Placental transfer of free fatty acids: factors affecting transfer across the guinea-pig placenta.

Using an in situ perfusion of the fetal side of the guinea-pig placenta the quantitative effects of changes in the perfusate flow rate and albumin concentration, and of changes in the transplacental free fatty acid (FFA) concentration gradient on FFA transfer across the placenta were investigated. Unidirectional transfer from mother to perfusate was assessed by the transfer of [14C]palmitic acid given as a constant infusion to the mother. The results of bidirectional FFA flux were assessed by measuring total unlabelled FFA accumulated by the perfusate during a single passage through the placenta. In 4 animals a factorial experiment was performed using perfusate in which the albumin concentration was changed from 1 to 3g/dl, whilst the flow was maintained at either 1 or 3 ml/min. A change from 1 to 3g/dl albumin averaged over both flow rates caused a significant increase in labelled palmitate, and unlabelled net FFA transfer, both for each ml of perfusuate (P less than 0.05, P less than 0.001) and for each minute of perfusion (P less than 0.005, P less than 0.001, respectively). A change from 1 to 3 ml/min flow rate averaged over both albumin concentrations caused a significant (P less than 0.001) decrease in labelled palmitate but no change in unlabelled FFA transfer per ml, but caused significant increases in labelled and unlabelled (P less than 0.05, P less than 0.001) FFA transfer per minute. From the results of these experiments and 32 more non-factorial experiments it was found that, whilst maternal and perfusate FFA concentrations were in the normal range, maternal plasma FFA levels significantly correlated (P less than 0.01) with net FFA transfer to the perfusate. When maternal plasma FFA levels became elevated then this relationship broke down. Inflowing perfusate FFA levels significantly (P less than 0.001) negatively correlated with net transfer, which changed direction and resulted in FFA transfer from perfusate to maternal blood when inflowing perfusate levels exceeded 1.7 microM/ml. It was concluded that the increases reported in fetal plasma albumin and flow rate as gestation advances enable the fetus to obtain increasing amounts of circulating maternal lipid. If this lipid is not extracted by the fetus then the reduction in the transplacental gradient will reduce net fat transfer to the fetus.