Foetal-maternal production and transfer of cortisol during the last days of gestation in the guinea-pig.

The metabolism and the rate of transfer of cortisol across the placenta in pregnant guinea-pigs and foetuses were studied by constant intravenous infusions of tritium-labelled cortisol. Estimates of endogenous and radioactive plasma cortisol levels were used to calculate the following parameters at four stages before parturition (days 62, 64, 66 and 67; parturition occurring at day 68): metabolic clearance rate; production rate; adrenal secretory rate; transfer rate from mother to foetus and from foetus to mother; irreversible removal rate; the fraction of cortisol derived from the other in the foetal and maternal vascular compartments; the fraction of secreted and recycled cortisol involved in the transfer. The metabolic clearance rate and the rates of production and secretion of cortisol were higher in the mother than in the foetus between days 62 and 67 of gestation. About 90% of the foetal cortisol was of maternal origin. The fraction of maternal cortisol of foetal origin increased in the last days of gestation.