Differential subcellular localization of ACTH and alpha-MSH in corticotropes of the rat anterior pituitary.

Specific antisera to alpha-melanotropin (alpha-MSH) and corticotropin (ACTH 1-39) were used to obtain immunocytochemical evidence for the differential localization of alpha-MSH and ACTH in the secretory granules of corticotropes of rat anterior pituitary. The specificity of the antisera was established by binding 131I-labeled alpha-MSH and ACTH 1-39 to their respective antisera. Double-labeling immunocytochemistry (for alpha-MSH, ferritin; for ACTH, colloidal gold) was performed. Some secretory granules were labeled with ferritin particles (alpha-MSH), whereas others contained gold particles (ACTH). Only a few granules showed both ACTH and alpha-MSH. In typical corticotropes (stellate in form with a small number of secretory granules aligned along the cell periphery) only some of the secretory granules that were labeled with anti-ACTH serum were also immunoreactive to anti-alpha-MSH. In atypical corticotropes (polygonal in shape and containing a large number of secretory granules) almost all of the immunoreactive ACTH secretory granules were also positive to anti-alpha-MSH serum. An intermediate type of corticotrope was observed containing a small number of secretory granules, almost all of which were labeled with anti-alpha-MSH. Thus, rat anterior pituitary corticotropes may be classified into three types according to the distribution and content of alpha-MSH. The light-microscopic immunocytochemistry provided similar results.