Endocrine changes of reproductive functions in aging male animals

Hormone secretion is age dependent, endocrine patterns of young bulls do not reflect the ''typical'' adult animal. Basal concentrations, frequency and amplitude of LH release are higher in fertile old than in young bulls approaching a ''castrate type'' secretion due to a vanishing negative feedback of gonadal steroids. Basal FSH and response to GnRH challenges are inhibited in young bulls in which regularly semen is collected and are much higher (also in comparison to relative changes of LH) in older animals. Mean, basal and maximal concentrations of gonadal testosterone and estradiol-17 beta increase with age reaching an upper limit in response to altered gonadotrophic stimulation. The age depen-ding increases of FSH and estradiol-17 beta are about linear. Certain thresholds in the slope may be used as diagnostic tools predicting subfertility. The aging Sertoli cell contributes to a vanishing inhibin mediated FSH suppression and to a progressive impairment of sperm cell production. Semen collection (short-term stress) induces more pronounced responses of prolactin, GH and cortisol in a higher number of young than old bulls whereas some young and most older bulls do not exhibit major responses to semen collection. Pulsatile and mean gonadotrophin secretion as well as gonadal steroids and metabolic hormones may be severely sup-presssed by food reduction stress. Some individual bulls react less to chronic stress than others. Stimulation of the somatotrophic axis of bulls results in higher fertility. We conclude that functional age is independent of life age.