Divergent gonadotropin-gonadal dose-responsive coupling in healthy young and aging men

Sleep, testosterone and cortisol balance, and ageing men

Sleep serves important biological functions, and influences health and longevity through endocrine and metabolic related systems. Sleep debt, circadian misalignment and sleep disruption from obstructive sleep apnea is widespread in modern society and accumulates with life because recovery sleep is not completely restorative. Accumulated disordered sleep throughout life impacts the ageing process and the development of age-related diseases. When epidemiological and interventional studies are considered collectively, sleep loss and lower sleep duration are associated with lower morning, afternoon and 24-h testosterone; as well as higher afternoon, but not morning or 24-h cortisol. These reciprocal changes imbalances anabolic-catabolic signaling because testosterone and cortisol are respectively the main anabolic and catabolic signals in man. Fixing testosterone-cortisol balance by means of a novel dual-hormone clamp mitigates the induction of insulin resistance by sleep restriction and provided the first proof-of-concept that the metabolic harm from sleep loss can be ameliorated by approaches that do not require sleeping more. Obstructive sleep apnea is associated with lower testosterone, even after controlling for age and obesity whereas the conclusion that continuous positive airway pressure therapy has no effect on testosterone is premature because available studies are underpowered and better-quality studies suggest otherwise. High dose testosterone therapy induces OSA, but more physiological dosing may not; and this effect may be transient or may dissipate with longer term therapy. Studies investigating the origin of the diurnal testosterone rhythm, the effect of circadian misalignment on testosterone-cortisol balance, and methods to mitigate metabolic harm, are required.

First-in-human trial assessing the pharmacokinetic-pharmacodynamic profile of a novel recombinant human chorionic gonadotropin in healthy women and men of reproductive age

The purpose of this first-in-human trial was to examine the safety, pharmacokinetics (PK), and pharmacodynamics (PD) of a novel recombinant human chorionic gonadotropin (rhCG; FE 999302, choriogonadotropin beta) to support its clinical development for various therapeutic indications. The single and multiple dose PK of choriogonadotropin beta (CG beta) were evaluated in women and the single dose PK and PD of CG beta were compared to those of CG alfa in men. CG beta was safe and well-tolerated in all 84 healthy subjects. In women, the area under the curve (AUC) and the peak serum concentration (C_max ) increased approximately dose proportionally following single and multiple doses of CG beta. The apparent clearance (CL/F) was ~ 0.5 L/h, the mean terminal half-life (t_½ ) ~ 45 h and the apparent distribution volume (V_z /F) ~ 30 L. After single administration in men, the mean AUC was 1.5-fold greater for CG beta than for CG alfa. Mean C_max and V_z /F were comparable for the 2 preparations. In accordance with the differences in AUC, the CL/F was lower for CG beta (CL/F 0.5 vs. 0.8 L/h), explained by a longer t_½ (47 vs. 32 h). Serum testosterone levels induced by a single dose rhCG reflected the PK profiles with a slight delay, resulting in 59% higher AUC for CG beta. The PK parameters for CG beta were comparable in men and in women. In conclusion, the PK differs between the two rhCG preparations, causing higher exposure and a higher PD response for CG beta, which may require relatively lower therapeutic doses.

Impact of age, sex and body mass index on cortisol secretion in 143 healthy adults

CONTEXT

Studies on 24-h cortisol secretion are rare. The impact of sex, age and adiposity on cortisol levels, often restricted to one or a few samples, are well recognized, but conflicting.

OBJECTIVE

To investigate cortisol dynamics in 143 healthy men and women, spanning 7 decades and with a 2-fold body mass index (BMI) range with different analytic tools.

SETTING

Clinical Research Unit.

DESIGN

Cortisol concentrations in 10-min samples collected for 24 h. Outcomes were mean levels, deconvolution parameters, approximate entropy (ApEn, regularity statistic) and 24-h rhythms.

