Characterization of the LH Surge in Middle-Aged Female Rats1

Abstract

Serum luteinizing hormone (LII) was measured throughout the afternoon and evening (1500-0130 h) of proestrus in young (4.5 month old) and middle-aged (10.5 month old) female rats displaying regular 4-day estrous cycles as judged by vaginal smears. An LI-I surge was measured in 22 of 24 young females. In 20 of the young females, the maximum concentration of LI-I in the blood was measured at 1800 h. The LH surge in middle-aged females was more variable in amplitude, and in 21 of the 29 animals in which a surge was observed it occurred at 1930 h or later.