Reduced pulsatile luteinizing hormone and testosterone secretion with aging in the male rat

To identify possible age-dependent changes in the feedback relationship between the brain-pituitary and testes, we examined the minute-to-minute patterns of plasma luteinizing hormone (LH) and testosterone (T) in intact, young male rats and compared these profiles to those of old animals. Young (3 mo; n=11) and old (22 mo; n=12) Spraque-Dawley rats were fitted with indwelling venous catheters and between 24 and 48 hr later, were bled without anesthesia, by remote sampling, at 10-min intervals for 8 h. Blood samples of 400 μl were withdrawn, and an equivalent volume of a blood replacement mixture was infused after each sample. Plasma LH and T levels in each sample were measured by radioimmunoassay (RIA). Plasma T levels in old animals failed to show the transient oscillations observed in young animals. Mean plasma T levels were 50% lower in old compared to young animals (P<0.001). Plasma patterns of LH in old animals, like their younger counterparts, showed statistically significant episodic increases, whose apparent pulse frequency was inappropriately low for their circulating T level (although not statistically different from the young group). Pulse amplitude in the old animals was 66% lower in the old compared to the young group (P<0.015). We conclude that age-associated alterations in brain mechanisms governing LH secretion underlie these endocrine changes.