RESULTS

Total 24-h cortisol secretion rates estimated by deconvolution analysis were sex, age and BMI independent. Mean 24-h cortisol concentrations were lower in premenopausal women than those in men of comparable age (176 ± 8.2 vs 217 ± 9.4 nmol/L, P = 0.02), but not in subjects older than 50 years. This was due to lower daytime levels in women, albeit similar in the quiescent overnight period. Aging increased mean cortisol by 10 nmol/L per decade during the quiescent secretory phase and advanced the acrophase of the diurnal rhythm by 24 min/decade. However, total 24-h cortisol secretion rates estimated by deconvolution analysis were sex, age and BMI independent. ApEn of 24-h profiles was higher (more random) in premenopausal women than those in men (1.048 ± 0.025 vs 0.933 ± 0.023, P = 0.001), but not in subjects older than 50 years. ApEn peaked during the daytime.

CONCLUSION

Sex and age jointly determine the 24-h cortisol secretory profile. Sex effects are largely restricted to age <50 years, whereas age effects elevate concentrations in the late evening and early night and advance the timing of the peak diurnal rhythm.

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.

Clinical management of infertile men with nonobstructive azoospermia

The clinical management of men with nonobstructive azoospermia (NOA) seeking fertility has been a challenge for andrologists, urologists, and reproductive medicine specialists alike. This review presents a personal perspective on the clinical management of NOA, including the lessons learned over 15 years dealing with this male infertility condition. A five-consecutive-step algorithm is proposed to manage such patients. First, a differential diagnosis of azoospermia is made to confirm/establish that NOA is due to spermatogenic failure. Second, genetic testing is carried out not only to detect the males in whom NOA is caused by microdeletions of the long arm of the Y chromosome, but also to counsel the affected patients about their chances of having success in sperm retrieval. Third, it is determined whether any intervention prior to a surgical retrieval attempt may be used to increase sperm production. Fourth, the most effective and efficient retrieval method is selected to search for testicular sperm. Lastly, state-of-art laboratory techniques are applied in the handling of retrieved gametes and cultivating the embryos resulting from sperm injections. A coordinated multidisciplinary effort is key to offer the best possible chance of achieving a biological offspring to males with NOA.

Testosterone and Cardiovascular Disease

Cardiovascular disease [CVD] is a leading cause of mortality accounting for a global incidence of over 31%. Atherosclerosis is the primary pathophysiology underpinning most types of CVD. Historically, modifiable and non-modifiable risk factors were suggested to precipitate CVD. Recently, epidemiological studies have identified emerging risk factors including hypotestosteronaemia, which have been associated with CVD. Previously considered in the realms of reproductive biology, testosterone is now believed to play a critical role in the cardiovascular system in health and disease. The actions of testosterone as they relate to the cardiac vasculature and its implication in cardiovascular pathology is reviewed.

Noninvasive analytical estimation of endogenous GnRH drive: analysis using graded competitive GnRH-receptor antagonism and a calibrating pulse of exogenous GnRH

Homeostatic control of endocrine systems proceeds via feedforward (agonistic, stimulatory) and feedback (antagonistic, inhibitory) interactions mediated via implicit dose-response functions. However, neither the feedback/feedforward pathways nor the dose-response interfaces are directly observed in vivo. Thus, the goal was to formulate and estimate an ensemble construct of time-varying feedback/feedforward interactions among GnRH, LH, and testosterone (T) in the male gonadal axis. The new analytical model revises and extends an earlier construct by: 1) allowing systemic T concentrations to inhibit hypothalamic GnRH output; 2) estimating GnRH outflow after injection of a calibrating pulse of biosynthetic GnRH; 3) framing the pituitary response to GnRH as a secretory burst, rather than continuous LH release; and 4) regressing feedback and feedforward ensemble parameters on age, rather than evaluating age dichotomously. Application of this methodology in 21 men aged 23-72 yr unveiled age-related 1) diminution of GnRH efficacy normalized for the decline in free T with age (P = 0.016), 2) potentiation of maximal T feedback onto (inhibition of) GnRH secretion (P = 0.006), and 3) accentuation of hypothalamic GnRH's sensitivity to T repression (P = 0.003). Outcomes were specific, because injected GnRH agonist and antagonist concentrations were invariant of age. We conclude that combining experimental and analytical strategies may provide a noninvasive means to investigate and decipher feedback determinants of unobserved endocrine signal(s).

