Peripheral estrogen receptor-alpha selectively modulates the waveform of GH secretory bursts in healthy women

Relative effects of estrogen, age, and visceral fat on pulsatile growth hormone secretion in healthy women

Growth hormone (GH) secretion is subject to complex regulation. How pre- and postmenopausal age (PRE, POST), estradiol (E(2)) availability, and abdominal visceral fat (AVF) jointly affect peptidyl-secretagogue drive of GH secretion is not known. To this end, healthy PRE (n = 20) and POST (n = 22) women underwent a low- vs. high-E(2) clamp before receiving a continuous intravenous infusion of GH-releasing hormone (GHRH) or GH-releasing peptide (GHRP-2). According to analysis of covariance, PRE and POST women achieved age-independent hypo- and euestrogenemia under respective low- and high-E(2) clamps. All four of age (P < 0.001), E(2) status (P = 0.006), secretagogue type (P < 0.001), and an age x peptide interaction (P = 0.014) controlled pulsatile GH secretion. Independently of E(2) status, POST women had lower GH responses to both GHRH (P = 0.028) and GHRP-2 (P < 0.001) than PRE women. Independently of age, GHRP-2 was more stimulatory than GHRH during low E(2) (P = 0.011) and high E(2) (P < 0.001). Stepwise forward-selection multivariate analysis revealed that computerized tomographic estimates of AVF explained 22% of the variability in GHRH action (P = 0.002), whereas age and E(2) together explained 60% of the variability in GHRP-2 drive (P < 0.001). These data establish that age, estrogen status, and AVF are triple covariates of continuous peptide-secretagogue drive of pulsatile GH secretion in women. Each factor must be controlled for to allow valid comparisons of GH-axis activity.

Sensitivity and specificity of pulse detection using a new deconvolution method

Quantifying pulsatile secretion from serial hormone concentration measurements (deconvolution analysis) requires automated, objective, and accurate detection of pulse times to ensure valid estimation of secretion and elimination parameters. Lack of validated pulse identification constitutes a major deficiency in the deconvolution field, because individual pulse size and number reflect regulated processes that are critical for the function and response of secretory glands. To evaluate deconvolution pulse detection accuracy, four empirical models of true-positive markers of pituitary (LH) pulses were used. 1) Sprague-Dawley rats had recordings of hypothalamic arcuate nucleus multiunit electrical activity, 2) ovariectomized ewes underwent sampling of hypothalamo-pituitary gonadotropin-releasing hormone (GnRH pulses), 3) healthy young men were infused with trains of biosynthetic LH pulses after GnRH receptor blockade, and 4) computer simulations of pulsatile LH profiles were constructed. Outcomes comprised sensitivity, specificity, and receiver-operating characteristic curves. Sensitivity and specificity were 0.93 and 0.97, respectively, for combined empirical data in the rat, sheep, and human (n = 156 pulses) and 0.94 and 0.92, respectively, for computer simulations (n = 1,632 pulses). For simulated data, pulse-set selection by the Akaike information criterion yielded slightly higher sensitivity than by the Bayesian information criterion, and the reverse was true for specificity. False-positive errors occurred primarily at low-pulse amplitude, and false-negative errors occurred principally with close pulse proximity. Random variability (noise), sparse sampling, and rapid pulse frequency reduced pulse detection sensitivity more than specificity. We conclude that an objective automated pulse detection deconvolution procedure has high sensitivity and specificity, thus offering a platform for quantitative neuroendocrine analyses.

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.

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

BACKGROUND

Sex steroids influence GH secretion in complex ways.

HYPOTHESIS

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

CONTEXT

The study was conducted in a tertiary medical center.

SUBJECTS

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

METHODS

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

OUTCOMES

We measured basal and pulsatile GH secretion.

RESULTS

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

CONCLUSION

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

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.