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Am J Physiol Regul Integr Comp Physiol 289: R721-R728, 2005. First published May 12, 2005; doi:10.1152/ajpregu.00138.2005
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APPETITE, OBESITY, DIGESTION, AND METABOLISM

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

Peter Y. Liu,1 Paul Y. Takahashi,2 Pamela D. Roebuck,1 Ali Iranmanesh,3 and Johannes D. Veldhuis1

1Endocrine Research Unit, Department of Internal Medicine, Mayo School of Graduate Medical Education, General Clinical Research Center and 2Division of Primary Care Internal Medicine, Department of Internal Medicine, Mayo Clinic, Rochester, Minnesota; and 3Endocrine Service, Research and Development, Salem Veterans Affairs Medical Center, Salem, Massachusetts

Submitted 24 February 2005 ; accepted in final form 6 May 2005


   ABSTRACT
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 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
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.

gonadotropin releasing hormone; luteinizing hormone; aging; male; androgen; secretion

THE MECHANISTIC BASIS OF DECLINING testosterone (Te) availability in the healthy aging male appears to be multifactorial (49, 54). Putative mechanisms include reduced hypothalamic gonadotropin releasing hormone (GnRH) secretion, decreased testicular steroidogenesis, and altered sex-steroid negative feedback (11, 13–15, 32, 45, 49, 54). For example, data obtained in the old male rat are consistent with attenuation of both neuronal GnRH outflow and Leydig cell Te secretion (2, 3, 6, 14, 18, 19, 41, 46, 47). Diminished GnRH drive and decreased testicular steroidogenesis may contribute to relative androgen depletion in elderly men also. Indirect clinical evidence in this regard includes: 1) normal or minimally elevated LH concentrations in the face of low Te concentrations (32, 33, 59, 62, 65); 2) normal or enhanced luteinizing hormone (LH) secretion when stimulated by exogenous GnRH pulses (35, 67); 3) reduced Te concentration responses to a pharmacological human chorionic gonadotropin (hCG) stimulus (16, 30, 38, 43, 65) or to recombinant human LH (36); and 4) reduced Leydig cell number (39). In contrast, the elimination kinetics of LH and unbound (free) Te are comparable in young and older subjects (23, 25, 36). Whether altered sex-steroid negative feedback further accentuates the aging-related decrease in LH and Te secretion is not clear (34, 37, 48, 64).

The interconnected nature of the hypothalamo-pituitary-gonadal axis makes verifying putative deficits in the individual regulation of GnRH, LH, and Te secretion difficult in vivo (23, 24, 34, 37, 62, 65). Analytical estimation of endogenous LH-driven Te secretion illustrates a noninvasive approach to examine such issues (20, 25). Ensemble model-based predictions provide another investigative strategy (21, 22, 24). In contrast, no direct experimental assessment exists of how age modifies feedforward drive of pulsatile Te secretion by endogenous LH pulses. Appraising the actions of endogenous LH secretory bursts is significant physiologically, because 1) the properties of LH release may be altered in aging, and the implications of such changes may not be determinable from the effects of fixed exogenous stimuli; and 2) the half-life of injected hCG is >15-fold longer and its lutropic potency multifold greater than that of LH, thereby downregulating Leydig cell steroidogenesis in the human and animal (49, 54). In principle, optimal experiments would require extraction of plasma LH to faithfully represent in vivo isoforms (4, 58), suppression of ongoing LH secretion, and reinfusion of aging pattern-specific pulses of purified LH in the donor animal.

As an alternative strategy to probe endogenous LH-Te feedfoward, the present paradigm enforced four different strata of pulsatile LH output by graded inhibition of hypothalamic GnRH action (9, 26, 40). To this end, we administered saline and three escalating doses of a potent, specific, and long-acting GnRH-receptor antagonist on separate randomly ordered days to 18 normal men whose ages spanned 23–72 yr. The rationale was to suppress mean LH concentrations and the amplitude of endogenous LH pulses stepwise without disrupting expected coupling between LH and Te secretion (20, 34, 40). The hypothesis was that increasing age would attenuate the feedforward relationship between pulsatile LH and pulsatile Te secretion over a wide range of endogenous LH drive.


   METHODS
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Sampling protocol. Eighteen men aged 23 to 72 yr (mean 45, 2–4 men/decade) participated in the study after providing written voluntary informed consent approved by the Mayo Clinic Institutional Review Board. Participants were healthy community-dwelling men within 20% of ideal body weight, who had not undertaken recent transmeridian travel (within 10 days) or consumed alcohol, caffeine, or systemic medications within eight biological half-lives. Detailed medical inventory excluded a history of infertility, systemic disease, recent weight change (exceeding 2 kg in the preceding 6 wk), hormonal therapy, or psychoactive drug use. Outpatient screening was unremarkable in relation to medical history (particularly libido and erectile function), physical examination (including testis size), and fasting morning (0800) biochemical tests of renal, hepatic, hematological, and metabolic function (plasma glucose and thyroid hormones).

