Testosterone Secretion in the Rat in Response to Chorionic Gonadotrophin: Alterations with Age

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Abstract
Both prevailing blood concentrations of testosterone and Leydig cell response to gonadotropin (up to 1 h after injection) are reduced in the aged male rat. Although resting levels of plasma testosterone in aged (24-26 mo. old) Sprague-Dawley rat are also depressed compared with young (3-4 mo.) or mature (12 mo.) animals of the same strain, s.c. injection with human chorionic gonadotropin for 3 days restores secretory function, producing testosterone levels indistinguishable from those of similarly stimulated younger rats. In short term experiments, old rats did show a diminished testosterone secretory response to human chorionic gonadotropin 1 h after a single i.v. injection, consistent with previous reports, but restoration of normal stimulated levels was observed by 2 h, and persisted up to 24 h. These findings differ from the demonstrated intrinsic testicular hyporesponsiveness to gonadotropin of aged men, and probably represent a state of chronic understimulation of the aged Leydig cells, due to low prevailing levels of LH [luteinizing hormone].