Orchestrated partitioning of maternal nutrients during ovine pregnancy

Ovine placental lactogen (oPL) is postulated to be involved in the repartitioning of maternal nutrients during pregnancy, through its effect on insulin metabolism. Ovine pancreatic insulin responses to exogenous glucose are depressed during pregnancy and this depression becomes more pronounced as gestation advances. In addition, under the hormonal environment of rising oPL and growth hormone (oGH) concentrations, maternal whole body glucose irreversible loss (GIL) increases. The percentage of GIL accounted for by uterine glucose uptake also increases with advancing gestation and increasing litter size. Regression analysis of oPL concentration with glucose uterine uptake as a percentage of GIL, accounts for 39% of variation. Maternal oPL concentrations which increase with gestational age, were significantly greater in multiple bearing ewes and ewes subjected to reduced metabolisable energy (ME) intakes. It is postulated that through actions on pancreatic sensitivity, oPL plays a major role as a homeorhetic control during pregnancy. Elevated oPL concentrations were strongly associated with continually depressed pancreatic insulin secretory ability. The reduction in pancreatic sensitivity to glucose was not as a result of elevation in GH or non-esterified fatty acid (NEFA) concentrations. Muscle insulin receptor number and affinity were found to increase with increasing litter size, suggesting that pregnancy associated insulin resistance occurs predominantly in adipose tissue. During ovine pregnancy there is a specific stimulation of maternal gluconeogenesis. As gestation advances, an increasingly greater proportion of this glucose is partitioned to the gravid uterus. The development of insulin resistance, together with the suppression of pancreatic activity, ensures the preferential uptake of glucose by non-insulin dependent tissues over insulin dependent tissues. These activities favour uterine glucose uptake, decrease adipose glucose uptake, and also promote adipose mobilisation and hepatic gluconeogenesis, so as to meet the increasing energy requirement of pregnancy. It is postulated that through these effects on insulin secretion and associated adipose tissue mobilisation factors, oPL plays a major role in homeorhesis during pregnancy