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

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.

Changes in structural and metabolic muscle characteristics following exercise-based interventions in patients with COPD: a systematic review

Patients with COPD suffer from lower-limb muscle dysfunction characterized by lower muscle oxidative capacity and muscle mass. Exercise-based training is expected to attenuate lower-limb intramuscular characteristics, but a detailed systematic approach to review the available evidence has not been performed yet. PUBMED and PEDro databases were searched. Twenty-five studies that implemented an exercise-based training program (aerobic and/or resistance training, high intensity interval training, electrical or magnetic stimulation) and reported muscle biopsy data of patients with COPD were critically appraised. The coverage of results includes changes in muscle structure, muscle protein turnover regulation, mitochondrial enzyme activity, oxidative and nitrosative stress, and inflammation after exercise-based training interventions. Study design and training modalities varied among studies, which partly explains the observed heterogeneous response in muscle characteristics. Gaps in the current knowledge are identified and recommendations for future research are made to enhance our knowledge on exercise training effects in patients with COPD.

Growth Hormone Dynamics in Healthy Adults Are Related to Age and Sex and Strongly Dependent on Body Mass Index

BACKGROUND

Studies on 24-hour growth hormone (GH) secretion are rare. The influences of sex, age, and adiposity are well recognized but generally derived from specific, selected subject groups, not spanning sexes, many age decades, and a range of body weights.

OBJECTIVE

Our goal was to investigate GH dynamics in a group of 130 healthy adult subjects, both men and women, across 5 age decades as well as a 2.5-fold range of body mass index (BMI) values.

METHODS

GH was measured by a sensitive immunofluorometric assay. Secretion parameters were quantified by automated deconvolution and relative pattern randomness by approximate entropy (ApEn).

RESULTS

The median age was 40 years (range 20-77). The median BMI was 26 (range 18.3-49.8). Pulsatile 24-hour GH secretion was negatively correlated with age (p = 0.002) and BMI (p < 0.0001). Basal GH secretion negatively correlated with BMI (p = 0.003) but not with age. The sex- dependent GH secretion (greater in women) was no longer detectable after 50 years of age. Insulin-like growth factor (IGF)-1 levels were lower in women over 50 years of age compared with men of a similar age. ApEn showed an age-related increase in both sexes and was higher in premenopausal and postmenopausal women than in men of comparable age (p < 0.0001). A single fasting GH measurement is not informative of 24-hour GH secretion.

CONCLUSIONS

BMI dominates the negative regulation of 24-hour GH secretion across 5 decades of age in this up till now largest cohort of healthy adults who underwent 24-hour blood sampling. Sex also impacts GH secretion before the age of 50 years as well as its regularity at all ages. Differences in serum IGF-1 partly depend on the pre- or postmenopausal state. Finally, a single GH measurement is not informative of 24-hour GH secretion.

Type I insulin-like growth factor as a liver reserve assessment tool in hepatocellular carcinoma

Chronic liver diseases (CLDs) encompass a wide range of illnesses, including nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, and viral hepatitis. Deterioration of liver capacity, with subsequent progression into cirrhosis and hepatocellular carcinoma (HCC), ultimately leads to a further decrease in the hepatic reserve. The Child-Turcotte-Pugh scoring system is the standard tool for assessing underlying liver reserve capacity in routine practice and in clinical trials of CLD and HCC. In this review, we highlight the clinical significance of insulin-like growth factor-I (IGF-I) and the growth hormone (GH) signaling pathway in HCC. IGF-I could be a marker for liver reserve capacity in CLDs and HCC in clinical practice. This approach could improve the risk assessment and stratifications of patients on the basis of their underlying liver reserve, either before active treatment in routine practice or before they are enrolled in clinical trials.

Use of localized human growth hormone and testosterone injections in addition to manual therapy and exercise for lower back pain: a case series with 12-month follow-up

Objective

The objective of this case series was to investigate the feasibility and safety of a novel method for the management of chronic lower back pain. Injections of recombinant human growth hormone and testosterone to the painful and dysfunctional areas in individuals with chronic lower back pain were used. In addition, the participants received manual therapies and exercise addressing physical impairments such as motor control, strength, endurance, pain, and loss of movement. Pain ratings and self-rated functional outcomes were assessed.

