78 Regression of an accessory corpus luteum during pregnancy affects placental function in lactating dairy cows

In cattle, progesterone (P₄) produced by the corpus luteum (CL) is crucial for initiating and sustaining pregnancy. To increase P₄ concentration, human chorionic gonadotropin (hCG) administration on Day 5 postoestrus has been used to induce ovulation of the first wave dominant follicle and formation of an accessory CL (ACL) in cattle. Regression of ACL formed by hCG treatment has been reported in pregnant cows. The decrease in plasma P₄ is expected when ACL is regressed. Relationship between pregnancy-specific protein B (PSPB), which present at the placenta-uterine interface and the P₄ level in circulation of pregnant cows was reported. Therefore, the placental function can be influenced by the ACL regression. The objectives of the present study were: (1) to investigate whether the characteristics of original CL and ACL are involved in the occurrence of ACL regression in pregnant cows and (2) to evaluate the relationship between ACL regression and placental function. Lactating Holstein cows were artificially inseminated at day of oestrus (Day 0), followed by 1500 IU of hCG administration on Day 5. Pregnancy diagnosis and ACL formation were confirmed by ultrasonography at Day 30. In experiment 1, pregnant cows (n = 132) were divided into two groups as follows: (1) ipsilateral ACL (iACL), in which ACL was ipsilateral to an original CL, and (2) contralateral ACL (cACL), in which ACL was contralateral to an original CL. Sizes of original CL and ACL was analysed at Day 35, 42, 49, 56, and 63. In experiment 2, pregnant cows with cACL were further subdivided into two groups as follows: (1) with regression of ACL (with regression; n = 5), or (2) without regression of ACL (without regression; n = 5). To investigate the effects of ACL regression on the placental function, fetal crown–rump length and concentrations of plasma P₄ and PSPB were analysed at Day 35, 42, 49, 56, and 63. The rates of ACL regression from Days 35 to 63 were compared between the iACL and cACL groups using the chi-squared test. Sizes of original CL and ACL, plasma P₄ concentration, PSPB, and fetal size were compared between specific groups (iACL vs cACL or with regulation vs without regulation) using two-way repeated-measures ANOVA. Post hoc analysis was performed using the Tukey Games-Howell test. In experiment 1, ACL regression was more frequent in cACL (34.3%, 23/67, P < 0.05) than iACL (7.7%, 5/65). Sizes of original CL and ACL at Day 35, respectively, were not different between iACL and cACL regardless of whether the ACL underwent regression or not. In experiment 2, PSPB decreased following ACL regression, concomitantly with decreased levels of P₄. There was no difference in fetal size between with and without regulation. Present study suggests that the characteristics of the CL might not influence the regression of the ACL during pregnancy in cows, because the size of ACL at Day 35 was not related to occurrence of ACL regression. Although decreased levels of PSPB were shown with regression of ACL during pregnancy, which may indicate potential alterations in placental function, these changes may not significantly affect fetal growth.