Volunteers were admitted to the General Clinical Research Center for four separate overnight inpatient studies each scheduled at least 7 days apart. Blood samples (1.0 ml) were withdrawn every 10 min beginning at 1800 for a total of 18 h through a forearm intravenous catheter for later assay of serum LH and Te concentrations. Blood was allowed to clot at room temperature, and sera were frozen at –20°C. In each study session, one dose of 0 (saline), 0.1, 0.3, or 1.0 mg/m2 ganirelix was administered subcutaneously at 2000 (2 h after beginning blood sampling) in a prospectively randomized double-blind fashion.

GnRH-receptor antagonist. Ganirelix is a potent, specific, and competitive GnRH-receptor antagonist used in ovulation induction protocols (1, 42). Time course studies in young men have indicated that ganirelix suppresses pulsatile LH and Te secretion by >80% in 4–8 h and maintains comparable suppression thereafter for an additional 20–24 h (52). The duration of inhibition is consistent with the plasma half-life of this peptide of 15 ± 2 h (42). Consequently, the plateau of LH and Te output was taken to be 8 h in duration beginning 8 h after ganirelix or saline injection.

Assays. LH and Te concentrations were measured in duplicate by automated immunochemiluminometry (ACS Corning, Bayer, Tarrytown, NY; see Refs. 53, 55, and 61). The LH standard was 80/552 defined against the Second International Reference Preparation. Concentration-dependent intra-assay coefficients of variation (CVs) averaged 4.7, 3.5, and 3.8% and interassay CVs 8, 3.7, and 4.7% at LH concentrations of 4.4, 18.2, and 38.8 IU/l, respectively. Procedural sensitivity was 0.05 IU/l. Te was quantitated analogously at median intra- and interassay CVs of 6.8 and 8.3% and a sensitivity of 8 ng/dl. Free Te concentrations were computed directly from measured albumin and sex hormone binding globulin concentrations, as described previously (35).

End points. Model-free end points were absolute minima and maxima of LH and Te concentration time series within the 16-h interval after saline/ganirelix administration. Minima and maxima were calculated from a five-point moving average.

Model-based outcomes were the sum of LH and Te secretory-burst mass values (pulsatile LH and Te secretion) over the 8-h interval, comprising inclusively 9–16 h after injection of saline or ganirelix. This time window encompasses stable (time-invariant) suppression of LH and Te concentrations (52). Stability was defined as a zero slope of the linear regression of LH or Te concentrations on time.

Analytical methods. Pulsatile secretion was estimated by multiparameter deconvolution analysis of each 18-h concentration profile (50). The entire 18-h concentration profile was deconvolved since, at any given time, the blood concentration reflects both current and prior secretion that is undergoing biexponential elimination. Hence, the most accurate estimates of pulsatile secretion are obtained by deconvolution of the full concentration series. However, statistical comparisons were made on values contained within the last 8 h of sampling. The deconvolution model assumed underlying Gaussian-like secretory bursts superimposed on time-invariant (nonpulsatile) basal LH and Te secretion, statistically conditioned on preestimated pulse times (57), and directly measured biexponential kinetics of LH and Te disappearance in men (17, 56). Deconvolution data are given as the mass of LH (IU) and Te (ng) secreted in bursts per unit distribution volume (liters for LH and dl for Te) in each subject over the 8-h time interval. Apparent distribution volumes of LH approximate the plasma space (51, 52), and those of Te vary among study techniques (7, 17).

Statistics. The primary postulate is that age decreases the capability of varying amplitudes of endogenously sustained LH pulses to stimulate burst-like Te secretion. To estimate feedforward drive as a function of LH output across the four experimental clamps, logarithmically transformed pulsatile Te secretion rates were regressed linearly on pulsatile LH secretion rates in each volunteer. The resultant slope estimate represents fractional stimulation of pulsatile Te production by a unit increase in pulsatile LH secretion. To test the impact of age on fractional LH-Te stimulation, the 18 slope estimates were regressed linearly on age.

Two-way ANCOVA was used to test the joint impact on pulsatile Te secretion of pulsatile LH secretion (3 fixed effects) and age (1 random effect), wherein the covariate was the response to saline. Homogeneity of slopes was tested at P < 0.05 by an F-statistic (27). Data are given as means ± SE or ± SD (slopes). Analyses were performed using SAS version 8.02 (SAS Institute, Cary, NC).


   RESULTS
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All 18 men completed all 4 visits. Potentially, 15,696 separate LH and Te measurements were available, of which ~0.5% were missing. Mean LH and total Te concentrations determined over the last 4 h of sampling on the saline day in the cohort of 18 men were 3.3 ± 0.36 IU/l and 471 ± 39 ng/dl (respective normal ranges are 1.2–10 IU/l and 300–950 ng/dl). Mean concentrations of LH and total Te did not differ with age, consistent with other reports in healthy populations (29). However, the logarithm of calculated free and bioavailable Te concentrations decreased linearly with age (both P < 0.025). Figure 1 illustrates Te concentration profiles in one individual aged 25 (left) and another aged 64 (right) yr, reflecting each of the four interventions studied. Data represent measurements made every 10 min beginning 2 h before (1800) and continuing for 16 h after subcutaneous injection of randomly ordered doses of ganirelix vs. saline. Increasing doses of the GnRH-receptor inhibitor progressively suppressed Te concentrations.