Analytical construct of reversible desensitization of pituitary-testicular signaling: illustrative application in aging

Luteinizing hormone (LH) administered in pharmacological amounts downregulates Leydig cell steroidogenesis. Whether reversible downregulation of physiological gonadotropin drive operates in vivo is unknown. Most of the analytical models of dose-response functions that have been constructed are biased by the assumption that no downregulation exists. The present study employs a new analytical platform to quantify potential (but not required) pulsatile cycles of LH-testosterone (T) dose-response stimulation, desensitization, and recovery (pulse-by-pulse hysteresis) in 26 healthy men sampled every 10 min for 24 h. A sensitivity-downregulation hysteresis construct predicted marked hysteresis with a median time delay to LH dose-response inflection within individual T pulses of 23 min and with median T pulse onset and recovery LH sensitivities of 1.1 and 0.10 slope unit, respectively (P < 0.001). A potency-downregulation model yielded median estimates of one-half maximally stimulatory LH concentrations (EC(50) values) of 0.66 and 7.5 IU/l for onset and recovery, respectively (P < 0.001). An efficacy-downregulation formulation of hysteresis forecasts median LH efficacies of 20 and 8.3 ng·dl(-1)·min(-1) for onset and offset of T secretory burst, respectively (P = 0.002). Segmentation of the LH-T data by age suggested greater sensitivity, higher EC(50) (increased LH potency), and markedly (2.7-fold) attenuated LH efficacy in older individuals. Each of the three hysteresis models yielded a marked (P < 0.005) reduction in estimated model residual error compared with no hysteresis. In summary, model-based analyses allowing for (but not requiring) reversible pituitary-gonadal effector-response downregulation are consistent with a hypothesis of recurrent, brief cycles of LH-dependent stimulation, desensitization, and recovery of pulsatile T secretion in vivo and an age-associated reduction of LH efficacy. Prospective studies would be required to prove this aging effect.

Reverse engineering of metabolic networks, a critical assessment

Inferring metabolic networks from metabolite concentration data is a central topic in systems biology. Mathematical techniques to extract information about the network from data have been proposed in the literature. This paper presents a critical assessment of the feasibility of reverse engineering of metabolic networks, illustrated with a selection of methods. Appropriate data are simulated to study the performance of four representative methods. An overview of sampling and measurement methods currently in use for generating time-resolved metabolomics data is given and contrasted with the needs of the discussed reverse engineering methods. The results of this assessment show that if full inference of a real-world metabolic network is the goal there is a large discrepancy between the requirements of reverse engineering of metabolic networks and contemporary measurement practice. Recommendations for improved time-resolved experimental designs are given.

Kinetics of removal of intravenous testosterone pulses in normal men

BACKGROUND

Testosterone is secreted into the bloodstream episodically, putatively distributing into total, bioavailable (bio) nonsex hormone-binding globulin (nonSHBG-bound), and free testosterone moieties. The kinetics of total, bio, and free testosterone pulses are unknown. Design Adrenal and gonadal steroidogenesis was blocked pharmacologically, glucocorticoid was replaced, and testosterone was infused in pulses in four distinct doses in 14 healthy men under two different paradigms (a total of 220 testosterone pulses).

METHODS

Testosterone kinetics were assessed by deconvolution analysis of total, free, bioavailable, SHBG-bound, and albumin-bound testosterone concentration-time profiles.

RESULTS

Independently of testosterone dose or paradigm, rapid-phase half-lives (min) of total, free, bioavailable, SHBG-bound, and albumin-bound testosterone were comparable at 1.4+/-0.22 min (grand mean+/-S.E.M. of geometric means). Slow-phase testosterone half-lives were highest for SHBG-bound testosterone (32 min) and total testosterone (27 min) with the former exceeding that of free testosterone (18 min), bioavailable testosterone (14 min), and albumin-bound testosterone (18 min; P<0.001). Collective outcomes indicate that i) the rapid phase of testosterone disappearance from point sampling in the circulation is not explained by testosterone dose; ii) SHBG-bound testosterone and total testosterone kinetics are prolonged; and iii) the half-lives of bioavailable, albumin-bound, and free testosterone are short.