Study design

This is a case series involving consecutive patients with chronic lower back pain who received the intervention of injections of recombinant human growth hormone and testosterone, and attended chiropractic and/or physical therapy. Outcomes were measured at 12 months from the time of injection.

Setting

A community based hospital affiliated office, and a private practice block suite.

Participants

A total of 60 consecutive patients attending a pain management practice for chronic lower back pain were recruited for the experimental treatment. Most participants were private pay.

Interventions

Participants who provided informed consent and were determined not to have radicular pain received diagnostic blocks. Those who responded favorably to the diagnostic blocks received injections of recombinant human growth hormone and testosterone in the areas treated with the blocks. Participants also received manipulation- and impairment-based exercises.

Outcome measures

Outcomes were assessed at 12 months through pain ratings with the Mankowski Pain Scale and the Oswestry Disability Index.

Results

Of the 60 patients recruited, 49 provided informed consent, and 39 completed all aspects of the study. Those patients receiving the intervention reported a significant decrease in pain ratings (P<0.01) and a significant improvement in self-rated Oswestry Disability Index scores (P<0.01). In addition, in the Oswestry Disability Index results, 41% of the patients demonstrated a 50% or greater change in their disability score. Of the subjects who withdrew from the study, one was due to the pain created by the injections and nine were for nonstudy factors.

Conclusion

The intervention appeared to be safe and the results provide a reasonable expectation that the intervention would be beneficial for a population of individuals with chronic nonradicular lower back pain. Due to the design of the study, causality cannot be inferred, but the results do indicate that further study of the intervention may be warranted.

Sex steroids, GHRH, somatostatin, IGF-I, and IGFBP-1 modulate ghrelin's dose-dependent drive of pulsatile GH secretion in healthy older men

CONTEXT

Ghrelin is a potent endogenous stimulator of GH secretion. However, clinical factors that regulate ghrelin dose-responsiveness are incompletely defined.

OBJECTIVE

The aim of the study was to test the multipathway hypothesis that testosterone (T) and estradiol, GHRH, and somatostatin (SS) jointly modulate ghrelin's action.

DESIGN/PARTICIPANTS/SETTING

Healthy older men (n = 21) participated in a double-blind, prospectively randomized, placebo (Pl)-controlled study in a Clinical Translational Research Center.

INTERVENTIONS

To create a range of sex-steroid milieus, men received leuprolide + Pl (n = 10) or leuprolide + T addback (n = 11). Sixteen to 21 d later, subjects received three separate randomly ordered overnight constant i.v. infusions of saline, GHRH, and SS. Interactions between the peptide clamp and ghrelin were tested by superimposed injections of four randomly ordered bolus i.v. doses of ghrelin (0.03, 0.135, 0.60, and 2.7 μg/kg). GH was measured every 10 min, and GH responses were assessed by nonlinear dose-response analysis. Linear associations were assessed by stepwise regression.

OUTCOME MEASURES/RESULTS

The descending numerical order of ghrelin efficacy (maximal GH secretory-burst mass; micrograms/liter) was 107 (GHRH + Pl), 104 (GHRH + T), 73 (saline + T), 73 (SS + T), 60 (saline + Pl), and 52 (SS + Pl) [means], wherein SS + T exceeded SS + Pl. GHRH and IGF binding protein-1 augmented, whereas IGF-I attenuated ghrelin potency. Age and IGF-I decreased ghrelin/GHRH synergy. Ghrelin sensitivity was independent of interventions.

CONCLUSIONS

These studies introduce composite regulatory effects of sex hormones, GHRH, SS, IGF binding protein-1, and IGF-I on ghrelin dose-responsiveness, suggesting multipathway modulation of GH-secretagogue action.

Hormone replacement therapy and physical function in healthy older men. Time to talk hormones?

