Gonadotropin and lipoprotein receptor levels in rat luteal cells of early pregnancy

Dispersed cells from rat corpora lutea isolated on pregnancy days, 5, 7, 10 and 13, yielded three distinct sub-populations of luteal cells when centrifuged in a continuous, 0-40%, Metrizamide density gradient. Bands I and II (refract, ind. 1.340-1.346) constituted highly enriched, functional, luteal cell fractions. The more dense fraction (Band III, refract, ind. 1.348-1.355), while contaminated with non-luteal cells, also contained cells capable of progesterone production. Progesterone synthesis by all three sub-populations was stimulated by LH, and each sub-population bound [¹²⁵I]-labeled hCG and [¹²⁵I]-labeled HDL with high affinity. However, the concentrations of LH/hCG and lipoprotein binding sites were greater in Band I + II than in Band III. Progesterone secretion during the selected days of pregnancy, being high on days 5 and 13 with a nadir on day 10, was shown to correlate with the concentration of LH/hCG-binding sites present on Band I + II cells. This correlation was not seen when comparing Band III cells. The concentration of HDL-binding sites did not vary significantly with the sub-populations of luteal cells isolated on the selected days of pregnancy.