Immunocytochemical localization of chromogranin A and secretogranin II in female rat gonadotropes.

Ultrastructures of pituitary gonadotropes are known to show a prominent sex-related difference: typical male rat gonadotropes contain both large- and small-sized granules, whereas typical female rat gonadotropes appear to exhibit uniformly small-sized granules. Our preceding studies have demonstrated that two representative granins, chromogranin A (CgA) and secretogranin II (SgII), are separately localized to each type of granule in male rat gonadotropes. To clarify whether or not there is a certain relationship between granin proteins and characteristic features of secretory granules in female rat gonadotropes, we examined the expression levels and immunocytochemical localizations of CgA and SgII in the cells. Northern blot and immunoblot analyses demonstrated that both CgA and SgII were synthesized and stored in the female pituitary, although the amount of CgA was much lower in the female than that in the male pituitary. Immunocytochemical observations clarified that gonadotropes in the female pituitary possessed intermediate secretory granules containing both CgA and SgII, in addition to solely CgA-positive and SgII-positive ones. However, secretory granules containing CgA in the female gonadotropes were much smaller in size and appeared less frequently than those in the male cells, whereas no sexual difference was discerned in SgII-positive granules. Moreover, the size and appearance of CgA-positive secretory granules varied depending on stages of the estrous cycle. These findings suggest that the size and appearance of secretory granules containing CgA are closely associated with the expression and storage levels of CgA in the pituitary.