Changes in gonadotrophin binding to rat ovaries during sexual maturation

Changes in LH and FSH binding to rat ovaries during sexual maturation were measured by in vitro binding assay and autoradiography. Ovaries were obtained from rats at various ages ranging from 5 till 35 days of age. Animals older than 35 days of age were classified in three groups: anoestrus (uterine weight below 60 mg), early pro-oestrus (uterine weight 60-120 mg) and pro-oestrus (uterus distended with fluid). Measurements were made of specific binding of radioiodinated hCG and rFSH to ovarian homogenates (day 5-35) and to granulosa cells isolated from the largest follicles (day 35-pro-oestrus). The earliest age at which specific hCG and FSH binding could be demonstrated was day 11. Thereafter hCG binding to ovarian homogenates, expressed in CPM/microgram DNA, increased with age. Specific binding of hCG to granulosa cells increased from 1000 CPM/microgram DNA at day 35 to 4000 CPM/microgram DNA at pro-oestrus. FSH binding to ovarian homogenates increased till day 21 and remained fairly constant thereafter. Autoradiography revealed the presence of hCG binding to interstitial tissue and thecal cells. hCG binding to granulosa cells was only found in large pre-ovulatory follicles present at early pro-oestrus and pro-oestrus. FSH binding was observed to granulosa cells of all types of follicles, but not to interstitial and thecal cells. The results demonstrate that during sexual maturation FSH binding to granulosa cells remains constant, whereas LH binding to interstitium and theca and ultimately to granulosa cells increases with age suggesting an increased responsiveness of the ovaries to LH.