MATERNAL ENDOCRINE STATUS IN RELATION TO PREGNANCY OUTCOME IN RAPIDLYGROWING ADOLESCENT SHEEP

It has previously been reported that high nutrient intakes which promo te rapid maternal growth throughout pregnancy are associated with poor pregnancy outcome when compared with normally growing adolescent anim als. The present study examined the maternal plasma concentrations of a number of putative endocrine regulators of nutrient partitioning bet ween the maternal and fetal compartments in relation to placental and fetal growth in this novel experimental paradigm. Embryos were recover ed on day 4 after oestrus fi-om superovulated adult ewes that had been inseminated using semen from a single sire and synchronously transfer red, in singleton, to the uterus of peripubertal adolescent recipients (n=38), which had been induced to ovulate at 32 weeks of age Give wei ght 47.4+/-0.4 kg). Post-transfer, the adolescent recipients were offe red a high (n=21) or moderate (n=17) level of a complete diet calculat ed to achieve rapid (RMG) or normal (NMG) maternal growth rates. After day 100 of gestation, the feed intake of the NMG group was adjusted w eekly to meet the increasing nutrient demands of the gravid uterus. Pr egnancy rate following embryo transfer was higher (P<0.05) in the RMG (90%) than in the NMG (59%) group. For ewes delivering live young at t erm, liveweight gain during the first 100 days of gestation was 294 +/ - 12.9 and 84 +/- 4.7 g/day for the RMG (n=16) and NMG (n=10) groups r espectively, and body condition score immediately prior to parturition was higher in RMG than in NMG ewes (2.9 +/- 0.04 vs 1.9 +/- 0.15 scor e units respectively, P<0.001). For the RMG and NMG groups respectivel y, mean placental weight was 327 +/- 18.1 and 485 +/- 16.6 g with lamb birth weights of 3.49 +/- 0.13 and 4.82 +/- 0.21 kg (P<0.001). The re duction in placental mass in the RMG group reflected a decrease in the number (P<0.001) and size (P<0.01) of the fetal cotyledons. The durat ion of gestation was shorter (P<0.001) and colostrum yield at parturit ion lower (P<0.001) in the RMG group. Maternal insulin concentrations, determined three times weekly, were higher (P<0.001) throughout gesta tion in the RMG group and irrespective of treatment group were negativ ely correlated (P<0.01) with placental weight and lamb birth weight. H igh glucose levels throughout gestation and a decreased response to an exogenous insulin challenge on day 95 of gestation implied a degree o f insulin resistance in the RMG group but, in spite of these high mate rnal glucose concentrations, the reduced size of the placenta probably constrained fetal growth. Maternal IGF-I levels determined weekly, we re elevated (P<0.001) during the second and third trimester in RMG ver sus NMG groups and a sustained elevation in maternal tri-iodothyronine and thyroxine concentrations was evident in the RMG group from mid-ge station. In contrast, GH pulse frequency and mean GH concentrations, d etermined on day 68 and 122 of gestation, were lower (P<0.05) in the R MG group, and irrespective of treatment group, were correlated negativ ely with feed intake and positively with placental weight and colostru m yield at parturition. Progesterone concentrations were lower in the RMG group during the second and third trimesters (P<0.001) and, irresp ective of treatment group, were positively associated (P<0.001) with p lacental weight, gestation length and colostrum yield. These results s uggest that in pregnant adolescent sheep on high dietary intakes, elev ated insulin and IGF-I levels ensure that the anabolic drive to matern al tissue synthesis is established during early gestation at the expen se of placental growth. The consequent restriction in placental transp ort capacity is the primary limitation to fetal growth and reduced GH and placental progesterone secretion may impair colostrum production.