Improving physical function and mobility in a continuously expanding elderly population emerges as a high priority of medicine today. Muscle mass, strength/power, and maximal exercise capacity are major determinants of physical function, and all decline with aging. This contributes to the incidence of frailty and disability observed in older men. Furthermore, it facilitates the accumulation of body fat and development of insulin resistance. Muscle adaptation to exercise is strongly influenced by anabolic endocrine hormones and local load-sensitive autocrine/paracrine growth factors. GH, IGF-I, and testosterone (T) are directly involved in muscle adaptation to exercise because they promote muscle protein synthesis, whereas T and locally expressed IGF-I have been reported to activate muscle stem cells. Although exercise programs improve physical function, in the long-term most older men fail to comply. The GH/IGF-I axis and T levels decline markedly with aging, whereas accumulating evidence supports their indispensable role in maintaining physical function integrity. Several studies have reported that the administration of T improves lean body mass and maximal voluntary strength in healthy older men. On the other hand, most studies have shown that administration of GH alone failed to improve muscle strength despite amelioration of the detrimental somatic changes of aging. Both GH and T are anabolic agents that promote muscle protein synthesis and hypertrophy but work through separate mechanisms, and the combined administration of GH and T, albeit in only a few studies, has resulted in greater efficacy than either hormone alone. Although it is clear that this combined approach is effective, this review concludes that further studies are needed to assess the long-term efficacy and safety of combined hormone replacement therapy in older men before the medical rationale of prescribing hormone replacement therapy for combating the sarcopenia of aging can be established.

Integrating GHS into the Ghrelin System

Oligopeptide derivatives of metenkephalin were found to stimulate growth-hormone (GH) release directly by pituitary somatotrope cells in vitro in 1977. Members of this class of peptides and nonpeptidyl mimetics are referred to as GH secretagogues (GHSs). A specific guanosine triphosphatate-binding protein-associated heptahelical transmembrane receptor for GHS was cloned in 1996. An endogenous ligand for the GHS receptor, acylghrelin, was identified in 1999. Expression of ghrelin and homonymous receptor occurs in the brain, pituitary gland, stomach, endothelium/vascular smooth muscle, pancreas, placenta, intestine, heart, bone, and other tissues. Principal actions of this peptidergic system include stimulation of GH release via combined hypothalamopituitary mechanisms, orexigenesis (appetitive enhancement), insulinostasis (inhibition of insulin secretion), cardiovascular effects (decreased mean arterial pressure and vasodilation), stimulation of gastric motility and acid secretion, adipogenesis with repression of fat oxidation, and antiapoptosis (antagonism of endothelial, neuronal, and cardiomyocyte death). The array of known and proposed interactions of ghrelin with key metabolic signals makes ghrelin and its receptor prime targets for drug development.

Preservation of GHRH and GH-releasing peptide-2 efficacy in young men with experimentally induced hypogonadism

BACKGROUND

Somatostatin (SS), GHRH, GH-releasing peptide (GHRP), and the sex-steroid milieu regulate GH secretion.

OBJECTIVE

To test whether GHRH and GHRP remain effective secretagogs in the face of short-term hypogonadism.

DESIGN

Prospective, randomized double-blind.

METHODS

Healthy young men (n=24) received a GnRH agonist twice 3 weeks apart followed by placebo (n=13, Pl) or testosterone (n=11, testosterone) addback.

SUBJECTS

were then given consecutive i.v. infusions of l-arginine (to restrain SS outflow) and a maximally effective dose of GHRH or GHRP-2 (to test corresponding secretagog pathways).

RESULTS

GH secretion stimulated by l-arginine/GHRH and by l-arginine/GHRP-2 was unaffected by combined testosterone/estradiol (E(2)) depletion. The low testosterone/E(2) milieu decreased basal (nonpulsatile) GH secretion (P=0.038), without altering fasting pulsatile GH secretion or IGF1 or IGF-binding protein (IGFBP)-3 concentrations. IGFBP-1 (P<0.0001) and abdominal visceral fat (AVF, P=0.017) correlated negatively with fasting basal GH secretion. By contrast, IGF1 (P=0.0012) and IGFBP-3 (P=0.015) correlated positively with fasting pulsatile GH secretion. AVF (P=0.0024) was a negative determinant, and IGF1 a positive determinant (P=0.018), of GHRH-driven GH pulses. Responses to GHRP-2 were unrelated to any of these factors.

CONCLUSION

l-arginine/GHRP-2 appears to be an especially robust stimulus of GH secretion, since efficacy is unmodified by profound short-term hypogonadism, a range of AVF estimates, and a spectrum of IGF1, IGFBP-1, and IGFBP-3 concentrations. Whether robustness also applies to chronic hypogonadism is not known.

Testosterone and growth hormone in older men: for better or for worse?

