Immunocytochemical evidence of intragranular acetylation of alpha-MSH in the melanotrophic cells of the rabbit.

Melanotrophic cells of the pars intermedia of the rabbit were studied at the electron-microscopic level by means of the immuno-gold technique with the use of antisera against gamma 3-MSH and alpha-MSH. Both antibodies labelled all secretory vesicles stored in the peripheral cytoplasm, but secretory vesicles and intrasaccular condensations in the Golgi area reacting for gamma 3-MSH were not labelled with the antibody against alpha-MSH. Since this antibody appears to recognise only the acetylated form of alpha-MSH, the present observations suggest that acetylation occurs at a stage subsequent to the Golgi packaging, during maturation of the secretory vesicles. Thus, the morphological evidence supports biochemical data in favour of intragranular processing of opiomelanocortin in melanotrophic cells.