alpha-Melanocyte-stimulating hormone-like peptides in the intermediate lobe of the rat pituitary gland: characterization of content and release in vitro.

Reverse phase high performance liquid chromatography (HPLC) followed by RIA of the chromatographic fractions was used to separate and quantify, respectively, the alpha MSH-like peptides stored in the intermediate lobe (IL) of the rat pituitary gland and released from IL cells in vitro. Immunoreactive material eluting with the same HPLC retention time as N,O-diacetyl alpha MSH accounted for approximately 80% of the total immunoreactive alpha MSH (IR-alpha MSH) in either the neurointermediate lobe or dispersed IL cells. The remainder of the IR-alpha MSH coeluted with either synthetic desacetyl alpha MSH or alpha MSH. Furthermore, the predominant alpha MSH-like compound released in vitro from dispersed IL cells eluted from the HPLC column with the same retention time as synthetic N,O-diacetyl alpha MSH. Treatment of dispersed IL cells with drugs known to enhance (l-isoproterenol or A 23187) or to inhibit (apomorphine or lisuride) the release of IR-alpha MSH revealed that N,O-diacetyl alpha MSH was the primary form released. Finally, an evaluation of the stability of the alpha MSH-like peptides indicated that N,O-diacetyl alpha MSH was readily converted to alpha MSH in the presence of 0.1 N hydrochloric acid.