Contribution of maternal circulation to blood-borne progesterone in the fetus. I. Studies on human subjects.

Tritiated progesterone was infused intravenously at a constant rate into three pregnant volunteers in labor, for at least two hours before delivery. Blood samples from a maternal peripheral vein and from unbilical vessels were taken at birth. Comparison of the specific activities of progesterone in these samples indicated that 10 per cent or less of the hormone in fetal circulation is derived from transfer of maternally circulating progesterone. After consideration of reported values of umbilical vein blood flow at term and measured arteriovenous differences in concentrations of progesterone in umbilical vessels, the secretion rate of the placental hormone toward the fetus was estimated to be about 1/10 of the rate of secretion of progesterone toward the maternal circulation. About 1 per cent of the maternally circulating hormone was found to cross the placenta.

Comparison of results from in situ and in vitro perfusions of rabbit uterus with labeled estrone and estradiol.

The interconversion of labeled estrone (E1) and estradiol (E2) in rabbit uterus was studied in vivo and in vitro, in order to evaluate the influence of incubation conditions upon the preferred direction of the reversible conversion of E2 to E1. In a typical experiment, one uterine horn was perfused intraluminally in situ with a mixture of [3H]E2 and [14C]E1, while the other horn was dissected and used for in vitro perfusions with the same tracers, both intraluminally and on tissue slices. The ratios of rate constants corresponding to the oxidative and reductive reactions were found to be similar under the three perfusion conditions, as calculated from the isotopic data. The rate constant of conversion of E1 to E2 was 10-20 times larger than the rate constant of the opposite reaction. Equal E2 nuclear saturation levels were obtained by perfusion of the uterine tissue with excess [3H]E2 in situ and in vitro after slicing.