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The Journal of Clinical Endocrinology & Metabolism Vol. 89, No. 11 5542-5548
Copyright © 2004 by The Endocrine Society

Age and Secretagogue Type Jointly Determine Dynamic Growth Hormone Responses to Exogenous Insulin-Like Growth Factor-Negative Feedback in Healthy Men

Johannes D. Veldhuis, Judith Y. Weltman, Arthur L. Weltman, Ali Iranmanesh, Eugenio E. Muller and Cyril Y. Bowers

Division of Endocrinology and Metabolism, Department of Internal Medicine (J.D.V.), Mayo Medical and Graduate Schools of Medicine, General Clinical Research Center, Mayo Clinic, Rochester, Minnesota 55905; Exercise Physiology Laboratory (J.Y.W., A.L.W.), General Clinical Research Center, University of Virginia Health System, Charlottesville, Virginia 22908; Endocrine Service, Medical Section (A.I.), Salem Veterans Affairs Medical Center, Salem, Virginia 24153; Department of Pharmacology/Chemical Toxicology (E.E.M.), University of Milan, Milan, Italy; and Division of Endocrinology and Metabolism, Department of Internal Medicine (C.Y.B.), Tulane University Medical Center, New Orleans, Louisiana 70112-2699

Address all correspondence and requests for reprints to: Johannes D. Veldhuis, Division of Endocrinology and Metabolism, Department of Internal Medicine, Mayo Medical and Graduate Schools of Medicine, General Clinical Research Center, Mayo Clinic, Rochester, Minnesota 55905. E-mail: veldhuis.johannes{at}mayo.edu.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
The primary cause of waning GH and IGF-I concentrations in healthy aging adults is not established. To test the postulate that age influences negative feedback by IGF-I in a secretagogue-specific fashion, 17 normal men (nine young and eight older) each completed eight randomly ordered injections of placebo or recombinant human (rh) IGF-I (20 µg/kg sc), followed by saline/rest, aerobic exercise, GHRH (1 µg/kg iv bolus), or GH-releasing peptide-2 (1 µg/kg iv bolus) stimulation. GH secretion was monitored by sampling blood every 10 min for 7 h, high-sensitivity immunochemiluminometric assay, and deconvolution analysis conditioned on prior pulse-onset times and biexponential kinetics. Analysis of covariance showed that age (P = 0.028), secretagogue (P < 0.001), and rhIGF-I (P < 0.005) individually determine pulsatile GH secretion and exhibit a strong 3-fold interaction (P < 10–5). Post hoc comparisons revealed that elderly subjects manifest less IGF-I inhibition of a maximal GHRH stimulus (P = 0.013 vs. young), blunted initial IGF-I suppression of fasting GH release (P = 0.038), and impaired IGF-I feedback on the regularity of GH secretion (P = 0.023). Age stratum did not influence peak IGF-I and nadir GH concentrations or rhIGF-I-induced inhibition of GH secretion stimulated by exercise or GH-releasing peptide-2.

In summary, experimental elevation of IGF-I concentrations unmasks reduced rhIGF-I-dependent feedback inhibition of fasting and GHRH-stimulated GH secretion in healthy older men, indicating that aging selectively modulates the autoinhibition process.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
RECENT PATHOPHYSIOLOGICAL STUDIES establish that systemic concentrations of IGF-I exert negative feedback on GH secretion (1, 2). For example, in murine models, transgenic silencing of hepatic IGF-I gene expression reduces peripheral IGF-I concentrations by 70–75% and elevates GH concentrations by 4- to 10-fold (3, 4). In young women and men, injection of a selective GH-receptor antagonist lowers IGF-I concentrations by 34% and stimulates pulsatile and basal GH secretion by 1.8-fold within 60–70 h (5). Conversely, infusion of recombinant human (rh) IGF-I in healthy fasting individuals and in patients harboring an inactivating mutation of the GH receptor (Laron syndrome) rapidly suppresses fasting and secretagogue-stimulated GH release (6, 7, 8, 9, 10). Additionally, partial truncational mutation of the IGF-I gene in one boy triggered marked outpouring of GH, which was suppressible by treatment with rhIGF-I (11).

Available data indicate that IGF-I may inhibit GH secretion by way of combined feedback actions on the hypothalamus and pituitary gland (10, 12, 13, 14, 15, 16). Inferred mechanisms include stimulation of somatostatin outflow, repression of GHRH secretion, and direct inhibition of somatotrope GH synthesis and release (1, 2). Accordingly, controlled IGF-I elevation provides an experimental means to probe negative feedback on central hypothalamopituitary drive of GH secretion.

