Sexual maturation in female rats: time-related changes in the isoelectric focusing pattern of anterior pituitary follicle-stimulating hormone.

Anterior pituitary glands (AP) were obtained from female rats at 5, 15, 18, 21 and 29 days of age, at the time of vaginal opening (VO) and during adulthood on proestrus. The multiple species of follicle-stimulating hormone (FSH) present within the AP were separated by the technique of polyacrylamide gel isoelectric focusing (PAG-IEF) and measured with the NIAMDD rat FSH radioimmunoassay kit. AP's obtained from immature female rats prior to VO contained elevated levels of total FSH as well as all of the species of AP FSH observed in adult rats (and hamsters). However, the majority of the FSH immunoactivity migrated to the most acidic portion of the gel (isoelectric point [pI] value=4.2-3.8). At the time of VO and during adulthood, a decrease in total AP FSH was observed. In addition, a shift in the relative proportions of certain FSH species occurred. The AP's of adult animals contained relatively greater amounts of more basic (pI values 6.0-5.0) forms of FSH compared with immature animals. When each of the AP FSH species isolated from adult animals was tested in a radioligand receptor assay, the most acidic (pI=4.2-3.8) failed to interact with the receptor preparation, while those with pI values from 6 to 4.7 were able to compete with [125I]-labeled FSH for receptor binding in a parallel fashion. Thus, the observed shift in the PAG-IEF FSH profiles to more basic (and biologically active) forms may represent a change in the composition of AP FSH that serves an important role in the maturation process leading to ovulatory cyclicity.