α-Melanocyte-Stimulating Hormone-Like Peptides in the Intermediate Lobe of the Rat Pituitary Gland: Characterization of Content and Release in Vitro

Abstract

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