Clinical investigations point to possible modulation of IGF-I-dependent feedback by age and/or gender (8, 9, 10, 16). However, in relation to age, studies have reported discrepant outcomes that might be related to gender, secretagogue used, analytical method applied, and/or experimental design, e.g. whether successive IGF-I infusions are given in randomized order (17) or in a fixed escalating sequence (18).

To investigate the impact of age and secretagogue type on IGF-I-enforced negative feedback on GH secretion, we have implemented a prospectively randomized, double-blind, parallel-cohort intervention in healthy young and older men comprising both injection of rhIGF-I vs. saline to test negative feedback and stimulation of GH secretion by each of saline, GHRH, GH-releasing peptide (GHRP)-2, and aerobic exercise on separate mornings in the fasting state.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Seventeen healthy community-living men enrolled in and completed each of eight study sessions (see Protocol design). The median age in young men (n = 9) was 29 yr (range, 9–27 yr), and the median body mass index was 25 kg/m2 (range, 20–32 kg/m2). Corresponding values in older men (n = 8) were 59 yr (range, 50–73 yr) and 29 kg/m2 (range, 24–33 kg/m2). Participants provided voluntary written informed consent approved by the Institutional Review Board. The protocol was reviewed by the U.S. Food and Drug Administration and assigned an investigator-initiated new drug file for combined experimental use of rhIGF-I, GHRH, and GHRP-2. Exclusion criteria included known or suspected cardiac, cerebrovascular, peripheral arterial, or venous thromboembolic disease; alcohol or drug abuse; recent transmeridian travel exceeding three time zones (within 10 d); night-shift work; significant weight change (≥ 2 kg in 3 wk); acute or chronic organ-system disease; psychiatric illness requiring medical treatment; concomitant or recent (within 6 half-lives) use of neuroactive medications; anemia; and failure to provide written informed consent. Some enrollees continued to take multivitamins, ferrous sulfate, or topical ophthalmic or dermatological ointments. Inclusion criteria were the capability to perform acute aerobic exercise, an unremarkable medical history and physical examination, and normal screening laboratory tests of hepatic, renal, endocrine, metabolic, and hematological function.

Protocol design

The design was a prospectively randomized, placebo-controlled, patient-blinded, within-subject crossover parallel intervention in two age strata. Each subject undertook eight sampling sessions, which were scheduled at least 3 d apart. For any individual, all admissions were completed within 3 months.

Volunteers were admitted to the General Clinical Research Center on the evening before study to allow overnight adaptation to the unit. To obviate food-related confounds, subjects received a constant evening meal (turkey sandwich or vegetarian alternative) of 500 kcal containing 55% carbohydrate, 15% protein, and 30% fat at 1800 h. Participants remained fasting overnight and until 1400 h the next day. Caffeinated beverages, cigarette use, daytime sleep, and vigorous exercise were not allowed.

Infusion and sampling paradigm

At 0600 h the next morning, iv catheters were inserted in (contralateral) forearm veins (Fig. 1Go). Blood was withdrawn at 0700 h for later assay of estradiol, testosterone, SHBG, and IGF-I. A sample (1.5 ml) was obtained each hour to measure IGF-I concentrations. Additional blood (1.0 ml) was collected every 10 min from 0700–1400 h for GH determinations. At 0800 h, after 1 h of baseline sampling, saline (0.5 ml) or rhIGF-I (20 µg/kg; Genentech, South San Francisco, CA) was administered once sc. At 1100 h (3 h after rhIGF-I injection), a single bolus of saline, GHRH (1.0 µg/kg, Geref; Serono, Rockland, MA), or GHRP-2 (1 µg/kg) was injected iv. Alternatively, aerobic exercise was initiated at 1100 h. The exercise stimulus comprised 30 min of cycle ergometry at an intensity set midway between the volunteer’s previously determined individual lactate threshold and maximal rate of oxygen consumption (19).



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  		FIG. 1. Paradigm to test rhIGF-I-induced feedback inhibition in healthy men. Volunteers were studied on separate randomly ordered mornings after fasting overnight (see Subjects and Methods). Placebo or rhIGF-I (20 µg/kg sc) was injected after 1 or 1.5 h (in three young men given saline and GHRH) of baseline sampling and 3 h before stimulation with saline, exercise, GHRH, or GHRP-2. Blood was withdrawn concomitantly every 10 min for 7 h. Burst-like GH secretion was quantitated by high-sensitivity immunochemiluminometric assay and biexponential deconvolution analysis (see Subjects and Methods). GCRC, General Clinical Research Center.