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  		Fig. 1. Illustrative testosterone (Te) concentration profiles in two healthy men aged 25 yr (left) and 64 yr (right) monitored by sampling blood every 10 min for 2 h before and 16 h after sc injection of saline or 0.1, 0.3, and 1.0 mg/m2 ganirelix on separate randomly ordered days at least 1 wk apart. Ganirelix is a potent selective and long-lived competitive inhibitor of the gonadotropin releasing hormone (GnRH) receptor, used here to achieve graded reductions in pulsatile luteinizing hormone (LH) and Te secretion (see METHODS). Zero on the x-axis scale corresponds to a clock time of 1800. Ganirelix was administered at 130 min (2000).

 
Figure 2 summarizes dose-dependent inhibition of maximal (peak) and minimal (nadir) Te concentrations by the GnRH-receptor antagonist in the entire study cohort (P < 0.001 vs. saline effect for each). Comparable inhibitory effects were observed for LH (P < 0.001). The time delay to reach minimal LH and Te concentrations was prolonged significantly compared with saline (P < 0.01) but was independent of ganirelix dose [global mean 760 ± 58 (LH) and 817 ± 46 (Te) min after ganirelix vs. 583 ± 85 (LH) and 308 ± 89 (Te) min after saline injection].



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  		Fig. 2. Absolute maximal (A) and minimal (B) Te concentrations (determined by a 5-point moving average) over the 16-h interval after injection of ganirelix in individual doses of 0, 0.1, 0.3, and 1.0 mg/m2 in healthy men. P values were determined by repeated-measures 1-way ANOVA. Data are means ± SE (n = 18 men). Means with unshared (unique) alphabetic superscripts differ significantly by the post hoc Tukey test. Means with shared (common) superscripts do not differ.

 
Deconvolution analysis was used to estimate pulsatile LH and Te secretion over the 8-h window before the end of sampling. This interval reflects stable suppression by ganirelix (52, 60). Table 1 gives mean pulsatile LH and Te secretion at each plateau and demonstrates that the inhibitory effect of ganirelix was dose dependent for both pulsatile Te (P < 0.01) and pulsatile LH (P < 0.001) secretion.


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  		Table 1. Dose-dependent impact of ganirelix on pulsatile LH and Te secretion in men

 
Figure 3 illustrates log-linear regression of the four strata of pulsatile Te secretion (dependent variable) on pulsatile LH secretion (independent variable) in the 18 individual subjects (ages shown). Each slope provides an estimate of the fractional response of pulsatile Te secretion to a unit increase in pulsatile LH secretion. The mean in the cohort was 9.4 ± 3.8 (±SD) x 10–3 slope units. To test whether increasing age attenuates LH feedforward on pulsatile Te secretion, the set of 18 slope estimates was regressed linearly on age. As shown in Fig. 4, slope values fell significantly with increasing age (r = –0.721, P < 0.001). The cohort slope (±SD) on age was –7.3 ± 1.8 x 10–4. Age accounted for 52% (r2) of the variance in fractional LH drive of pulsatile Te secretion.



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  		Fig. 3. Regression of the natural logarithm of pulsatile Te secretion on pulsatile LH secretion monitored over the 8-h interval beginning 8 h after ganirelix administration in healthy men of the indicated ages (boxed values). Each slope (boldface number), determined by least-squares linear regression, is an estimate of the sensitivity of pulsatile Te secretion to a unit increase in pulsatile LH secretion in that subject.

 


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  		Fig. 4. Regression of the sensitivity of pulsatile Te secretion to a unit increase in pulsatile LH secretion (slope term in Fig. 3) on age in 18 healthy men. Error bars are SDs of the individual slope estimates. Numerical values are the square of the correlation coefficient, corresponding P value, and cohort slope (±SD) on age. Data are from 18 men.

 
To test the assumption that age does not determine the relative potency of ganirelix in inhibiting pulsatile Te secretion, we regressed the log of the Te/LH secretion ratio on ganirelix dose in each subject (Fig. 5A). The resultant set of inhibitory slopes (n = 18) was then regressed linearly on age (Fig. 5B). Age did not alter the effect of ganirelix on the LH-to-Te secretion ratio. Statistical power was >95% to detect a 30% contribution of age to the variance in the pulsatile LH-to-Te secretion ratio at P ≤ 0.05. In addition, we examined the slopes obtained in Fig. 5A to test whether the mean slope significantly differed from zero. The mean of these slopes was 1.54 ± 0.21 (median 1.46; P < 0.01), indicating that greater ganirelix blockade enhanced the pulsatile Te-to-LH secretion ratio. This outcome is also consistent with the greater percentage fall in LH (85%) than Te (35%) secretion in response to GnRH-receptor antagonism (Table 1), thus suggesting that some Te secretion is not acutely LH dependent.