Evaluation of: Sattler FR, Castaneda-Sceppa C, Binder EF et al. Testosterone and growth hormone improve body composition and muscle performance in older men. J. Clin. Endocrinol. Metab. 94(6), 1991-2001 (2009). In men, testosterone and IGF1 levels decline with increasing age, and lower levels of these hormones are associated with poorer health. A recent, randomized clinical trial demonstrates the additive effects of testosterone and growth hormone supplementation to increase lean body mass, reduce fat mass and improve muscle strength in older men. These findings highlight the interaction between these two hormones and provide a model for further evaluation of combined therapy to explore other end points, such as cardiovascular risk.

Factors other than sex steroids modulate GHRH and GHRP-2 efficacies in men: evaluation using a GnRH agonist/testosterone clamp

BACKGROUND

Sex steroids are prominent regulators of pulsatile GH secretion.

HYPOTHESIS

An experimentally controlled sex-steroid milieu will permit detection of nonsteroidal factors that determine GH secretion.

SUBJECTS

Eleven young (age, 24 +/- 0.99 yr) and 11 older (64 +/- 2.4 yr) men participated in the study.

LOCATION

The study was conducted at a tertiary medical center.

METHODS

The study consisted of GnRH-agonist down-regulation of the gonadal axis followed by fixed-dose testosterone (T) replacement (leuprolide/T clamp) and consecutive infusion of l-arginine and GHRH or GH-releasing peptide-2 (GHRP-2) to quantify peptide-secretagogue efficacies.

OUTCOMES

The experimental leuprolide/T clamp yielded statistically age-comparable total, bioavailable, and free T and estradiol (E(2)) concentrations. In this controlled milieu, sequential l-arginine/GHRH infusion stimulated 1.4-fold more (P = 0.021) and l-arginine/GHRP-2 1.3-fold more (P = 0.045) GH release in young than older men. Abdominal visceral fat (AVF) correlated negatively with both GHRH (P = 0.0006; R(2) = 0.39) and GHRP-2 (R(2) = 0.29) efficacy, whereas IGF-I positively predicted the same endpoints (R(2) = 0.25 to 0.30). In multivariate analysis, AVF emerged as a dominant negative determinant of GHRH efficacy (P = 0.002; R(2) = 0.41) and IGF-I as a primary positive determinant of GHRP-2 efficacy (P = 0.007; R(2) = 0.31).

CONCLUSION

During fixed T/E(2) availability, AVF contributes 41% of the GH-response variability to maximal GHRH drive, whereas IGF-I accounts for 31% of that for GHRP-2. Accordingly, a statistically equalized sex-steroid milieu permits dissection of age-independent and T/E(2)-independent modulators of GHRH and GHRP efficacy in men.

Aromatase and 5alpha-reductase inhibition during an exogenous testosterone clamp unveils selective sex steroid modulation of somatostatin and growth hormone secretagogue actions in healthy older men

BACKGROUND

How endogenous testosterone (Te), 5alpha-dihydrotestosterone (DHT), and estradiol (E(2)) regulate pulsatile GH secretion is not understood.

HYPOTHESIS

Conversion of Te to androgenic (Te-->DHT) or estrogenic (Te-->E(2)) products directs GH secretion. SUBJECTS AND LOCATION: Healthy older men (N = 42, ages 50-79 yr) participated at an academic medical center.

METHODS

We inhibited 5alpha-reduction with dutasteride and aromatization with anastrozole during a pharmacological Te clamp and infused somatostatin (SS), GHRH, GH-releasing peptide-2 (GHRP-2), and L-arginine/GHRH/GHRP-2 (triple stimulus) to modulate GH secretion.

ENDPOINTS

Deconvolution-estimated basal and pulsatile GH secretion was assessed.

RESULTS

Administration of Te/placebo elevated Te by 2.8-fold, DHT by 2.6-fold, and E(2) concentrations by 1.9-fold above placebo/placebo. Te/dutasteride and Te/anastrozole reduced stimulated DHT and E(2) by 89 and 86%, respectively. Stepwise forward-selection regression analysis revealed that 1) Te positively determines mean (P = 0.017) and peak (P < 0.001) GH concentrations, basal GH secretion (P = 0.015), and pulsatile GH secretion stimulated by GHRP-2 (P < 0.001); 2) Te and E(2) jointly predict GH responses to the triple stimulus (positively for Te, P = 0.006, and negatively for E(2), P = 0.031); and 3) DHT correlates positively with pulsatile GH secretion during SS infusion (P = 0.011). These effects persisted when abdominal visceral fat was included in the regression.

