Serum LH Levels Following Multiple LHRH Injections in Aging Rats

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Abstract
The effect of 3 serial injections of 500 ng of LHRH [luteinizing hormone releasing hormone] on serum LH [luteinizing hormone] was tested in young (4-5 mo.) and aged (24-28 mo.) male and female Long-Evans rats. Aged female groups were rats showing constant estrous or pseudopregnant vaginal cytology (at least 10 days of cornified or leukocytic vaginal smears, respectively) which were compared to young female rats at estrous and diestrous stages of their ovarian cycle. The increase in serum LH following the 1st LHRH stimulation was less in the aged than in the young groups. Serum LH concentrations in the aged female group were progressively increased following the 2nd and 3rd LHRH injections. The increase in serum LH following the 2nd and 3rd LHRH injections was similar in young and aged female groups. LH concentration in blood samples collected before LHRH injection was progressively increased in both aged female groups. Although LH was greater in young than aged males after the 1st LHRH injection, LH concentrations after the 2nd and 3rd injections were similar in both ages. Aged rat pituitaries can sustain higher serum LH concentrations than are normally found in their circulation. Failure of hypothalamic hormonal stimulation of LH release may contribute to the loss of reproductive function in aging. The increase in LH following multiple LHRH injections in aged female rats reflects increased pituitary responsiveness following the priming effect of the 1st stimulation and reduced LH clearance.