CONCLUSION

A frequent-sampling strategy comprising an experimental hormone clamp, estimation of hormone concentrations as bound and free moieties, mimicry of physiological pulses, and deconvolution analysis may have utility in estimating the in vivo kinetics of other hormones, substrates, and metabolites.

Endocrine pulse identification using penalized methods and a minimum set of assumptions

The detection of hormone secretion episodes is important for understanding normal and abnormal endocrine functioning, but pulse identification from hormones measured with short interval sampling is challenging. Furthermore, to obtain useable results, the model underlying hormone secretion and clearance must be augmented with restrictions based on biologically acceptable assumptions. Here, using the assumption that there are only a few time points at which a hormone is secreted, we used a modern penalized nonlinear least-squares setup to select the number of secretion events. We did not assume a particular shape or frequency distribution for the secretion pulses. Our pulse identfication method, VisPulse, worked well with luteinizing hormone (LH), cortisol, growth hormone, or testosterone. In particular, applying our modeling strategy to previous LH data revealed a good correlation between the modeled and measured LH hormone concentrations, the estimated secretion pattern was sparse, and the small and structureless residuals indicated a proper model with a good fit. We benchmarked our method to AutoDecon, a commonly used hormone secretion model, and performed releasing hormone infusion experiments. The results of these experiments confirmed that our method is accurate and outperforms AutoDecon, especially for detecting silent periods and small secretion events, suggesting a high-secretion event resolution. Method validation using (releasing hormone) infusion data revealed sensitivities and selectivities of 0.88 and 0.95 and of 0.69 and 0.91 for VisPulse and AutoDecon, respectively.

Age-dependent regression analysis of male gonadal axis

The mechanisms by which aging progressively depletes testosterone (Te) availability in the male are unknown. Accordingly, the objective was to estimate brain gonadotropin-releasing hormone (GnRH) outflow (release and action), which cannot be observed directly, on the basis of downstream effects on pituitary luteinizing hormone (LH) secretion. LH, in turn, feeds forward on (stimulates) gonadal Te secretion, which then feeds back on (inhibits) GnRH-driven LH secretion. LH and Te concentrations were measured repetitively (every 10 min) over 18 h during graded pharmacological blockade of endogenous GnRH outflow in 24 healthy 20- to 72-yr-old men. Data were analyzed using a new age-dependent regression model of GnRH-LH-Te interactions to estimate pulsatile LH secretion and elimination, GnRH outflow, LH feedforward, and Te feedback. By incorporating regression on age within the dose-response model, we show that aging erodes all three primary forward and reverse pathways linking the brain, pituitary gland, and testes. Aging is associated with concomitant deficits in GnRH --> LH feedforward, LH --> Te feedforward, and Te --> GnRH/LH feedback. The analytical formalism should be generalizable to other ensemble regulatory systems, such as those that control growth, reproduction, stress adaptations, and glucose metabolism.

Sex defines the age dependence of endogenous ACTH-cortisol dose responsiveness

Sex influences adrenal glucocorticoid responses to ACTH in experimental animals. Whether similar sex differences operate in humans is unknown. To test this notion, we estimated ACTH-cortisol dose-response properties analytically in 48 healthy adults (n = 22 women, n = 26 men), ages 18-77 yr, body mass index (BMI) 18-32 kg/m(2), previously studied at two medical centers. Plasma ACTH and cortisol concentrations were measured every 10 min for 24 h. The 145 sample pairs were used in each subject to estimate ACTH-cortisol drive via a logistic function. Statistical analyses revealed that 24-h cortisol secretion (>82% pulsatile) fell in men (r = -0.38, P = 0.028) and rose in women (r = +0.37, P = 0.045) with age (P = 0.01 sex effect). The mechanisms involved decreased ACTH efficacy with age in men (r = -0.35, P = 0.04), and increased ACTH efficacy with age in women (r = +0.42, P = 0.025) [P = 0.009 sex effect]. ACTH potency diminished with higher BMI in men (r = +0.38, P = 0.029) and in the cohort as a whole (r = 0.34, P = 0.0085). These outcomes demonstrate that sex, age, and BMI modulate selective properties of endogenous ACTH-cortisol drive in humans, thereby indicating the need to control these three major variables in experimental comparisons.

