Effect of maternal serum insulin on umbilical extraction of glucose and lactate in fed and fasted sheep

Abstract

In 18 chronically instrumented pregnant ewes in late gestation, umbilical extractions of glucose and lactate were determined before and during the continuous infusion of ovine insulin, 0.25 mU/kg . min, into one uterine artery. Studies were conducted in both well-nourished and fasting ewes. Exogenous insulin increased the umbilical extraction of glucose without altering the umbilical extraction of lactate over the range of concentrations of maternal blood glucose encountered in the fed and fasted states. Moreover, the efflux of lactate into the uterine venous circulation in two additional ewes was not altered by the administration of insulin. These studies support the hypothesis that maternal circulating insulin binds to insulin receptors on the microvillous brush border of the placenta to effect an increase in the carrier-mediated transfer of glucose rather than cause an intraplacental decrease in the catabolism of glucose to lactate. The results suggest that, as maternal levels of blood glucose and serum insulin spontaneously rise in concert over the physiologic range, both factors may contribute to the increasing umbilical extraction of glucose. Furthermore, these observations raise the possibility that decreased binding of insulin by placental insulin receptors, which is reported to occur in placentas from diabetic women, may be accompanied by a relatively decreased umbilical uptake of glucose for a given maternal concentration of glucose, but not of lactate.