CONCLUSION

The present outcomes suggest a tetrapartite model of GH regulation in men, in which systemic concentrations of Te, DHT, and E(2) along with abdominal visceral fat determine the selective actions of GH secretagogues and SS.

Effect of aromatizable and unaromatizable androgen replacement in hypogonadal men on GH responsiveness

OBJECTIVES

Although studies have clearly demonstrated that oestrogen replacement affects GH responsiveness by causing relative GH resistance, the effect of androgen replacement is unknown. Circumstantial evidence only suggests that androgen replacement may increase GH sensitivity and/or responsiveness. To examine the impact of androgens on GH responsiveness, hypogonadal men underwent the IGF-1 generation test in the unreplaced state, replaced with testosterone (T) and also replaced with dihydrotestosterone (DHT), its nonaromatizable metabolite.

DESIGN AND PATIENTS

Twelve hypogonadal men with a normal GH axis were recruited. Each subject in random order had 4 weeks off T (NoRx), 4 weeks on T gel (TG) and 4 weeks on DHT gel (DHTG) applied daily, with 1 week washout between each preparation. An IGF-1 generation test using a subcutaneous injection of 7 mg of GH was performed at the end of each of these 4-week phases.

MEASUREMENTS

Serum GHBP, total and free IGF-1, IGFBP-3 and acid-labile subunit (ALS) levels were measured at baseline and 24 h (peak) after GH administration.

RESULTS

Despite a decrease in GHBP during the TG and DHTG phases, there were no observed differences in baseline, peak or increment (peak - baseline) total or free IGF-1 between the NoRx, TG or DHTG phases.

CONCLUSIONS

There is no evidence of fluctuation in GH responsiveness in hypogonadal men, untreated or replaced with T or DHT alone. This implies that the increased level of oestradiol as a consequence of T replacement in hypogonadal men does not impact significantly on GH responsiveness, nor is there evidence of an androgen effect with elevated DHT levels as a consequence of either T or DHT replacement.

Testosterone supplementation in older men restrains insulin-like growth factor's dose-dependent feedback inhibition of pulsatile growth hormone secretion

BACKGROUND

Pulsatile GH secretion declines in older men. The causal mechanisms are unknown. Candidates include deficient feedforward (stimulation) by endogenous secretagogues and excessive feedback (inhibition) by GH or IGF-I due to age and/or relative hypoandrogenemia.

HYPOTHESIS

Testosterone (T) supplementation in healthy older men will restrain negative feedback by systemic concentrations of IGF-I.

SUBJECTS

Twenty-four healthy men (ages, 50 to 75 yr; body mass index, 24 to 30 kg/m(2)) participated in the study.

METHODS

We performed a prospectively randomized, double-blind, placebo-controlled assessment of the impact of pharmacological T supplementation on GH responses to randomly ordered separate-day injections of recombinant human IGF-I doses of 0, 1.0, 1.5, and 2.0 mg/m(2).

ANALYSIS

Deconvolution and approximate entropy analyses of pulsatile, basal, and entropic (pattern-sensitive) modes of GH secretion were conducted.

RESULTS

Recombinant human IGF-I injections 1) elevated mean and peak serum IGF-I concentrations dose-dependently (both P < 0.001); 2) suppressed pulsatile GH secretion (P = 0.003), burst mass (P = 0.025), burst number (P = 0.005), interpulse variability (P = 0.032), and basal GH secretion (P = 0.009); and 3) increased secretory pattern regularity (P = 0.020). T administration did not alter experimentally controlled IGF-I concentrations, but it elevated mean GH concentrations (P = 0.015) and stimulated pulsatile GH secretion (frequency P = 0.037, mass per burst P = 0.038). Compared with placebo, T attenuated exogenous IGF-I's inhibition of GH secretory-burst mass (P < 0.038) without restoring pulse number, basal secretion, or pattern regularity.

CONCLUSION

The capability of systemic T to mute IGF-I feedback on pulsatile GH secretion suggests a novel mechanism for augmenting GH production.