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  		Fig. 5. A: increasing doses of ganirelix (x-axis) elevate the logarithmic ratio of pulsatile Te/pulsatile LH secretion by a median of 46% in healthy men. Data are presented as in Fig. 3. B: lack of impact of age on ganirelix's dose-dependent increase in the ratio of pulsatile Te/LH secretion (slope ± SD in A) in 18 men. The data format is otherwise that of Fig. 4. P = NS denotes P > 0.05.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
The present data support the postulate that age significantly attenuates pulsatile LH-dependent drive of episodic Te secretion in normal men. This inference depends on deconvolution and regression analysis of four experimentally controlled graded decrements in burst-like LH and Te secretion in each of 18 subjects. Varying plateaus of pulsatile LH output were achieved by selective antagonism of hypothalamic GnRH-dependent secretion of LH and thereby gonadal Te. Statistical contrasts demonstrated that the negative impact of age on endogenous LH-driven burst-like Te secretion accounts for 52% of the interindividual variability in pulsatile Te production in this cohort of volunteers aged 23–72 yr.

Multiple loci of hypothalamo-pituitary-testicular impairment appear to underlie the gradual fall in free and bioavailable Te concentrations in aging men (29, 54). The present analyses consider particularly the LH-Te interface in healthy individuals. LH and Te are both secreted in pulses (12, 25, 33, 50, 66). LH secretory bursts represent a relevant, but not necessarily exclusive, feedforward signal to the testis, inasmuch as discrete pulses of infused LH or hCG stimulate Te secretion in the rat, dog, sheep, horse, and human (10, 20, 31, 52, 63). On the basis of this physiological background, we used escalating doses of a potent and long-acting selective GnRH-receptor antagonist to enforce stepwise reductions in pulsatile LH output. The GnRH blocker suppressed both pulsatile Te secretion rates (model based) and peak Te concentrations (model free) in a dose-dependent fashion. Analyses of the paired LH and Te time series indicated that any given amplitude of LH pulses is less effective in driving burst-like Te secretion in the aging than young male. This inference extends earlier model-based mathematical predictions using cross-correlation (linear) and logistic (nonlinear) analyses of concomitant LH and Te concentration profiles in cohorts of men in the age extrema >60 vs. <30 yr (25, 34). Analogously, in one study, administration of pulses of GnRH (100 ng/kg) intravenously every 90 min for 14 days elevated mean LH concentrations comparably in five older and five young men but failed to increase bioavailable and free Te concentrations maximally in older individuals (35). The current data differ by way of demonstrating that 1) the decline in pulsatile Te secretion in aging men occurs continuously over six decades of life, and 2) impaired testicular steroidogenic responsiveness operates over a 5.6-fold range of pulsatile LH secretion in healthy individuals.

Increasing doses of the GnRH-receptor antagonist elevated the ratio of pulsatile Te to pulsatile LH secretion by ~50% independently of age. This finding excludes a direct inhibitory effect of ganirelix on Leydig cells, which inference mirrors that for other agents in this class (40). Greater suppression of LH than Te secretion could indicate that a basal-like component of Te production is sparingly dependent on LH stimulation (8, 20, 25, 28, 33, 44, 66).

By way of qualifications, first, no comprehensive studies are available that directly quantitate the dose-responsive actions of purified or biosynthetic LH on pulsatile Te secretion over a span of ages in the intact experimental animal or human. When available, such data will ultimately be important to compare with the accompanying estimates based on endogenous LH pulses. Second, our use of saline and three nonzero doses of the GnRH-receptor antagonist (72 study sessions) allowed linear approximation of testicular steroidogenic sensitivity to pulsatile LH drive. Comprehensive estimation of nonlinear dose-response properties (potency, sensitivity, and efficacy) would require achieving supraphysiological LH pulses to verify attainment of maximal Leydig cell steroidogenesis. Third, aging attenuates the mass of LH contained in secretory bursts (and hence blunts the incremental amplitude of LH peaks) by 35–50, and disrupts the orderliness of the LH release process (41, 59, 61, 65). The precise impact of chronically reduced amplitudes and irregular patterns of LH secretion on gonadal steroidogenesis is not known. Thus further studies will be required to assess whether prolonged normalization of LH pulse amplitude and regularity will enhance Te secretion in the older male. Recent experimental data suggest that reduced Te production in the aged male rat is not reversible by in vivo delivery of fixed LH pulses for 5 days (14). On the other hand, sustained withdrawal of endogenous LH in the young adult rat potentiates LH-dependent Te secretion in the older animal (5). Such data suggest that prolonged exposure to LH in adulthood may induce deleterious intragonadal factors, such as oxidative stress-related products of steroidogenesis, transforming growth factor-{beta}, or inflammatory cytokines (5, 14, 44).

In summary, an experimental paradigm comprising randomly ordered, separate-day administration of saline and varying doses of a selective GnRH-receptor antagonist delineates a prominent age-related decline in the apparent sensitivity of burst-like Te secretion to endogenous LH pulses in normal men. Longitudinal studies and exogenous LH dose-response analyses will be required to corroborate the inferred decline in Leydig cell responsiveness with age in men, and basic laboratory investigations will be needed to establish the precise biochemical bases of reduced LH-stimulated Te biosynthesis in the older male.