The aging male hypothalamic-pituitary-gonadal axis: pulsatility and feedback

Aging results in insidious decremental changes in hypothalamic, pituitary and gonadal function. The foregoing three main anatomic loci of control are regulated by intermittent time-delayed signal exchange, principally via gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH) and testosterone/estradiol (Te/E(2)). A mathematical framework is required to embody these dynamics. The present review highlights integrative adaptations in the aging male hypothalamic-pituitary-gonadal axis, as assessed by recent objective ensemble models of the axis as a whole.

Leydig cell aging and the mechanisms of reduced testosterone synthesis

In males, serum testosterone levels decline with advancing age. Though part of a complex process, this age-related decline in testosterone appears to occur, in part, due to a significant decline in the ability of aged Leydig cells to produce testosterone maximally in response to luteinizing hormone (LH). The structure of the molecular machinery responsible for the synthesis of testosterone is described, and placed in the context of Leydig cell biology. Multiple parameters related to the synthesis of testosterone by the Leydig cell have been observed to change with age. Relationships among these changes are reviewed. A discussion of potential causes of the age-related decline in Leydig cell steroidogenic capacity presents a model in which the inability of aged cells to adequately respond to hormonal stimulation results in cellular regression with concomitant decline in maximal testosterone output.

Motivations and methods for analyzing pulsatile hormone secretion

Endocrine glands communicate with remote target cells via a mixture of continuous and intermittent signal exchange. Continuous signaling allows slowly varying control, whereas intermittency permits large rapid adjustments. The control systems that mediate such homeostatic corrections operate in a species-, gender-, age-, and context-selective fashion. Significant progress has been made in understanding mechanisms of adaptive interglandular signaling in vivo. Principal goals are to understand the physiological origins, significance, and mechanisms of pulsatile hormone secretion. Key analytical issues are: 1) to quantify the number, size, shape, and uniformity of pulses, nonpulsatile (basal) secretion, and elimination kinetics; 2) to evaluate regulation of the axis as a whole; and 3) to reconstruct dose-response interactions without disrupting hormone connections. This review will focus on the motivations driving and the methodologies used for such analyses.

Cross-talk between G protein-coupled and epidermal growth factor receptors regulates gonadotropin-mediated steroidogenesis in Leydig cells

Gonadal steroid production is stimulated by gonadotropin binding to G protein-coupled receptors (GPCRs). Although GPCR-mediated increases in intracellular cAMP are known regulators of steroidogenesis, the roles of other signaling pathways in mediating steroid production are not well characterized. Recent studies suggest that luteinizing hormone (LH) receptor activation leads to trans-activation of epidermal growth factor (EGF) receptors in the testes and ovary. This pathway is critical for LH-induced steroid production in ovarian follicles, probably through matrix metalloproteinase (MMP)-mediated release of EGF receptor (EGFR) binding ectodomains. Here we examined LH and EGF receptor cross-talk in testicular steroidogenesis using mouse MLTC-1 Leydig cells. We demonstrated that, similar to the ovary, trans-activation of the EGF receptor was critical for gonadotropin-induced steroid production in Leydig cells. LH-induced increases in cAMP and cAMP-dependent protein kinase (PKA) activity mediated trans-activation of the EGF receptor and subsequent mitogen-activated protein kinase (MAPK) activation, ultimately leading to StAR phosphorylation and mitochondrial translocation. Steroidogenesis in Leydig cells was unaffected by MMP inhibitors, suggesting that cAMP and PKA trans-activated EGF receptors in an intracellular fashion. Interestingly, although cAMP was always needed for steroidogenesis, the EGFR/MAPK pathway was activated and necessary only for early (30-60 min), but not late (120 min or more), LH-induced steroidogenesis in vitro. In contrast, 36-h EGF receptor inhibition in vivo significantly reduced serum testosterone levels in male mice, demonstrating the physiologic importance of this cross-talk. These results suggest that GPCR-EGF receptor cross-talk is a conserved regulator of gonadotropin-induced steroidogenesis in the gonads, although the mechanisms of EGF receptor trans-activation may vary.

