Immunoreactive γ-Melanotropin in Rat Pituitary and Plasma: A Partial Characterization*

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Abstract
A RIA for the γMSH region of proopiomelanocortin has been established and validated. The antiserum was raised to synthetic bovine γMSH, and it cross-reacts well with larger polypeptides, including γ3 MSH and 16 K fragment, which contain the γMSH sequence. Gel filtration chromatography of extracts prepared from the anterior and neurointermediate lobes of rat pituitary and from rat plasma reveal two heterogeneous immunoreactive (IR-) forms of γMSH in each venue which elute in molecular weight peaks of approximately 11,000 (UK) and 6,000 (6 K). Addition experiments indicate that the smaller material is not an artifact generated from the larger form during preparation. By the criterion of retention on affinity columns of Concanavalin A-agarose, these peptides are glycosylated. The 6 K form represents 8–17% of the total IR-γMSH in the anterior lobe and about 30% of that in the neurointermediate lobe. IRγMSHs are released by dispersed rat pituitary cells in response to several of the same secretagogues which modulate the secretion of other proopiomelanocortin-derived peptides. Changes in the plasma and anterior lobe content of IR-γMSHs and ACTH resulting from perturbation of the pituitary-adrenal axis are concordant. In particular, the plasma concentration of 6 K IRγMSH increases dramatically after imposed stress.