SERUM CONCENTRATIONS OF IMMUNOACTIVE AND BIOACTIVE LUTEINIZING-HORMONE (LH) IN GILTS .2. 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 pre pubertal gilts were relocated and exposed to boars to stimulate the on set of puberty. Puberty was defined as the first sign of behavioral es trus and occurrence of a preovulatory surge of LH. Blood samples were taken every 2 h, beginning on the third or fifth day after boar exposu re; sampling was continued for 24 h after each gilt had exhibited estr us, or for 7 days, or until two-thirds of the gilts in a group attaine d puberty. Fifty percent of the total number of gilts exposed to boars attained puberty within 7 days of first boar exposure. Total area und er 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 foll icular phases of the estrous cycle. Blood samples were collected every 20 min for a 6-h period of spontaneous secretion; then GnRH was injec ted and blood samples were collected every 20 min for 4 h from gilts d uring the luteal (n = 16) and follicular (n = 15) phases, respectively . BLH baseline tended to be greater (p = 0.07) during the follicular p hase than during the luteal phase. Mean peak amplitude of naturally oc curring peaks of ILH and BLH was greater (p < 0.05) during the luteal phase than during the follicular phase. The B:I ratio of the GnRH-indu ced peaks was greater (p = 0.04) during the follicular than during the luteal phase. Greater baseline concentrations of BLH during the folli cular phase may be important for follicular growth and development.