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Division of Endocrinology and Metabolism (J.D.V.), Department of Internal Medicine, Mayo Medical and Graduate Schools of Medicine, General Clinical Research Center, Mayo Clinic, Rochester, Minnesota 55905; Endocrine Service (A.I.), Medical Section, Salem Veterans Affairs Medical Center, Salem, Virginia 24153; and Department of Statistics (D.M.K.), University of Virginia, Charlottesville, Virginia 22908
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, Mayo Clinic, 200 First Street Southwest, Rochester, Minnesota 55905. E-mail: veldhuis.johannes{at}mayo.edu.
The present study tests the intuition that successful aging in men is marked by: 1) impaired feedforward by endogenous LH concentrations (con) of testosterone (Te) secretion (sec); and/or 2) attenuated feedback by unmanipulated Te con of LH sec. The goal was to assess both implicit linkages analytically without disrupting normal pathway coupling. This strategy required: 1) assay of paired LH and Te con sampled every 10 min for 24 h in 13 older (O) (ages 60–78 yr) and 13 young (Y) (ages 18–30 yr) men; 2) deconvolution-based estimation of LH and Te sec rates; 3) lag-specific cross-correlation analyses of the relationships between LH and Te con and sec; and 4) statistical contrasts by age stratum. Salient outcomes were: 1) O and Y men maintain comparable LH con drive of Te sec, viz maximal r = +0.51 and r = +0.52, respectively, at an optimal time lag of 50 min (both P < 0.001 against random LH and Te associations); 2) elderly subjects exhibit reduced Te con inhibition of LH sec [minimal r = –0.008 (O) vs. r = –0.10 (Y), P < 0.01 at a time lag of 40 min]; 3) mean (24-h) LH con do not differ by age; and 4) molar Te/sex hormone-binding globulin con are lower in the elderly than in Y individuals (P < 0.01).
In conclusion, noninvasive analyses predict that attenuation of endogenous Te feedback restraint on the hypothalamo-pituitary unit may be an early biological marker of adaptive changes in the GnRH-LH-Te ensemble axis in the healthy O male.
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