Androgens may mediate a relative preservation of IGF-I levels in overweight and obese women despite reduced growth hormone secretion

CONTEXT

Obesity is characterized by reduced GH secretion, but data regarding IGF-I levels and their determinants are conflicting.

OBJECTIVES

The objectives were to determine whether IGF-I levels are reduced and to investigate determinants of GH and IGF-I in healthy overweight and obese women.

DESIGN

A cross-sectional study was performed.

SETTING

The study was conducted at a General Clinical Research Center.

STUDY PARTICIPANTS

Thirty-four healthy women without pituitary/hypothalamic disease participated, including 11 lean [body mass index (BMI) <25 kg/m(2)], 12 overweight (BMI > or =25 kg/m(2) and <30 kg/m(2)), and 11 obese (BMI > or =30 kg/m(2)) women of comparable age (overall mean age, 30.7 +/- 7.8 yr).

INTERVENTION

There was no intervention.

MAIN OUTCOME MEASURES

The main outcome measures were frequent sampling (every 10 min for 24 h) for GH, peak GH after GHRH-arginine stimulation, IGF-I, IGF binding protein-3, estrone, estradiol, testosterone, free testosterone, SHBG, homeostasis model assessment of insulin resistance, and abdominal fat.

RESULTS

Mean 24-h GH and peak stimulated GH were lower in overweight than lean women and lowest in obese women. Mean IGF-I levels trended lower in obese, but not overweight, compared with lean women. Free testosterone was positively associated with IGF-I (R = 0.36, P = 0.04) but not with GH measures. Visceral fat was the only determinant of mean 24-h GH (R(2) = 0.66, P < 0.0001) and of peak stimulated GH (R(2) = 0.63, P < 0.0001), and mean 24-h GH accounted for 39% of the variability of IGF-I (P = 0.0002), with an additional 28% (P < 0.0001) attributable to free testosterone levels.

CONCLUSIONS

Despite a linear decrease in GH secretion and peak stimulated GH levels with increasing BMI in healthy overweight and obese women, IGF-I levels were not commensurately reduced. Androgens may contribute to this relative preservation of IGF-I secretion in overweight and obese women despite reduced GH secretion.

Estrogen supplementation selectively enhances hypothalamo-pituitary sensitivity to ghrelin in postmenopausal women

CONTEXT

Sex-steroid hormones amplify pulsatile GH secretion by unknown mechanisms. Ghrelin is the most potent natural GH secretagogue discovered to date. A plausible unifying postulate is that estradiol (E(2)) enhances hypothalamo-pituitary sensitivity to ghrelin (a physiological effect). The hypothesis is relevant to understanding the basis of hyposomatotropism in aging and other relatively hypogonadal states.

OBJECTIVE

Our objective was to test the hypothesis that E(2) supplementation potentiates ghrelin's stimulation of pulsatile GH secretion.

SETTING

The study was conducted at an academic medical center.

SUBJECTS

Healthy postmenopausal women (n = 20) were included in the study.

INTERVENTIONS

Separate-day iv infusions of saline vs. five graded doses of ghrelin were performed in volunteers prospectively randomly assigned to receive (n = 8) or not receive (n = 12) transdermal E(2) for 21 d were performed.

MEASURES

GH secretion was estimated by deconvolution analysis and abdominal visceral fat mass determined by computerized axial tomography were calculated.

RESULTS

E(2) supplementation augmented ghrelin's stimulation of basal (nonpulsatile) GH secretion by 3.6-fold (P = 0.022), increased GH responses to low-dose ghrelin by 2.9-fold (P = 0.035), did not alter ghrelin efficacy, and elicited more regular patterns of acylated ghrelin concentrations during saline infusion (P = 0.033). Abdominal visceral fat negatively determined responses to ghrelin (R = -0.346; P < 0.005).

CONCLUSIONS

Transdermal E(2) supplementation potentiates GH secretion stimulated by physiological but not pharmacological concentrations of acylated ghrelin, and concomitantly regularizes patterns of bioactive ghrelin secretion in postmenopausal women. Accordingly, the estrogen milieu appears to control sensitivity of the hypothalamopituitary unit to acylated ghrelin.

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.