Aging and hormones of the hypothalamo-pituitary axis: gonadotropic axis in men and somatotropic axes in men and women

Neuroendocrinology of the aging (male) gonadal and (male and female) somatotropic axes will be reviewed. A companion chapter discusses reproductive hormonal changes in aging women. Both the gonadal and growth-hormone/insulin-like growth factor (GH/IGF-I) axes function as ensembles. The ensembles comprise tripartite interactions among the brain (hypothalamus), anterior pituitary gland (gonadotrope and somatotrope cells) and target organs (testis, liver, muscle, fat and brain). Compelling evidence indicates that combined hypothalamic and gonadal adaptations operate in the reproductive axis of older men, and multiple hypothalamic adaptations prevail in the GH axis of elderly men and women. Evolving investigative methods allow more precise parsing of the particular mechanisms that subserve such age-related changes, and suggest novel interventional strategies to evaluate the physiological impact of the dynamic alterations discerned in aging individuals.

Age attenuates testosterone secretion driven by amplitude-varying pulses of recombinant human luteinizing hormone during acute gonadotrope inhibition in healthy men

CONTEXT

Whether testosterone (Te) depletion in aging men reflects deficits in the testis, hypothalamus, and/or pituitary gland is unknown.

OBJECTIVE

Our objective was to quantify the impact of age on gonadal Te secretion driven by amplitude-varying pulses of recombinant human LH (rhLH) in the absence of confounding by endogenous hypothalamo-pituitary signals.

DESIGN

This was a double-blind, placebo-controlled study.

SETTING

The setting was an academic medical center.

SUBJECTS

Fifteen healthy community-dwelling men ages 22-78 yr were included in the study.

INTERVENTION

Saline or four separate rhLH doses were each infused twice iv in randomized order as one pulse every 2 h over 20 h to stimulate Te secretion, after LH secretion was suppressed by a GnRH-receptor antagonist, ganirelix.

MAIN OUTCOME

LH and Te concentrations were determined in blood samples collected every 5 min. Maximal and minimal (as well as mean) Te responses were regressed linearly on age to reflect LH peak and nadir (and average) effects, respectively.

RESULTS

The ganirelix/rhLH paradigm yielded serum LH concentrations of 4.6 +/- 0.22 IU/liter (normal range 1-9). By regression analysis, age was associated with declines in rhLH pulse-stimulated peak and nadir (and mean) concentrations of total Te (P = 0.0068), bioavailable Te (P = 0.0096), and free Te (P = 0.013), as well as lower Te/LH concentration ratios (P < 0.005). Deconvolution analysis suggested that the half-life of infused LH increases by 12%/decade (P = 0.044; R(2) = 0.28).

CONCLUSIONS

Infusion of amplitude-varying pulses of rhLH during gonadal-axis suppression in healthy men unmasks prominent age-related deficits in stimulated total (39%), bioavailable (66%), and free (63%) Te concentrations, and a smaller age-associated increase in LH half-life. These data suggest that age-associated factors reduce the efficacy of LH pulses.

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.

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.

Age-specific changes in the regulation of LH-dependent testosterone secretion: assessing responsiveness to varying endogenous gonadotropin output in normal men

Pulsatile and thus total testosterone (Te) secretion declines in older men, albeit for unknown reasons. Analytical models forecast that aging may reduce the capability of endogenous luteinizing hormone (LH) pulses to stimulate Leydig cell steroidogenesis. This notion has been difficult to test experimentally. The present study used graded doses of a selective gonadotropin releasing hormone (GnRH)-receptor antagonist to yield four distinct strata of pulsatile LH release in each of 18 healthy men ages 23-72 yr. Deconvolution analysis was applied to frequently sampled LH and Te concentration time series to quantitate pulsatile Te secretion over a 16-h interval. Log-linear regression was used to relate pulsatile LH secretion to attendant pulsatile Te secretion (LH-Te drive) across the four stepwise interventions in each subject. Linear regression of the 18 individual estimates of LH-Te feedforward dose-response slopes on age disclosed a strongly negative relationship (r = -0.721, P < 0.001). Accordingly, the present data support the thesis that aging in healthy men attenuates amplitude-dependent LH drive of burst-like Te secretion. The experimental strategy of graded suppression of neuroglandular outflow may have utility in estimating dose-response adaptations in other endocrine systems.