 
Hormone assays

GH concentrations were measured in duplicate by modified ultrasensitive automated immunochemiluminometry (Nichols Institute Diagnostics, San Clemente, CA) using 22-kDa rhGH as standard (20, 21). Cross-reactivity with 20-kDa GH was 30%. Sensitivity was 0.005 µg/liter (defined as 3 SDs above the zero-dose tube). Median intra- and interassay coefficients of variation were 5.2 and 6.3%, respectively. No sample concentration in the present study fell less than 0.020 µg/liter. LH, FSH, testosterone, estradiol, and SHBG were assayed as described (21, 22). Total IGF-I concentrations were assayed in batch by RIA after acid-ethanol extraction, with a sensitivity of 30 µg/liter and intra- and interassay coefficients of variation of 6.8 and 8.4%, respectively (Nichols Institute Diagnostics, San Juan Capistrano, CA) (22).

Analysis of pulsatile GH secretion above basal

The amount of GH secreted in bursts (nonbasal) was quantitated by modified biexponential deconvolution analysis (23). Two-component GH kinetics were defined by a rapid-phase half-life of 3.5 min, a slow-phase half-life of 20.8 min, and a fractional (slow/total) decay amplitude of 0.63. These values were determined directly earlier (24). Pulse times were set a priori by low-threshold Cluster analysis (25). A conditional model was required to distinguish burst-like GH secretion (µg/liter·3 h post secretagogue) reliably from all three of the time-invariant basal release, incomplete dissipation of prior secretory bursts, and decay of hormone concentrations across the stimulus-response interval (26, 27).

Statistical comparisons

The null hypothesis posits that age stratum and type of intervention do not determine GH secretory-burst mass. Data were analyzed by three-way analysis of covariance comprising two factors (age: young vs. older) by three factors (exercise, GHRH, and GHRP-2) by two factors (placebo and rhIGF-I). The covariate was the corresponding within-subject response to saline. Logarithmic transformation was used to limit the dispersion of residual variance and to address the biological assumption that stimulated GH secretion is asymptotically maximal (1). Responses were expressed as an absolute value (µg/liter·3 h), a within-subject decrement (µg/liter·3 h), or fractional (%) decrease induced by rhIGF-I over placebo. The decrement (or fractional inhibition) was calculated as the algebraic difference in secretagogue-stimulated GH secretory-burst mass observed after administration of placebo vs. rhIGF-I (divided by the response to the secretagogue alone). Post hoc contrasts were assessed by Tukey’s test at overall experiment-wise protected P < 0.05 (28).

Linear regression analysis was used to estimate the rate (slope) of initial decline of GH concentrations during the first 2-h interval (inclusive of 30–150 min) after the onset of inhibition by rhIGF-I injection. Slopes were contrasted by cohort-specific 95% statistical confidence intervals.

The approximate entropy (ApEn) statistic was applied to quantitate rhIGF-I-induced orderliness of GH release (29). Lower values of statistically normalized ApEn (m = 1, r = 0.85) denote greater feedback effectiveness, as established theoretically and empirically (30, 31).

Data are cited as the arithmetic mean ± SEM.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Table 1Go summarizes mean baseline (0700 h) concentrations of GH, IGF-I, testosterone, estradiol, and SHBG in the young and older cohorts. Concentrations of GH and IGF-I and the molar ratios of testosterone to SHBG and estradiol to SHBG were lower and the concentrations of FSH and SHBG were higher in elderly men compared with young men.


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  		TABLE 1. Fasting hormone concentrations in healthy young and older men

 
Figure 2Go depicts mean GH concentration profiles obtained by sampling every 10 min in nine young and eight older men before and after sc administration of placebo (Fig. 2AGo) or rhIGF-I (Fig. 2BGo). Plots highlight the time course of the GH response to each stimulus (saline, exercise, GHRH, and GHRP-2) administered 3 h after placebo or IGF-I administration. The absolute nadir (lowest value of a three-point moving average) GH concentration detected after rhIGF-I injection was comparable by age (0.039 ± 0.006 µg/liter in young men and 0.041 ± 0.008 µg/liter in older men; P = not significant).



