Muting of Androgen Negative Feedback Unveils Impoverished Gonadotropin-Releasing Hormone/Luteinizing Hormone Secretory Reactivity in Healthy Older Men1

Plasma bioavailable testosterone concentrations decline in healthy older men without a uniformly commensurate rise in serum LH concentrations, which disparity is consistent with a hypothesis of relative hypogonadotropism. Likewise, preserved gonadotrope responsiveness to exogenous GnRH stimulation, despite an attenuated amplitude of endogenous LH pulses, points to reduced hypothalamic GnRH feedforward signaling in aging males. To appraise GnRH/LH secretory reserve more directly in older men, we have compared daily LH secretion, driven by profound short-term blockade of androgen biosynthesis by oral ketoconazole administration, in nine young (ages, 18-32 yr) and seven older (ages, 60-73 yr) volunteers. The ability to unleash endogenous GnRH-driven LH secretion in response to acute testosterone withdrawal was quantitated by sampling blood every 10 min, for 24 h, followed by high-precision immunoradiometric assay. The resultant serum LH concentration profiles were analyzed by: 1) model-free discrete peak detection (Cluster) analysis; 2) the approximate entropy statistic to quantitate pattern regularity; and 3) 24-h rhythmic (cosinor) analysis. At baseline, mean and integrated (24-h) serum LH concentrations were similar in both age cohorts. However, Cluster analysis established an elevated LH peak frequency [18 +/- 0.86 (older) vs. 13 +/- 1.3 pulses/24 h (young), P = 0.0028] and a reduced incremental LH pulse area [37 +/- 6.9 (older) vs. 106 +/- 20 (young) IU/L x min, P = 0.016] in older men. Approximate entropy calculations also revealed more irregular LH release patterns in older men before intervention (P = 0.00089). Feedback stress, achieved by ketoconazole-induced androgen deprivation, unmasked further neuroregulatory defects in older volunteers, who failed to equivalently increase the: 1) mean (24-h) serum LH concentration [i.e. to 5.0 +/- 0.99 (older men) vs. 9.0 +/- 1.1 (young) IU/L, P = 0.000071]; 2) maximal LH peak height (to 6.1 +/- 1.1 vs. 10.4 +/- 1.2 IU/L, P = 0.00043); 3) incremental LH pulse area (to 41 +/- 8.8 vs. 87 +/- 20 IU/L x min, P = 0.016); 4) interpeak nadir serum LH concentration (to 4.0 +/- 0.77 vs. 7.9 +/- 1.0 IU/L, P < 10(-6)); 5) the quantitable irregularity of LH release (P = 0.00089); and 6) the mesor of 24-h rhythmic LH secretion (P = 0.000062). In summary, experimental imposition of a novel hypoandrogenemic open-loop feedback stressor, for 48 h, to heighten hypothalamic GnRH feedforward drive, unveils impoverished augmentation of LH pulse mass, impaired orderliness of LH release, and diminished 24-h rhythmic LH secretion in older men. The foregoing trilogy of neuroregulatory defects identifies unequivocally attenuated hypothalamo-pituitary reactivity to muting of androgen negative feedback in the aging male.