Gonadal status and body mass index jointly determine growth hormone (GH)-releasing hormone/GH-releasing peptide synergy in healthy men

CONTEXT

Sex steroid hormones potentiate whereas increased body mass index (BMI) represses GH secretion. Whether sex steroids modify the negative effect of BMI on secretagogue-induced GH secretion in men is not known. The issue is important in designing GH-stimulation regimens that are relatively insensitive to both gonadal status and adiposity.

OBJECTIVE

Our objective was to compare the relationships between BMI and peptide-stimulated GH secretion in men with normal and reduced testosterone and estradiol availability.

SETTING

The study was performed at an academic medical center.

SUBJECTS

Healthy young men were included in the study.

INTERVENTIONS

Randomized separate-day iv infusion of saline and/or maximally effective doses of L-arginine/GHRH, L-arginine/GH-releasing peptide (GHRP)-2, and GHRH/GHRP-2 in eugonadal (n=12) and experimentally hypogonadal (n=10) men was performed.

OUTCOMES

Regression of paired secretagogue-induced GH responses on BMI was determined.

RESULTS

In eugonadal men, peak GH concentrations correlated negatively with BMI. In particular, BMI accounted for only 38% of the response variability after L-arginine/GHRH (P=0.0165), but 62% after GHRH/GHRP-2 (P=0.0012) and 65% after L-arginine/GHRP-2 (P=0.00075). In contrast, in hypogonadal men, GH responses were uncorrelated with BMI. The negative effects of BMI on peak GH responses in eugonadal and hypogonadal states differed most markedly after stimulation with GHRH/GHRP-2 (P=0.0019). This contrast was corroborated using integrated GH responses (P=0.0007).

CONCLUSIONS

Short-term experimental gonadal sex hormone depletion attenuates dual secretagogue-stimulated GH secretion in lean young men. The inhibitory effect of relative adiposity on GH secretion appears to predominate over that of acute sex steroid withdrawal.

Long-term testosterone supplementation augments overnight growth hormone secretion in healthy older men

Circulating testosterone (T) and GH/IGF-I are diminished in healthy aging men. Short-term administration of high doses of T augments GH secretion in older men. However, effects of long-term, low-dose T supplementation on GH secretion are unknown. Our objective was to evaluate effects of long-term, low-dose T administration on nocturnal GH secretory dynamics and AM concentrations of IGF-I and IGFBP-3 in healthy older men (65-88 yr, n = 34) with low-normal T and IGF-I. In a double-masked, placebo-controlled, randomized study we assessed effects of low-dose T supplementation (100 mg im every 2 wk) for 26 wk on nocturnal GH secretory dynamics [8 PM to 8 AM, Q(20) min sampling, analyzed by multiparameter deconvolution and approximate entropy (ApEn) algorithms]. The results were that T administration increased serum total T by 33% (P = 0.004) and E(2) by 31% (P = 0.009) and decreased SHBG by 17% (P = 0.002) vs. placebo. T supplementation increased nocturnal integrated GH concentrations by 60% (P = 0.02) and pulsatile GH secretion by 79% (P = 0.05), primarily due to a twofold increase in GH secretory burst mass (P = 0.02) and a 1.9-fold increase in basal GH secretion rate (P = 0.05) vs. placebo. There were no significant changes in GH burst frequency or orderliness of GH release (ApEn). IGF-I levels increased by 22% (P = 0.02), with no significant change in IGFBP-3 levels after T vs. placebo. We conclude that low-dose T supplementation for 26 wk increases spontaneous nocturnal GH secretion and morning serum IGF-I concentrations in healthy older men.

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

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

Regulation of growth hormone action by gonadal steroids

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

Modulation of growth hormone action by sex steroids

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

Clinical utility of measurements of insulin-like growth factor 1

Plasma insulin-like growth factor 1 (IGF-I) concentrations are regulated by genetic factors, nutrient intake, growth hormone (GH) and other hormones such as T4, cortisol and sex steroids. The accuracy of IGF-I measurement in diagnosing GH deficiency or excess depends, in part, on the relative contributions of each of these variables. Since their respective influence may vary widely between individuals, the establishment of well-defined normal ranges is necessary, which requires adequate numbers of normal individuals, in order for IGF-I measurements to have maximum utility. In states of GH deficiency, the influence of these non-GH-related factors predominates. Although IGF-I levels have utility as a screening test in children and young adults, they cannot be used as a stand-alone test for the diagnosis of GH deficiency. By contrast, in acromegaly, GH is the predominant determinant of IGF-I levels and, therefore, measurement of IGF-I is a very useful diagnostic test. In acromegaly, IGF-I levels are useful for assessing the relative degree of GH excess, because changes in IGF-I correlate with changes in symptoms and soft-tissue growth. IGF-I is also very useful in monitoring the symptomatic response to therapy.