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  		FIG. 2. GH concentrations (y-axis) in young (left) and older (right) men monitored every 10 min after sc injection of placebo (A) and rhIGF-I (B) at 70 min. The first observation was assigned a time of 10 min. Stimulation with saline or the indicated secretagogue was initiated at 250 min (see paradigm in Fig. 1Go). To highlight the burst-like release of GH, the timescale extends from 200–430 min inclusively. Each datum is the cohort mean ± SEM (n = 9 young men, n = 8 older men).

 
Statistical analysis of GH secretory-burst mass revealed a significant overall interaction among age, secretagogue, and rhIGF-I (P < 10–5). The individual effects of age, secretagogue, and rhIGF-I were significant at P = 0.028, P < 0.001, and P < 0.005, respectively. Post hoc comparisons of the mass of GH released (µg/liter·3 h) after placebo pretreatment disclosed the following descending rank orders of stimulation: GHRP-2 = GHRH > exercise > rest/saline in young subjects (P < 0.001); and GHRP-2 > GHRH = exercise = saline in older volunteers (P < 0.001). In the placebo setting in young men, exercise (P = 0.0079), GHRH (P < 0.001), and GHRP-2 (P = 0.0019) stimulated GH secretion significantly above saline/rest. In the same context in older men, only GHRP-2 was effectual over saline/rest (P < 0.01). In the absence of IGF-I pretreatment, stimulated GH secretory-burst mass was significantly lower in the older cohort for each of exercise (P = 0.012), GHRH (P = 0.0046), and GHRP-2 (P = 0.019).

Figure 3Go summarizes the effect of rhIGF-I vs. saline injection on GH secretory-burst mass determined after each stimulus. Among the four secretagogue types, inhibition by IGF-I was significant in young men for saline (P < 0.001), exercise (P = 0.026), GHRH (P = 0.011), and GHRP-2 (P = 0.015) and in older individuals for saline only (P = 0.018).



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  		FIG. 3. Comparison of the impact of placebo and rhIGF-I injection on saline, exercise, GHRH, and GHRP-2-stimulated GH secretory-burst mass (µg/liter·3 h) in young (top) and older (bottom) men. P values denote post hoc contrasts between responses to rhIGF-I and placebo within an age cohort for any given secretagogue. Data are the mean ± SEM (n = 9 young men, n = 8 older men). P = not significant (NS) signifies > 0.05.

 
To examine the impact of age on exogenous IGF-I-imposed negative feedback, we compared within-subject decrements in stimulated GH secretory-burst mass (µg/liter·3 h; Fig. 4Go). The intraindividual decrement induced by rhIGF-I over placebo adjusts partially for unequal absolute secretagogue responses among subjects. Secretagogue type (P < 10–4) and age (P = 0.002) individually determined the IGF-I-enforced decrement in burst-like GH secretion and exerted a combined (interactive) effect (P = 0.027). Post hoc contrasts revealed that older age reduced IGF-I-dependent inhibition of GHRH drive specifically (P = 0.0062). Age also tended nonsignificantly to impede IGF-I-induced suppression of the GH response to exercise (P = 0.079) but did not affect inhibition of the saline or GHRP-2 stimulus.



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  		FIG. 4. Influence of age stratum and secretagogue type on the absolute decrement in GH secretory-burst mass (µg/liter·3 h) enforced by injection of rhIGF-I compared with placebo in healthy men. The decrement was defined as the within-subject algebraic difference between the GH response to placebo and rhIGF-I. Data are given for saline, exercise, GHRH, and GHRP-2-stimulated GH secretion. Individual P values denote contrasts by age for the indicated secretagogue. Data are the mean ± SEM (n = 9 young men, n = 8 older men). P = not significant (NS) signifies > 0.05.

 
Fractional (%) suppression of secretagogue-stimulated GH secretory-burst mass was used as a normalizing measure of rhIGF-I feedback effectiveness. This estimate relates the IGF-I-induced within-subject decrement in GH secretion to the response to secretagogue alone (Table 2Go). In young men, the descending rank order of fractional inhibition (median) for the four stimuli was as follows: saline (87%) = GHRH (76%) = exercise (66%) > GHRP-2 (45%) (P < 0.001 overall interventional effect). In older volunteers, the corresponding rank order of fractional suppression was as follows: saline (76%) > GHRH (42%) = exercise (36%) = GHRP-2 (23%) (P = 0.033 overall). Statistical analysis disclosed that older compared with younger men had lesser fractional inhibition of the GHRH (but no other) stimulus by rhIGF-I (P = 0.013). Values for GHRP-2 trended to be significant (P = 0.08).