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.

Age diminishes the testicular steroidogenic response to repeated intravenous pulses of recombinant human LH during acute GnRH-receptor blockade in healthy men

Testosterone (Te) concentrations fall gradually in healthy aging men. Postulated mechanisms include relative failure of gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), and/or gonadal Te secretion. Available methods to test Leydig cell Te production include pharmacological stimulation with human chorionic gonadotropin (hCG). We reasoned that physiological lutropic signaling could be mimicked by pulsatile infusion of recombinant human (rh) LH during acute suppression of LH secretion. To this end, we studied eight young (ages 19-30 yr) and seven older (ages 61-73 yr) men in an experimental paradigm comprising 1) inhibition of overnight LH secretion with a potent selective GnRH-receptor antagonist (ganirelix, 2 mg sc), 2) intravenous infusion of consecutive pulses of rh LH (50 IU every 2 h), and 3) chemiluminometric assay of LH and Te concentrations sampled every 10 min for 26 h. Statistical analyses revealed that 1) ganirelix suppressed LH and Te equally (> 75% median inhibition) in young and older men, 2) infused LH pulse profiles did not differ by age, and 3) successive intravenous pulses of rh LH increased concentrations of free Te (ng/dl) to 4.6 +/- 0.38 (young) and 2.1 +/- 0.14 (older; P < 0.001) and bioavailable Te (ng/dl) to 337 +/- 20 (young) and 209 +/- 16 (older; P = 0.002). Thus controlled pulsatile rh LH drive that emulates physiological LH pulses unmasks significant impairment of short-term Leydig cell steroidogenesis in aging men. Whether more prolonged pulsatile LH stimulation would normalize this inferred defect is unknown.

Analysis of bidirectional pattern synchrony of concentration-secretion pairs: implementation in the human testicular and adrenal axes

The hypothalamo-pituitary-testicular and hypothalamo-pituitary-adrenal axes are prototypical coupled neuroendocrine systems. In the present study, we contrasted in vivo linkages within and between these two axes using methods without linearity assumptions. We examined 11 young (21–31 yr) and 8 older (62–74 yr) men who underwent frequent (every 2.5 min) blood sampling overnight for paired measurement of LH and testosterone and 35 adults (17 women and 18 men; 26–77 yr old) who underwent adrenocorticotropic hormone (ACTH) and cortisol measurements every 10 min for 24 h. To mirror physiological interactions, hormone secretion was first deconvolved from serial concentrations with a waveform-independent biexponential elimination model. Feedforward synchrony, feedback synchrony, and the difference in feedforward-feedback synchrony were quantified by the cross-approximate entropy (X-ApEn) statistic. These were applied in a forward (LH concentration template, examining pattern recurrence in testosterone secretion), reverse (testosterone concentration template, examining pattern recurrence in LH secretion), and differential (forward minus reverse) manner, respectively. Analogous concentration-secretion X-ApEn estimates were calculated from ACTH-cortisol pairs. X-ApEn, a scale- and model-independent measure of pattern reproducibility, disclosed 1) greater testosterone-LH feedback coordination than LH-testosterone feedforward synchrony in healthy men and significant and symmetric erosion of both feedforward and feedback linkages with aging; 2) more synchronous ACTH concentration-dependent feedforward than feedback drive of cortisol secretion, independent of gender and age; and 3) enhanced detection of bidirectional physiological regulation by in vivo pairwise concentration-secretion compared with concentration-concentration analyses. The linking of relevant biological input to output signals and vice versa should be useful in the dissection of the reciprocal control of neuroendocrine systems or even in the analysis of other nonendocrine networks.

