Because of the unpredictable occurrence of embryonic or foetal losses in a herd and the current increasing interest of monitoring pregnancies in bovine, it is important to monitor pregnancies at early stage. One way to create a model of foetal death is to use drugs like cloprostenol that induce mortality.
Lindell and co-workers  induced abortions in heifers and found differences on physical nature of the abortions and PG metabolite release between two different stages of pregnancies (pregnancies below 75 days and between 100 – 150 days). In the former group foetuses were delivered with intact foetal membranes with little or no bleeding whereas in the latter group the aborted foetuses were delivered prior to the membranes, which were retained for more than 24 hours. The proposed reason for the differences in the nature of the induced abortions was the differences in the degree of foetal membrane attachment. The findings of the present study are in agreement with the report of Lindell and co-workers , but heifer no. 3 is obviously falling between the two groups since she had blood tinged vaginal discharge and expelled foetus with intact foetal membrane.
Standing oestrus was observed in two of the heifers (nos. 1 & 2) within two days after abortion supporting the suggestion given by Lindell and co-workers . These authors suggested that abortion could be induced up to 80 days of pregnancy for practical reasons with little or no compromise at the subsequent reproductive performance at least in heifers. This is because such abortions are associated with only little or no uterine trauma. This idea is further supported by the return of PG metabolite release to the basal level immediately after abortion in these heifers unlike those heifers above 100 days of pregnancy as it was observed by Lindell and co-workers  and the current study.
Detection of PSPB or bPAG1 above the threshold levels in the maternal blood of cows or heifers is a good indicator of the presence of a live embryo or foetus with the exceptions during the postpartum period or for few days after embryonic/foetal death [5, 6]. Moreover, the plasma/serum levels of PSPB/bPAG1 fell steadily commencing within 24 hours of inoculation/injection  or embryonic/foetal mortality  following experimental Arcanobacterium pyogens infection or cloprostenol injection. On the other hand, in the current study the gradual fall of plasma bPAG1 concentration commenced after 48 hours of cloprostenol treatment (after expulsion of the foetuses) in three of the heifers and even later in the fourth heifer. The previous experiments involved heifers/cows at pregnancy stages less than 50 days whereas the current one above 60 days, which may explain the observed difference in the time of start of decline in plasma bPAG1 concentration. Szenci and co-workers  reported a half-life of 3.2 – 3.9 days of bPAG1 after cloprostenol induced embryonic mortality in heifers, which falls within the range of the current finding. Moreover, Semambo and co-workers  reported approximately seven days half-life of PSPB, which is roughly closer to the present finding. The minor differences observed in the half-life of bPAG1 among the reports of different workers may be due to differences in the stages of pregnancy at the time of induction of embryonic/foetal mortality. In the current study the plasma level of bPAG1 did not fall immediately after death of the foetus but it showed minor changes until the foetus was expelled. These minor changes in plasma bPAG1 concentration could be possibly explained by the effect of uterine contraction caused by the pulsatile release of the endogenous PGF2α (measured as the metabolite) and a continuity of the placental release of bPAG1 for a brief time even after foetal death. Moreover, the differences in the rate of decline of plasma bPAG1 concentration observed among the heifers at different stages of gestation period could also be possibly attributed to the increase in the plasma concentration and half-life of the glycoprotein with increasing stage of pregnancy.
The plasma PG metabolite level before cloprostenol treatment was in the basal level (below 300 pmol/L) but the level increased immediately after the treatment, which agrees with the results of Lindell and co-workers . However, the reason behind such immediate rise of the peripheral blood level of endogenous PG metabolite after cloprostenol injection is not well established and it needs further investigation.
In heifer no. 4, the pulsatile release of PG metabolite continued up to five days post PG injection even after expulsion of the foetus though the highest concentration of PG metabolite was 623 pmol/L. This plasma PG metabolite concentration is low as compared to the previous report of Lindell and co-workers , who reported massive release of PG metabolite up to 2500 pmol/L in heifers having the same pregnancy stage (heifer no. 4) at the time of abortion induced by cloprostenol. This difference in the level of PG metabolite could be partly attributed to the effect of retained foetal membranes in case of the latter heifer with higher level of PG metabolite. This is because cows with retained foetal membranes had significantly higher levels of PG metabolite than cows without retained foetal membranes during the immediate postpartum period .
In the current study, a sharp decline of plasma levels of progesterone (from above 20.7 nmol/L to less than 4 nmol/L) during 24 hours post cloprostenol injection was observed. This finding agrees with previous reports that indicated the luteolytic effect of cloprostenol in cyclic non-pregnant or pregnant cows/heifers [9, 20–22]. In the present study, the disruption of foeto-endometrial connection as a result of contraction of the uterus caused by the pulsatile release of the endogenous PG metabolite could be the possible cause for the occurrence of foetal deaths within 24 to 48 hours after cloprostenol injection. Szenci and co-workers  reported the occurrence of late embryonic mortalities within 24 and between 48 and 72 hours post Arcanobacterium pyogens inoculation and cloprostenol treatment, respectively. In another cloprostenol induced abortion study, the progesterone concentration dramatically declined to < 0.5 ng/mL within 24 hours of treatment .