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  		TABLE 2. Percentage inhibition of saline- and secretagogue-stimulated GH secretion by rhIGF-I over placebo in young and older men

 
Figure 5Go depicts linear regression plots of time-dependent initial suppression of GH concentrations during the 2-h interval comprising 30–150 min after injection of rhIGF-I. Slope comparisons revealed that older individuals manifest 55% less rapid inhibition of GH concentrations (P = 0.038 vs. young).



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  		FIG. 5. More rapid rate of linear decline in GH concentrations induced by sc injection of rhIGF-I 30 min earlier in young (n = 9, top solid line) than older (n = 8), bottom interrupted line). Slope values are given as the mean ± SEM and differ at P = 0.038.

 
The ApEn statistic was used as a model-free measure of rhIGF-I-imposed negative feedback. In mathematical and experimental systems, negative feedback enhances the regularity of output patterns (lowers ApEn, see Statistical comparisons). In young men, injection of rhIGF-I reduced GH ApEn values (P = 0.023), confirming feedback enhancement (Table 3Go). In older subjects, rhIGF-I did not decrease GH ApEn significantly, denoting the absence of detectable negative feedback in this age group.


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  		TABLE 3. ApEn analysis of the regularity of GH concentrations

 
Serum IGF-I concentrations were measured every hour before and after rhIGF-I injection. Peak values were comparable by age at 441 ± 36 µg/liter (young) and 471 ± 37 µg/liter (older).


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
The present clinical investigation demonstrates that age stratum and secretagogue type individually and jointly determine pulsatile (burst-like) GH secretion under negative feedback imposed by systemic IGF-I in healthy men. In particular, injection of rhIGF-I suppresses GH secretory-burst mass in the following descending rank order in young men (ages 19–27 yr): saline (median inhibition, 87%), GHRH (76%), exercise (66%), and GHRP-2 (45%); and, in older volunteers (ages 50–73 yr), the rank order is as follows: saline (76%), GHRH (42%), exercise (36%), and GHRP-2 (23%). Statistical comparisons indicated that aging men evince blunted feedback repression of GHRH-induced (but not saline/rest, exercise, or GHRP-2) GH secretory-burst mass. Impaired IGF-I feedback inhibition was affirmed by significant age-related attenuation of the absolute (µg/liter·3 h) and fractional (%) decrements enforced by rhIGF-I (vs. saline) of GHRH-stimulated GH secretion, the expected rhIGF-I- induced enhancement of the regularity of GH release, and the rate of initial suppression of unstimulated GH concentrations. In contrast, age did not influence absolute nadir GH and peak IGF-I concentrations attained after rhIGF-I administration. Accordingly, we infer that healthy older men manifest a selective diminution of IGF-I-dependent feedback suppression of GHRH-stimulated GH secretion.

Deconvolution analysis was used to distinguish burst-like GH secretion from concurrent basal hormone release, distribution, and elimination. To enhance statistical reliability, analyses were predicated on independently measured biexponential GH kinetics and a priori estimates of GH pulse-onset times (26, 27). Under this mathematical formalism, we infer that rhIGF-I lowers GH concentrations in both young and older men primarily by reducing GH secretory-burst mass and that age stratum and secretagogue type both determine the degree of repression of GH secretory-burst mass by systemic IGF-I. Regulation of secretory-burst mass is important mechanistically because pulsatile secretion constitutes most (>85%) of the total daily GH output and because aging principally attenuates pulsatile, and thereby total, GH secretion (20, 21, 32, 33).

Analyses of fasting GH secretion disclosed that older men manifest significant blunting of rhIGF-I-enforced initial suppression of GH concentrations. Because endogenous GHRH feedforward to somatotropes is crucial to maintain GH secretion (see Introduction), the foregoing outcome is consistent with the observed age-related reduction in IGF-I-negative feedback on exogenous GHRH drive. Attenuation of rhIGF-I-induced inhibition in the elderly male was supported by demonstrating loss of IGF-I feedback enhancement of the regularity of GH release patterns. The last result is significant as an independent, scale-invariant, and model-free marker of reduced IGF-I action on the hypothalamopituitary unit.

