HCG-binding capacity of the rat ovary during pregnancy.

During pregnancy, binding of 125I-labelled HCG to the rat ovary increases markedly from the 3rd to the 17th day of pregnancy but is reduced drastically at the end of pregnancy and after delivery. At the 17th day of pregnancy the HCG-binding capacity of the rat ovary is raised 76-fold in comparison to oestrous rats (17th day of pregnancy: 128 X 10(-15) mol/mg wet weight; oestrus: 1.67 X 10(-15) mol/mg wet weight). It is assumed that only LH receptors not occupied by endogenous hormone are covered by our assay. The high number of "free" ovarian HCG receptors during pregnancy apparently represents "Spare" receptors, the functional significance of which remains unclear. It is suggested that the metabolic activity of the ovarian luteal cells during pregnancy is not restricted by the number of HCG receptors present, but through the level of the endogenous gonadotrophic hormone and/or some unknown factors residing in the luteal cell.