   GRANTS
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REFERENCES
 
Studies were supported in part by National Institutes of Health Grant M01 RR-00585 to the General Clinical Research Center of the Mayo Clinic and Foundation and Grants RO1 AG-023133 and DK-060717. P. Y. Liu was supported by a Neil Hamilton Fairley Research Fellowship from the National Health and Medical Research Council of Australia (Grant ID 262025).


   ACKNOWLEDGMENTS
 
We thank Kris Nunez for manuscript support and Jason Kerkvliet for performing the ganirelix assays.


   FOOTNOTES
 

Address for reprint requests and other correspondence: J. D. Velduis, Endocrine Research Unit, Dept. of Internal Medicine, Mayo School of Graduate Medical Education, General Clinical Research Center, Mayo Clinic, Rochester, MN 55905 (e-mail: veldhuis.johannes{at}mayo.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
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 REFERENCES
 

  1. The Ganivelix Dose-Finding Study Group. A double-blind, randomized, dose-finding study to assess the efficacy of the gonadotrophin-releasing hormone antagonist ganirelix (Org 37462) to prevent premature luteinizing hormone surges in women undergoing ovarian stimulation with recombinant follicle stimulating hormone (Puregon). The ganirelix dose-finding study group. Hum Reprod 13: 3023–3031, 1998.[Abstract/Free Full Text]
  2. Bonavera JJ, Swerdloff RS, Sinha Hakim AP, Lue YH, and Wang C. Aging results in attenuated gonadotropin releasing hormone-luteinizing hormone axis responsiveness to glutamate receptor agonist N-methyl-D-aspartate. J Neuroendocrinol 10: 93–99, 1998.[CrossRef][Web of Science][Medline]
  3. Bourguignon JP, Gerard A, and Franchimont P. Age-related difference in the effect of castration upon hypothalamic LHRH content in male rat. Neuroendocrinology 38: 376–381, 1984.[Web of Science][Medline]
  4. Burgon PG, Stanton PG, and Robertson DM. In vivo bioactivities and clearance patterns of highly purified luteinizing hormone isoforms. Endocrinology 137: 4827–4836, 1996.[Abstract]
  5. Chen H and Zirkin BR. Long-term suppression of Leydig cell steroidogenesis prevents Leydig cell aging. Proc Natl Acad Sci USA 96: 14877–14881, 1999.[Abstract/Free Full Text]
  6. Chubb CE and Desjardins C. Testicular function and neuroendocrine activity in senescent mice. Am J Physiol Endocrinol Metab 247: E410–E415, 1984.
  7. Clark AF, Calandra RS, and Bird CE. Kinetics of [3H]-testosterone metabolism in normal young men: effects of medroxyprogesterone acetate (provera) administration. J Steroid Biochem 3: 837–842, 1972.[CrossRef][Web of Science][Medline]
  8. Darney, KJ Jr and Ewing L. Autoregulation of testosterone secretion in perfused rat testes. Endocrinology 109: 993–995, 1981.[Abstract/Free Full Text]
  9. Davis MR, Veldhuis JD, Rogol AD, Dufau ML, and Catt KJ. Sustained inhibitory actions of a potent antagonist of gonadotropin-releasing hormone in postmenopausal women. J Clin Endocrinol Metab 64: 1268–1274, 1987.[Abstract/Free Full Text]
  10. Eik-Nes KB. Secretion of testosterone in anesthetized dogs. Endocrinology 71: 101–106, 1962.[Web of Science][Medline]
  11. Feldman HA, Longcope C, Derby CA, Johannes CB, Araujo AB, Coviello AD, Bremner WJ, and McKinlay JB. Age trends in the level of serum testosterone and other hormones in middle-aged men: longitudinal results from the Massachusetts male aging study. J Clin Endocrinol Metab 87: 589–598, 2002.[Abstract/Free Full Text]
  12. Foresta C, Bordon P, Rossato M, Mioni R, and Veldhuis JD. Specific linkages among luteinizing hormone, follicle stimulating hormone, and testosterone release in the peripheral blood and human spermatic vein: evidence for both positive (feed-forward) and negative (feedback) within-axis regulation. J Clin Endocrinol Metab 82: 3040–3046, 1997.[Abstract/Free Full Text]
  13. Gruenewald DA, Naai MA, Hess DL, and Matsumoto AM. The Brown Norway rat as a model of male reproductive aging: evidence for both primary and secondary testicular failure. J Gerontol B Psychol Sci Soc Sci 49: B42–B50, 1994.
  14. Grzywacz FW, Chen H, Allegretti J, and Zirkin BR. Does age-associated reduced Leydig cell testosterone production in Brown Norway rats result from under-stimulation by luteinizing hormone? J Androl 19: 625–630, 1998.[Abstract/Free Full Text]
