Alterations In Testosterone Feedback Restraint

Whether or how sex-steroid negative feedback is altered in older men is controversial. For example, three clinical studies report heightened negative feedback by exogenous androgens in the aging male. These analyses were based on short-term i.v. infusion of testosterone and or dihydrotestosterone (DHT) for 4 d (33) or transdermal delivery of 5-a-DHT for 11 d (72) or testosterone for 12 mo (73). Two other investigations describe reduced androgen feedback efficacy in the older male after several weeks of im injection of high-dose testosterone (54,74). Two interventional studies using an estrogen- and or androgen-receptor antagonist and one biomathematical construct imply reduced LH secretory adaptations to muted sex-steroid feedback in elderly men (56,75,76). From another vantage, cross-correlation analysis of 24-h (paired) LH and testosterone concentration profiles identifies impaired negative feedback by endogenous androgen in older volunteers (29). Based on immunocytochemical detection, androgen (and estrogen) receptors may decline in the brain and pituitary gland of aged rats and in genital fibroblasts of older men (77,78).

Freely Varying Basal SDE Model

Freely Varying Basal SDE Model

Young Older

Freely Varying Basal a>

CO CO CD

Young Older

Freely Varying Basal a>

CO CO CD

P = 0.038 o v

o

T

a 5.46 ± 0.66

(5.2)

3.73 ± 0.58

9

¥

(2.8)

s

+

T

Older

Fig. 5. Comparison of luteinizing hormone (LH) secretory attributes in young and older men based on a variable-waveform model that allows a freely varying basal secretion rate. (A) Impact of age on LH pulse frequency (events/day) and LH secretory-burst mass (international unit secreted per pulse per liter of distribution volume). (Figure continues)

Young

Older

Fig. 5. Comparison of luteinizing hormone (LH) secretory attributes in young and older men based on a variable-waveform model that allows a freely varying basal secretion rate. (A) Impact of age on LH pulse frequency (events/day) and LH secretory-burst mass (international unit secreted per pulse per liter of distribution volume). (Figure continues)

Prominent adaptations to experimental androgen depletion in young men are accelerated frequency and reduced incremental amplitude of LH pulses (see Fig. 5A) and quantitatively irregular patterns of LH release (76,79-81). The neuroendocrine phe-notype in older men includes each of the foregoing feedback adjustments (27,28,30,31,55). Thus, a facile hypothesis is that lower bioavailable and free-testosterone concentrations in the elderly male are the proximate cause of unleashing anomalous LH secretion patterns. However, quite unlike the feedback-deprived state, basal, pulsatile and total LH secretion remain normal in many healthy older men (38,39) (see Fig. 5B). Accordingly, an important but unresolved mechanistic issue is whether neuroendocrine disturbances in aging individuals reflect expected feedback withdrawal resulting from hypoandrogenemia (37). This question becomes important, inasmuch as testosterone repletion in androgen-deficient young men reverses each of rapid-frequency, diminutive-amplitude, and disorderly LH secretion (76,79,80).

Was this article helpful?

0 0
100 Hair Growth Tips

100 Hair Growth Tips

100 Hair Growth Tips EVERY Balding Person Should Know. This Report

Get My Free Ebook


Post a comment