Glucocorticoid and progesterone receptors in yolk sac placenta

The parietal yolk sac (PYS) of the rat fetus at the 14th day of gestation contains glucocorticoid as well as progesterone receptors; both are present in the trophoblast cell layer. Following heat activation the receptors are capable of binding to deoxyribonucleic acid-(DNA-)cellulose. Glucocorticoid receptors, but not progesterone receptors, are also present in the visceral yolk sac (VYS) at the 14th day of gestation. Greater amounts (some 250 femtomoles/mg cytosol protein) of a glucocorticoid receptor are present in the VYS on the 17th day of gestation. The K d is approximately 4×10 −9 m; following activation it also binds to DNA—cellulose. The elution pattern of the activated VYS receptor from diethylaminoethyl-(DEAE-)Sephadex, however, is similar to that found with kidney and colon rather than that of liver (i.e., it resembles corticosteroid binder IB rather than binder II) indicating a possible role in transport. Although the receptors are separate entities, progesterone competes as effectively as corticosterone for binding to the glucocorticoid receptors in both the PYS and VYS, thus raising the question of the possible effect of changes in progesterone concentrations on the functioning of glucocorticoids during development.