  15. Harman SM, Metter EJ, Tobin JD, Pearson J, and Blackman MR. Longitudinal effects of aging on serum total and free testosterone levels in healthy men. Baltimore Longitudinal Study of Aging. J Clin Endocrinol Metab 86: 724–731, 2001.[Abstract/Free Full Text]
  16. Harman SM and Tsitouras PD. Reproductive hormones in aging men. I. Measurement of sex steroids, basal luteinizing hormone, and Leydig cell response to human chorionic gonadotropin. J Clin Endocrinol Metab 51: 35–40, 1980.[Abstract/Free Full Text]
  17. Horton R, Shinsako J, and Forsham PH. Testosterone production and metabolic clearance rates with volumes of distribution in normal adult men and women. Acta Endocrinol 48: 446–458, 1965.
  18. Jarjour LT, Handelsman DJ, and Swerdloff RS. Effects of aging on the in vitro release of gonadotropin-releasing hormone. Endocrinology 119: 1113–1117, 1986.[Abstract/Free Full Text]
  19. Karpas AE, Bremner WJ, Clifton DK, Steiner RA, and Dorsa DM. Diminished luteinizing hormone pulse frequency and amplitude with aging in the male rat. Endocrinology 112: 788–791, 1983.[Abstract/Free Full Text]
  20. Keenan DM, Alexander SL, Irvine CHG, Clarke IJ, Canny BJ, Scott CJ, Tilbrook AJ, Turner AI, and Veldhuis JD. Reconstruction of in vivo time-evolving neuroendocrine dose-response properties unveils admixed deterministic and stochastic elements in interglandular signaling. Proc Natl Acad Sci USA 101: 6740–6745, 2004.[Abstract/Free Full Text]
  21. Keenan DM and Veldhuis JD. Stochastic model of admixed basal and pulsatile hormone secretion as modulated by a deterministic oscillator. Am J Physiol Regul Integr Comp Physiol 273: R1182–R1192, 1997.[Abstract/Free Full Text]
  22. Keenan DM and Veldhuis JD. A biomathematical model of time-delayed feedback in the human male hypothalamic-pituitary-Leydig cell axis. Am J Physiol Endocrinol Metab 275: E157–E176, 1998.[Abstract/Free Full Text]
  23. Keenan DM and Veldhuis JD. Disruption of the hypothalamic luteinizing-hormone pulsing mechanism in aging men. Am J Physiol Regul Integr Comp Physiol 281: R1917–R1924, 2001.[Abstract/Free Full Text]
  24. Keenan DM and Veldhuis JD. Hypothesis testing of the aging male gonadal axis via a biomathematical construct. Am J Physiol Regul Integr Comp Physiol 280: R1755–R1771, 2001.[Abstract/Free Full Text]
  25. Keenan DM and Veldhuis JD. Divergent gonadotropin-gonadal dose-responsive coupling in healthy young and aging men. Am J Physiol Regul Integr Comp Physiol 286: R381–R389, 2004.[Abstract/Free Full Text]
  26. Kolp LA, Pavlou SN, Urban RJ, Rivier JC, Vale WW, and Veldhuis JD. Abrogation by a potent GnRH antagonist of the estrogen/progesterone-stimulated surge-like release of LH and FSH in postmenopausal women. J Clin Endocrinol Metab 75: 993–997, 1992.[Abstract]
  27. Kuehl RO. Split-Plot Designs. Statistical Principles of Research Design and Analysis. Belmont, CA: Duxbury, 1994, 473–498.
  28. Lee S, Miselis R, and Rivier C. Anatomical and functional evidence for a neural hypothalamic-testicular pathway that is independent of the pituitary. Endocrinology 143: 4447–4454, 2002.[Abstract/Free Full Text]
  29. Liu PY, Swerdloff RS, and Veldhuis JD. The rationale, efficacy and safety of androgen therapy in older men: future research and current practice recommendations. J Clin Endocrinol Metab 89: 4789–4796, 2004.[Abstract/Free Full Text]
  30. Longcope C. The effect of human chorionic gonadotropin on plasma steroid levels in young and old men. Steroids 21: 583–592, 1973.[CrossRef][Web of Science][Medline]
  31. Moger WH. Production of testosterone, 5 alpha-androstane-3 alpha, 17 beta-diol and androsterone by dispersed testicular interstitial cells and whole testes in vitro. J Endocrinol 80: 321–332, 1979.[Abstract/Free Full Text]
  32. Morley JE, Kaiser F, Raum WJ, Perry 3rd HM, Flood JF, Jensen J, Silver AJ, and Roberts E. Potentially predictive and manipulable blood serum correlates of aging in the healthy human male: progressive decreases in bioavailable testosterone, dehydroepiandrosterone sulfate, and the ratio of insulin-like growth factor I to growth hormone. Proc Natl Acad Sci USA 94: 7537–7542, 1997.[Abstract/Free Full Text]
