SIIp: A unique secretogranin/chromogranin of the pituitary released in response to gonadotropin-releasing hormone

A monoclonal antibody prepared by immunization of mice with a rat pituitary granule fraction stained a single band on a Western blot of pituitary homogenate (bovine, ovine, porcine, or rat) with an apparent mol wt of 78,000 (7.5% acrylamide gel in sodium dodecyl sulfate) and pI 5.0-5.1 (isoelectric focusing). Subcellular fractionation studies of rat pituitaries indicated that the determinant of the monoclonal antibody was markedly enriched in the secretory granule fraction, an observation that was independently confirmed by immunohistochemistry of intact cells. Immunohistochemistry also indicated that this determinant was selectively located in gonadotropes and thyrotropes. On Western blots, this band comigrated with adrenal secretogranin-II (SII; chromogranin-C), had the same N-terminal sequence (six amino acids), and was heat stable (95 C; 10 min). The pituitary protein containing the determinant for the monoclonal antibody could be precipitated by a polyclonal antibody prepared by immunization of rabbits with the C-terminal sequence of adrenal SII (triodecapeptide). Conversely, the monoclonal antibody precipitated the protein containing the determinant for the polyclonal antibody. While both the monoclonal and polyclonal antisera recognized the pituitary molecule, only the polyclonal antibody recognized SII from the adrenal. A RIA was established and used to assess the release pattern of this molecule from pituitary cell cultures. Release was stimulated by GnRH and blocked by a GnRH antagonist. Release was Ca²⁺ dependent and stimulated by either phorbol myristyl acetate (a protein kinase-C activator) or NaF (a G-protein activator). GHRH and TRH were not as effective secretogogues as GnRH. The observations that a unique form of SII is present in the pituitary gonadotrope and secreted in response to a specific endocrine stimulus present the possibility that this substance has an endocrine function. Further, the tissue specificity of the determinant suggests that it may be useful for the specific diagnosis and monitoring of pituitary tumors.