Co-localization of parathyroid hormone and secretory protein-I in bovine parathyroid glands: a double immunocytochemical study at the electron microscopical level.

Secretory protein-I (SP-I), also known as chromogranin A, is an acidic glycoprotein of unknown function that is found in large amount in the secretory granules of all endocrine and neuroendocrine cells, but not in exocrine or epithelial cells. It is cosecreted with parathyroid hormone (PTH) and by immunocytochemical staining has been reported to exist in the same subcellular structures of the gland. In the present study we have used the colloidal gold-double immunocytochemical technique at the ultrastructural level to precisely define the locales of SP-I and PTH in the bovine parathyroid cell. SP-I and PTH were co-localized to the same secretory granules. The patterns for both gold labels were diffuse ones throughout the granule and suggested that there was a general association of SP-I and PTH. There was little or no localization of the SP-I at the secretory granule membrane. The results support the concept that SP-I is responsible for stabilization of PTH within the secretory granules.