  33. Mulligan T, Iranmanesh A, Gheorghiu S, Godschalk M, and Veldhuis JD. Amplified nocturnal luteinizing hormone (LH) secretory burst frequency with selective attenuation of pulsatile (but not basal) testosterone secretion in healthy aged men: possible Leydig cell desensitization to endogenous LH signaling–a clinical research center study. J Clin Endocrinol Metab 80: 3025–3031, 1995.[Abstract/Free Full Text]
  34. Mulligan T, Iranmanesh A, Johnson ML, Straume M, and Veldhuis JD. Aging alters feedforward and feedback linkages between LH and testosterone in healthy men. Am J Physiol Regul Integr Comp Physiol 273: R1407–R1413, 1997.[Abstract/Free Full Text]
  35. Mulligan T, Iranmanesh A, Kerzner R, Demers LW, and Veldhuis JD. Two-week pulsatile gonadotropin releasing hormone infusion unmasks dual (hypothalamic and Leydig-cell) defects in the healthy aging male gonadotropic axis. Eur J Endocrinol 141: 257–266, 1999.[Abstract]
  36. Mulligan T, Iranmanesh A, and Veldhuis JD. Pulsatile iv infusion of recombinant human LH in leuprolide-suppressed men unmasks impoverished Leydig-cell secretory responsiveness to midphysiological LH drive in the aging male. J Clin Endocrinol Metab 86: 5547–5553, 2001.[Abstract/Free Full Text]
  37. Muta K, Kato K, Akamine Y, and Ibayashi H. Age-related changes in the feedback regulation of gonadotrophin secretion by sex steroids in men. Acta Endocrinol 96: 154–162, 1981.
  38. Nankin HR, Lin T, and Murono EP. The aging Leydig cell. III. Gonadotropin stimulation in men. J Androl 2: 181–186, 1981.[Abstract]
  39. Neaves WB, Johnson L, Porter JC, Parker, CR Jr, and Petty CS. Leydig cell numbers, daily sperm production, and serum gonadotropin levels in aging men. J Clin Endocrinol Metab 59: 756–763, 1984.[Abstract/Free Full Text]
  40. Pavlou SN, Veldhuis JD, Lindner J, Souza KH, Urban RJ, Rivier JE, Vale WW, and Stallard DJ. Persistence of concordant LH, testosterone and alpha subunit pulses following LHRH antagonist administration in normal men. J Clin Endocrinol Metab 70: 1472–1478, 1990.[Abstract/Free Full Text]
  41. Pincus SM, Mulligan T, Iranmanesh A, Gheorghiu S, Godschalk M, and Veldhuis JD. Older males secrete luteinizing hormone and testosterone more irregularly, and jointly more asynchronously, than younger males. Proc Natl Acad Sci USA 93: 14100–14105, 1996.[Abstract/Free Full Text]
  42. Rabinovici J, Rothman P, Monroe SE, Nerenberg C, and Jaffe RB. Endocrine effects and pharmacokinetic characteristics of a potent new gonadotropin-releasing hormone antagonist (Ganirelix) with minimal histamine-releasing properties: studies in postmenopausal women. J Clin Endocrinol Metab 75: 1220–1225, 1992.[Abstract]
  43. Reubens R, Dhondt M, and Vermeulen A. Further studies on Leydig cell response to human choriogonadotropin. J Clin Endocrinol Metab 39: 40–45, 1976.
  44. Saez JM. Leydig cells: endocrine, paracrine, and autocrine regulation. Endocr Rev 15: 574–626, 1994.[Abstract/Free Full Text]
  45. Shaar CJ, Euker JS, Riegle GD, and Meites J. Effects of castration and gonadal steroids on serum luteinizing hormone and prolactin in old and young rats. J Endocrinol 66: 45–51, 1974.[CrossRef][Web of Science]
  46. Sortino MA, Aleppo G, Scapagnini U, and Canonico PL. Different responses of gonadotropin-releasing hormone (GnRH) release to glutamate receptor agonists during aging. Brain Res Bull 41: 359–362, 1996.[CrossRef][Web of Science][Medline]
  47. Stearns EL, MacDonald JA, Kaufman BJ, Padua R, Lucman T, Winter JSD, and Faiman C. Declining testicular function with age, hormonal and clinical correlates. Am J Med 57: 761–766, 1974.[CrossRef][Web of Science][Medline]
  48. Tenover JS, Matsumoto AM, Plymate SR, and Bremner WJ. The effects of aging in normal men on bioavailable testosterone and luteinizing hormone secretion: response to clomiphene citrate. J Clin Endocrinol Metab 65: 1118–1125, 1987.[Abstract/Free Full Text]
  49. Urban RJ, Evans WS, Rogol AD, Kaiser DL, Johnson ML, and Veldhuis JD. Contemporary aspects of discrete peak detection algorithms. I. The paradigm of the luteinizing hormone pulse signal in men. Endocr Rev 9: 3–37, 1988.[Abstract/Free Full Text]
  50. Veldhuis JD, Carlson ML, and Johnson ML. The pituitary gland secretes in bursts: appraising the nature of glandular secretory impulses by simultaneous multiple-parameter deconvolution of plasma hormone concentrations. Proc Natl Acad Sci USA 84: 7686–7690, 1987.[Abstract/Free Full Text]
