Serum concentrations of immunoactive and bioactive luteinizing hormone (LH) in gilts. II. Biopotency of LH increases at puberty but remains constant throughout the estrous cycle.

To determine serum concentrations of immunoactive (I) and bioactive (B) LH during the pubertal surge of LH release, two groups of twelve prepubertal gilts were relocated and exposed to boars to stimulate the onset of puberty. Puberty was defined as the first sign of behavioral estrus and occurrence of a preovulatory surge of LH. Blood samples were taken every 2 h, beginning on the third or fifth day after boar exposure; sampling was continued for 24 h after each gilt had exhibited estrus, or for 7 days, or until two-thirds of the gilts in a group attained puberty. Fifty percent of the total number of gilts exposed to boars attained puberty within 7 days of first boar exposure. Total area under the ILH curve was greater (p < 0.01) than area under the BLH curve during the ovulatory rise in LH. A second experiment was conducted to compare serum concentrations of ILH and BLH during the luteal and follicular phases of the estrous cycle. Blood samples were collected every 20 min for a 6-h period of spontaneous secretion; then GnRH was injected and blood samples were collected every 20 min for 4 h from gilts during the luteal (n = 16) and follicular (n = 15) phases, respectively. BLH baseline tended to be greater (p = 0.07) during the follicular phase than during the luteal phase. Mean peak amplitude of naturally occurring peaks of ILH and BLH was greater (p < 0.05) during the luteal phase than during the follicular phase.(ABSTRACT TRUNCATED AT 250 WORDS)