Pituitary-Ovarian Relationships during the Estrous Cycle and the Influence of Parity in an Inbred Strain of Miniature Swine1

Pituitary-ovarian function was analyzed in a strain of miniature swine previously shown to produce a low ovulation rate resulting in the formation of only 8.6 corpora lutea (CL)/animal. Five multiparous (M) and four nulliparous (N) miniature pigs with a mean inbreeding coefficient of .39 were monitored for estrous behavior through four consecutive estrous cycles. Daily blood samples were collected from 5 d before to 5 d after the onset of the second, third and fourth estrus and at 48-h intervals during the remainder of the second and third estrous cycle. Laparoscopy was used to examine the ovaries 1 and 5 d after onset of the third estrus and 2 d after the beginning of the fourth estrus. For the entire group, temporal fluctuations among serum estradiol-17β, luteinizing hormone (LH) and progesterone concentrations and sexual behavior were similar to previously published data in standard swine breeds. Although the mean lengths of the estrous cycle were not different (P> .05) between parity subgroups (M, 23 ± 1.3 vs N, 22 ± .7 d), multiparous pigs were in estrus longer (P<.05) than nulliparous females (M, 3.7 ± .2 vs N, 2.2 ± .4 d). Parity subgroups were similar with respect to the mean number of follicles forming CL (M, 8.8 ± .7 vs N, 9.2 ± .2). Although an average of 6.2 ± 2.1 CL had formed by 24 h after onset of estrus in the nulliparous subgroup, no CL were detected in the multiparous subgroup at this time. Of the total of 12 LH peaks detected in nulliparous females, eight (67%) occurred on the day preceding estrus or the first day of estrus and averaged 8.6 ±1.3 ng/ml. In contrast, the majority (10/15, 67%) of LH peaks in multiparous pigs were produced on the second day of estrus and averaged 14.7 ± .9 ng/ml. The results integrate hormonalovarian events in an inbred strain of miniature pig. Furthermore, parity appeared to influence the timing of reproductive events in that nulliparous pigs 1) experienced a shorter estrus, 2) produced an earlier preovulatory LH release of lower magnitude and 3) ovulated earlier than multiparous females.