Relationship between progesterone and pregnacy associated glycoprotein concentrations in the maternal circulation durino early pregnancy in dairy cows

The corpus luteum (CL) plays a major role for the successful establishment of pregnancy, and CL function during early embryonic development and implantation seems to be crucial for a correct foetus-maternal interaction. Pregnancy-associated glicoproteins (PAGs) represent a large family of glycoproteins expressed in the mononucleate and binucleate cells of the trophectoderm of several species. In the bovine, PAGs are detectable in the maternal circulation from D 24 after fertilization until parturition, and they are unquestionable indicators of the presence of a viable embryo. However, data about the biological function and the control of secretion of these glycoproteins are scarce. In this study, we tested the hypothesis that the progesterone (P4) secreted by the CL around the time of embryo implantation may influence PAG secretion during early pregnancy. The outcome of 84 artificial inseminations (AI) was studied in 56 pluriparous cows by measuring PAG and P4 (RIA) in maternal plasma samples taken at AI (D 0) and at 21, 28, 30, 45, 60 and 90 D after AI. Confirmation of pregnancy was performed by trans-rectal examination after D 45. Fourty-four AIs (52.4%) were positive, while embryo loss was observed in 10 cases (11.9%). On D 21, plasma P4 was higher, although not significantly, in positive AIs (6.09±0.39 ng/ml) than in AIs followed by embryo loss (4.88±0.79 ng/ml). Plasma PAG concentrations were significantly higher (P<0.01) in AIs performed before 80 days post parturition and were excluded from subsequent analysis. Plasma PAG concentrations were significantly lower in AIs followed by embryo loss (P<0.001) between D 30 and D 60. In most cases, embryo loss occurred between D 30 and D 45. Hormone data obtained from positive AIs performed after 80 days post parturition (N=40) were used to study the relationships between plasma progesterone measured on D 21 and PAG plasma concentrations. AIs were grouped on the basis of P4 concentrations observed on D 21 into Low P4 Group (P4<6.0 ng/ml at D 21) and High P4 Group (P4≥6.0 ng/ml at D 21), and the relationship between plasma P4 and PAG was analysed by ANOVA. On average, AIs of the High P4 Group showed higher levels of circulating PAG between D 30 and D 60 and, in particular, plasma PAG was significantly greater on D 60 (High P4 Group: 5.62±0.63 ng/ml; Low P4 Group: 4.00±0.57 ng/ml; P<0.05). The results of the present study confirm the hypothesis that circulating P4 around the time of embryo implantation can affect PAG secretion in early pregnancy.