  51. Veldhuis JD, Fraioli F, Rogol AD, and Dufau ML. Metabolic clearance of biologically active luteinizing hormone in man. J Clin Invest 77: 1122–1128, 1986.[Web of Science][Medline]
  52. Veldhuis JD and Iranmanesh A. Pulsatile intravenous infusion of recombinant human luteinizing hormone under acute gonadotropin-releasing hormone receptor blockade reconstitutes testosterone secretion in young men. J Clin Endocrinol Metab 89: 4474–4479, 2004.[Abstract/Free Full Text]
  53. Veldhuis JD, Iranmanesh A, Godschalk M, and Mulligan T. Older men manifest multifold synchrony disruption of reproductive neurohormone outflow. J Clin Endocrinol Metab 85: 1477–1486, 2000.[Abstract/Free Full Text]
  54. Veldhuis JD, Iranmanesh A, and Keenan DM. An ensemble perspective of aging-related hypoandrogenemia in men. In: Male Hypogonadism: Basic, Clinical, and Theoretical Principles., edited by Winters SJ. Totowa, NJ: Humana, 2004, p. 261–284.
  55. Veldhuis JD, Iranmanesh A, Mulligan T, and Pincus SM. Disruption of the young-adult synchrony between luteinizing hormone release and oscillations in follicle-stimulating hormone, prolactin, and nocturnal penile tumescence (NPT) in healthy older men. J Clin Endocrinol Metab 84: 3498–3505, 1999.[Abstract/Free Full Text]
  56. Veldhuis JD and Johnson ML. Cluster analysis: a simple, versatile and robust algorithm for endocrine pulse detection. Am J Physiol Endocrinol Metab 250: E486–E493, 1986.[Abstract/Free Full Text]
  57. Veldhuis JD and Johnson ML. Specific methodological approaches to selected contemporary issues in deconvolution analysis of pulsatile neuroendocrine data. Methods Neurosci 28: 25–92, 1995.[CrossRef]
  58. Veldhuis JD, Urban RJ, Beitins I, Blizzard RM, Johnson ML, and Dufau ML. Pathophysiological features of the pulsatile secretion of biologically active luteinizing hormone in man. J Steroid Biochem 33: 739–750, 1989.[CrossRef][Web of Science][Medline]
  59. Veldhuis JD, Urban RJ, Lizarralde G, Johnson ML, and Iranmanesh A. Attenuation of luteinizing hormone secretory burst amplitude is a proximate basis for the hypoandrogenism of healthy aging in men. J Clin Endocrinol Metab 75: 52–58, 1992.[Web of Science]
  60. Veldhuis JD, Veldhuis NJ, Keenan DM, and Iranmanesh A. Age diminishes the testicular steroidogenic response to repeated intravenous pulses of recombinant human LH during acute GnRH-receptor blockade in healthy men. Am J Physiol Endocrinol Metab 288: E775–E781, 2005.[Abstract/Free Full Text]
  61. Veldhuis JD, Zwart A, Mulligan T, and Iranmanesh A. Muting of androgen negative feedback unveils impoverished gonadotropin-releasing hormone/luteinizing hormone secretory reactivity in healthy older men. J Clin Endocrinol Metab 86: 529–535, 2001.[Abstract/Free Full Text]
  62. Vermeulen A, Deslypere JP, and De Meirleir K. A new look at the andropause: altered function of the gonadotrophs. J Steroid Biochem 32: 163–165, 1989.[CrossRef][Web of Science][Medline]
  63. Voglmayr JK, Roberson C, and Musto NA. Comparison of androgen levels in ram rete testis fluid, testicular lymph and spermatic venous blood plasma: evidence for a regulatory mechanism in the seminiferous tubules. Biol Reprod 23: 29–39, 1980.[Abstract]
  64. Winters SJ, Sherins RJ, and Troen P. The gonadotropin-suppressive activity of androgen is increased in elderly men. Metabolism 33: 1052–1059, 1984.[CrossRef][Web of Science][Medline]
  65. Winters SJ and Troen P. Episodic luteinizing hormone (LH) secretion and the response of LH and follicle-stimulating hormone to LH-releasing hormone in aged men: evidence for coexistent primary testicular insufficiency and an impairment in gonadotropin secretion. J Clin Endocrinol Metab 55: 560–565, 1982.[Abstract/Free Full Text]
  66. Zhang FP, Pakarainen T, Poutanen M, Toppari J, and Huhtaniemi I. The low gonadotropin-independent constitutive production of testicular testosterone is sufficient to maintain spermatogenesis. Proc Natl Acad Sci USA 100: 13692–13697, 2003.[Abstract/Free Full Text]
  67. Zwart AD, Urban RJ, Odell WD, and Veldhuis JD. Contrasts in the gonadotropin-releasing dose-response relationships for luteinizing hormone, follicle-stimulating hormone, and alpha-subunit release in young versus older men: appraisal with high-specificity immunoradiometric assay and deconvolution analysis. Eur J Endocrinol 135: 399–406, 1996.[Abstract/Free Full Text]




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