Simplified mathematical models indicate that reciprocal interactions among GHRH, somatostatin, and GH are sufficient to confer self-renewing, high-amplitude GH secretory bursts (34, 35, 36). Negative feedback and time delay are key features of automaticity in such constructs. In analytical models of GH pulsatility, autoinhibition of GH secretion proceeds via reciprocal stimulation of somatostatin release and suppression of GHRH secretion (1, 2). The decline in GH concentrations after each pulse relieves central drive on somatostatinergic neurons, and thereby disinhibits release of GH by somatotropes and of GHRH by hypothalamic neurons (37). Whether comparable feedback-modulatory pathways operate for total, free, or bioavailable IGF-I concentrations over a short time scale is not known. However, the reported rapidity (30–90 min) of iv rhIGF-I-induced suppression of fasting GH concentrations is consistent with this consideration. Such mechanisms, if valid, predict that diminished rhIGF-I feedback inhibition as observed here in elderly men would damp the amplitude of rebound-like GH secretion (20, 21, 32, 33, 38, 39).

Several experimental findings may be relevant to interpreting the unique combination of attenuated stimulation of GH secretion by GHRH, GHRP-2, and exercise along with reduced IGF-I autofeedback in the older male. First, aging in the rodent is associated with elevations of both hypothalamic somatostatin and pituitary somatostatin-receptor gene transcripts (1, 2). If applicable to the human, accentuation of somatostatinergic restraint in older individuals would decrease fasting GH pulse amplitude and limit effectiveness of all three stimuli evaluated here. Low baseline GH secretory-burst mass would diminish detection of further decremental or fractional inhibition by rhIGF-I. Second, aging lowers hypothalamic GHRH gene expression (rat) and blunts GH secretion induced by exogenous GHRH (rat and human) (12, 13, 15, 16, 34, 35, 36, 40). One or both alterations may reduce the efficacy of GHRH and GHRP-2 because GHRP synergizes with GHRH. And third, in principle, aging and/or attendant reduction in the molar ratios of sex steroid to SHBG concentrations (Table 1Go) could attenuate central IGF-I receptor-effector signaling. This consideration is pertinent, given that IGF-I receptors are expressed in both the anterior pituitary gland and hypothalamus and inferentially mediate autonegative feedback by this peptide. How hypothalamic IGF-I receptor-effector signaling may change in the aging human is not known. However, the abundance of GH receptors in the central nervous system declines significantly in older individuals (41).

In confirmation of an earlier report, the present analyses show that rhIGF-I antagonizes stimulation by GHRP-2 (median decrease of 34%) less than stimulation by GHRH (by 76%) in young men (8). The accompanying data further demonstrate that IGF-I blocks feedforward by GHRP-2 only minimally in elderly men as well (viz. by 31%). The precise basis for lesser IGF-I-dependent suppression of GHRP than GHRH drive in both age groups has not been elucidated. However, in the intact rat or sheep, GHRP-receptor agonists evoke GH secretion via combined hypothalamopituitary actions, which collectively oppose autonegative feedback (viz. synergism with GHRH, acute stimulation of hypothalamic GHRH release, direct augmentation of somatotrope GH secretion, and antagonism of hypothalamopituitary inhibition by somatostatin) (15, 42, 43).

In summary, healthy elderly men compared with young men manifest significant failure of rhIGF-I to suppress maximal GHRH-stimulated GH secretion, decrease fasting GH concentrations rapidly, and enhance the quantifiable regularity of GH patterns. These age-related distinctions are selective in that absolute nadir GH and peak IGF-I concentrations and inhibition of the GHRP-2 and exercise stimulus by rhIGF-I do not evidently differ to the same extent. Age-dependent differences in the release of GH secretagogues has been inferred on the other grounds as well (17, 18, 33, 40, 44, 45, 46, 47, 48, 49). Accordingly, we conclude that, in normal men, both age and type of secretagogue determine hypothalamopituitary responses to negative feedback exerted by blood-borne IGF-I.


   Acknowledgments
 
We thank Kimberly Coulter and Gail Bierbaum for excellent assistance in manuscript preparation, Jonathan Kuipers for contributing to data analyses and illustrations, the General Clinical Research Center (GCRC) Core Assay facility and Veterans Affairs Assay Laboratory for performing the immunoassays, and the GCRC nursing staff for conducting the protocol. We thank Dr. Stacey M. Anderson, who was reimbursed to screen subjects, see them in the unit, and give peptide injections.


   Footnotes
 
This work was supported in part by Grants MO1 RR00847 and RR00585 to the General Clinical Research Centers of the University of Virginia and Mayo Clinic and Mayo Foundation from the National Center for Research Resources (Rockville, MD), and Grant R01 AG 19695 (to J.D.V.) from the National Institutes of Health (Bethesda, MD).

Abbreviations: ApEn, Approximate entropy; GHRP, GH-releasing peptide; rh, recombinant human.

Received February 13, 2004.

Accepted August 18, 2004.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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