Biological activity of LH during the peripartum period and the estrous cycle of the ewe

Abstract

Biological activity of luteinizing hormone (LH) is related to the degree of glycosylation of the glycoprotein hormone. The objectives of this study were to determine changes in biologically (BLH) and immunologically (ILH) active LH concentrations in plasma ( in vitro bioassay and radioimmunoassay, respectively) and in the ratio of BLH to ILH (B:I) during the peripartum period and during the estrous cycle of the ewe. Blood samples were collected daily 4 days before through 4 days after parturition and during one estrous cycle. Also, samples were collected at 15-min intervals for 6 hr on Days 3 and 12 of the estrous cycle to quantify the influence of an elevated plasma concentration of progesterone (P) on the episodic secretion profiles of BLH. Progesterone concentration was determined on the 4th days pre- and post-partum, on each day of the estrous cycle and at hourly intervals on Days 3 and 12 of the cycle to investigate the hypothesis of an inverse relationship between P and BLH. The BLH and ILH concentrations were low during the peripartum period, and the B:I ratio did not increase by the 4th day postpartum. Mean ILH concentration was greater (P<.05) in the postpartum than during the prepartum period. During the estrous cycle, mean daily B:I ratio was consistently above unity except for the day of estrus. The pre-ovulatory LH surge (BLH and ILH) was associated with a decrease (P<.05) in the mean B:I ratio to 0.065. Mean concentrations of BLH and ILH in plasma samples collected every 15 min on Day 12 were similar to Day 3 of the cycle. On Day 12 the frequency of BLH and ILH pulses decreased (P<.05), but the peak height was not different from Day 3. These data indicate that both biological and immunological activity of LH are suppressed during late gestation and the escape from the suppressive effects of gestation is more rapid for immunological than for biological activity of LH. In addition, the change in B:I ratio during estrus demonstrates differences in biological potency of LH associated with the preovulatory LH surge. We suggest that factors of placental and ovarian origin may influence biological activity of LH in the ewe.