo-pituitary axes.

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.

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.

Somatotropic axis in hypocretin-deficient narcoleptic humans: altered circadian distribution of GH-secretory events

Narcolepsy is a sleep disorder caused by impaired hypocretin (orexin) neurotransmission. Growth hormone (GH) secretion may be altered in narcolepsy for various reasons. Slow-wave sleep episodes, which are closely associated with GH-secretory events, are more randomly dispersed over 24 h in narcoleptics. Furthermore, hypocretins may inhibit pituitary GH release. We assessed the function of the somatotropic axis in narcolepsy by deconvolving 24-h (10-min sampling interval) plasma GH concentration profiles in seven hypocretin-deficient narcoleptic patients and in seven healthy controls matched for age, sex, and body weight. Both basal and pulsatile GH secretion rate and secretagogue-induced GH release were similar in patients and controls. However, narcoleptics secreted approximately 50% of their total production during the daytime, whereas controls secreted only 25% during the day. Also, the GH output pattern of narcoleptics was significantly less regular. We propose that hypocretin deficiency disrupts the circadian distribution of hypothalamic GH-releasing hormone release in narcoleptic patients to simultaneously cause daytime GH release and promote their propensity to fall asleep during the day.

Impact of pulsatility on the ensemble orderliness (approximate entropy) of neurohormone secretion

Regular patterns of neurohormone secretion are driven by underlying pulsatile and subordinate (feedback sensitive) dynamics. Measures of time-series orderliness, e.g., the approximate entropy (ApEn) statistic (Pincus SM. Proc Natl Acad Sci 88: 2297-2301, 1991), vividly discriminate pathological and physiological patterns of hormone release. To investigate how specific pulsatility features impact regularity estimates, we have examined the sensitivity of the ApEn metric to systematic variations in the frequency, amplitude, and half-life of simulated neurohormone pulse trains (Veldhuis JD, Carlson ML, and Johnson ML. Proc Natl Acad Sci 84: 7686-7690, 1987) and compared the impact of a high vs. low baseline luteinizing hormone (LH) pattern regularity state mimicking the normal female luteal phase and the young male, respectively. Shortening the interpulse interval length elevated ApEn in both pulsatility models, thereby signifying greater ensemble series irregularity. The frequency sensitivity of ApEn was robust to several complementary renditions of ApEn and to variations in experimental uncertainty, basal (nonpulsatile) LH secretion, and secretory burst amplitude. ApEn rose with increasing hormone half-life, especially in the face of low baseline variability emulated by midluteal LH secretion profiles. High variability of secretory burst amplitude, pulse duration, or interpeak intervals increased ApEn in the more orderly femalelike construct; in the highly irregular malelike LH pulse model, these variability changes had little effect on ApEn. In summary, the ensemble regularity statistic, ApEn, quantifies unequal pattern orderliness in neurohormone pulse trains with minimal dependence on mean pulse amplitude, interpulse baseline, or (subthreshold) sample uncertainty. Thus ApEn monitors changing secretory event frequency and interpulse variability with sensitivity to starting pattern regularity, providing a mechanistic linkage between model evolution and statistical change.