Erosion of endogenous testosterone-driven negative feedback on pulsatile luteinizing hormone secretion in healthy aging men

The present study tests the intuition that successful aging in men is marked by: 1) impaired feedforward by endogenous LH concentrations (con) of testosterone (Te) secretion (sec); and/or 2) attenuated feedback by unmanipulated Te con of LH sec. The goal was to assess both implicit linkages analytically without disrupting normal pathway coupling. This strategy required: 1) assay of paired LH and Te con sampled every 10 min for 24 h in 13 older (O) (ages 60-78 yr) and 13 young (Y) (ages 18-30 yr) men; 2) deconvolution-based estimation of LH and Te sec rates; 3) lag-specific cross-correlation analyses of the relationships between LH and Te con and sec; and 4) statistical contrasts by age stratum. Salient outcomes were: 1) O and Y men maintain comparable LH con drive of Te sec, viz maximal r = +0.51 and r = +0.52, respectively, at an optimal time lag of 50 min (both P < 0.001 against random LH and Te associations); 2) elderly subjects exhibit reduced Te con inhibition of LH sec [minimal r = -0.008 (O) vs. r = -0.10 (Y), P < 0.01 at a time lag of 40 min]; 3) mean (24-h) LH con do not differ by age; and 4) molar Te/sex hormone-binding globulin con are lower in the elderly than in Y individuals (P < 0.01).In conclusion, noninvasive analyses predict that attenuation of endogenous Te feedback restraint on the hypothalamo-pituitary unit may be an early biological marker of adaptive changes in the GnRH-LH-Te ensemble axis in the healthy O male.

Endogenous ACTH concentration-dependent drive of pulsatile cortisol secretion in the human

According to current regulatory concepts, pulsatile ACTH concentrations (CON) stimulate time-lagged cortisol secretion rates (SEC) via an implicit CON-SEC dose-response relationship. The present analyses reconstruct nonlinear properties of this in vivo agonist-response interface noninvasively in order to investigate pulse-by-pulse coupling consistency and to obviate the need to infuse isotopes or exogenous effectors, which may disrupt pathway interactions. This approach required an ensemble strategy of 1) measuring ACTH and cortisol CON in plasma sampled every 10 min for 24 h in 32 healthy adults, and 2) estimating simultaneously a) variable-waveform ACTH and cortisol SEC bursts superimposed upon fixed basal SEC; b) biexponential kinetics of ACTH and cortisol disappearance; c) nonequilibrium exchange of cortisol among free and cortisol-binding globulin (CBG)- and albumin-bound moieties; d) two SEC-burst shapes demarcated by a statistically defined day/night boundary; e) feedforward efficacy, potency, and sensitivity; and f) stochastic variability in feedforward measures over time. Thereby, we estimate 1) ACTH SEC (microg.l(-1).day(-1)) of 0.27 +/- 0.04 basal and 0.87 +/- 0.07 pulsatile (means +/- SE); 2) cortisol SEC (micromol.l(-1).day(-1)) of 0.10 +/- 0.01 basal and 3.5 +/- 0.20 pulsatile; 3) free cortisol half-lives (min) of 1.8 +/- 0.20 (diffusion/advection) and 4.1 +/- 0.30 (elimination) and a half-life of total cortisol of 49 +/- 2.4 and of ACTH of 20 +/- 1.3; 4) ACTH potency (EC(50), ng/l) of 26 +/- 2.4, efficacy (nmol.l(-1).min(-1)) 10 +/- 1.8, and sensitivity (slope units) 0.65 +/- 0.09; 5) night/day augmentation of ACTH and cortisol SEC-burst mass by 2.1- and 1.7-fold (median); 6) abbreviation of the modal time to maximal ACTH and cortisol SEC rates by 4.4- and 4.3-fold, respectively, after a change point clock time of 0205 (median); 7) in vivo percentage distribution of cortisol as 6% free, 14% albumin bound, and 80% CBG bound with an absolute free cortisol CON (nmol/l) 11.5 +/- 0.54; and 8) significant (mean CV) stochastic variability in feedforward efficacy (140%), potency (38%), and sensitivity (56%) within the succession of paired ACTH/cortisol pulses of any given subject. In conclusion, the present composite formulation illustrates a platform for dissecting mechanisms of in vivo regulation of effector-response properties noninvasively in the corticotropic axis of the uninfused individual.