Effects of testosterone supplementation on skeletal muscle fiber hypertrophy and satellite cells in community-dwelling older men

OBJECTIVE

In this study, we determined the effects of graded doses of testosterone on muscle fiber cross-sectional area (CSA) and satellite cell number and replication in older men.

PARTICIPANTS

Healthy men, 60-75 yr old, received a long-acting GnRH agonist to suppress endogenous testosterone production and 25, 50, 125, 300, or 600 mg testosterone enanthate im weekly for 20 wk.

METHODS

Immunohistochemistry, light and confocal microscopy, and electron microscopy were used to perform fiber typing and quantitate myonuclear and satellite cell number in vastus lateralis biopsies, obtained before and after 20 wk of treatment.

RESULTS

Testosterone administration in older men was associated with dose-dependent increases in CSA of both types I and II fibers. Satellite cell number increased dose dependently at the three highest doses (3% at baseline vs. 6.2, 9.2, and 13.0% at 125, 300, and 600 mg doses, P < 0.05). Testosterone administration was associated with an increase in the number of proliferating cell nuclear antigen+ satellite cells (1.8% at baseline vs. 3.9, 7.5, and 13% at 125, 300, and 600 mg doses, P < 0.005). The expression of activated Notch, examined only in the 300-mg group (baseline, 2.3 vs. 9.0% after treatment, P < 0.005), increased in satellite cells after testosterone treatment. The expression of myogenin (baseline, 6.2 vs. 20.7% after treatment, P < 0.005), examined only in the 300-mg group, increased significantly in muscle fiber nuclei after testosterone treatment, but Numb expression did not change.

CONCLUSIONS

Older men respond to graded doses of testosterone with a dose-dependent increase in muscle fiber CSA and satellite cell number. Testosterone-induced skeletal muscle hypertrophy in older men is associated with increased satellite cell replication and activation.

Modulation of the growth hormone-insulin-like growth factor (GH-IGF) axis by pharmaceutical, nutraceutical and environmental xenobiotics: an emerging role for xenobiotic-metabolizing enzymes and the transcription factors regulating their expression. A review

The growth hormone-insulin-like growth factor (GH-IGF) axis has gained considerable focus over recent years. One cause of this increased interest is due to a correlation of age-related decline in plasma GH/IGF levels with age-related degenerative processes, and it has led to the prescribing of GH replacement therapy by some practitioners. On the other hand, however, research has also focused on the pro-carcinogenic effects of high GH-IGF levels, providing strong impetus for finding regimes that reduce its activity. Whereas the effects of GH/IGF activity on the action of xenobiotic-metabolizing enzyme systems is reasonably well appreciated, the effects of xenobiotic exposure on the GH-IGF axis has not received substantial review. Relevant xenobiotics are derived from pharmaceutical, nutraceutical and environmental exposure, and many of the mechanisms involved are highly complex in nature, not easily predictable from existing in vitro tests and do not always predict well from in vivo animal models. After a review of the human and animal in vivo and in vitro literature, a framework for considering the different levels of direct and indirect modulation by xenobiotics is developed herein, and areas that still require further investigation are highlighted, i.e. the actions of common endocrine disruptors such as pesticides and phytoestrogens, as well as the role of xenobiotic-metabolizing enzymes and the transcription factors regulating their expression. It is anticipated that a fuller appreciation of the existing human paradigms for GH-IGF axis modulation gained through this review may help explain some of the variation in levels of plasma IGF-1 and its binding proteins in the population, aid in the prescription of particular dietary regimens to certain individuals such as those with particular medical conditions, guide the direction of long-term drug/nutraceutical safety trials, and stimulate ideas for future research. It also serves to warn athletes that using compounds touted as performance enhancing because they promote short-term GH release could in fact be detrimental to performance in the long-run.