Maternal factors modulating nutrient transfer to fetus.

Current knowledge of the modulation of maternal-fetal transfer of metabolites is reviewed and new data on the actual placental transport of D-glucose, L-alanine and glycerol in the rat are presented. Twenty-one day pregnant rats were infused with the 14C-labelled substrates throughout the left uterine artery. Radioactivity appearing in fetuses was corrected by the specific dilution of the tracer at maternal arterial plasma and the uterine artery blood flow to estimate placental transfer. This parameter appeared to be 127 mumol X kg-1 fetal b.w. X min-1 for D-glucose, 23 for L-alanine, and 1 for glycerol--values which are much higher than those described for larger species. There is a parallelism between the magnitude of transfer to fetus and arterial concentration in mother for each studied metabolite and actually variations in their plasmatic levels affect this transport process. This is clearly seen in the case of glucose where placental transfer is reduced during fasting hypoglycemia and greatly increased in diabetes. Placental transfer of L-alanine and blood flow to the placenta were reduced in both 48-hour starved and streptozotocin-induced diabetic late pregnant rats. Results show the main role of maternal nutrient concentration as a modulator of their transfer to fetus, the deleterious effect of reductions of uterine blood flow on placental transport of amino acids as well as the small placental transfer of glycerol as compared to either glucose or alanine.