EFFECTS OF BACKGROUND ADAPTATION ON ALPHA-MSH AND BETA-ENDORPHIN IN SECRETORY GRANULE TYPES OF MELANOTROPE CELLS OF XENOPUS-LAEVIS

Placing the clawed toad Xenopus laevis on a black background stimulate s the melanotrope cells in the pars intermedia of the pituitary gland to release proopiomelanocortin (POMC)-derived peptides, including alph a-MSH and N-acetyl-beta-endorphin. In this study three types of secret ory granules, electron-dense (similar to 130 nm empty set), moderately electron-dense (similar to 160 nm empty set) and electronlucent (simi lar to 180 nm empty set), have been identified in these cells. Apparen tly, only dark granules are formed by the Golgi apparatus and lucent g ranules release their contents via exocytosis. Immuno-electron microsc opy (immunogold double labelling) of glutaraldehyde-fixed and freeze-s ubstituted material shows that desacetyl-alpha-MSH and N-acetyl-beta-e ndorphin coexist in all three granule types. Quantification of immunos taining revealed that immunoreactivities to these peptides are lowest in the dark granules and highest in the light ones. It is proposed tha t intragranular processing of POMC to immunoreactive desacetyl-alpha-M SH and N-acetyl-beta-endorphin involves an increase in granule size an d a decrease in granule electron density. Black background-induced act ivation of the melanotrope cell is reflected by an increase in immunor eactivity of the secretory granules to each of the antisera. This sugg ests that cell activation stimulates the formation of peptides by intr agranular processing of POMC and/or of intermediate POMC-processing pr oducts. In addition, cell activation evoked an increase in the percent age of the granule population that reacts with anti-N-acetyl-beta-endo rphin, probably by stimulating intragranular acetylation of beta-endor phin. Apparently, this acetylation is a regulated event that occurs in the cytoplasm, independently from the acetylation of desacetyl-alpha- MSH which takes place near the plasmalemma at the time of granule exoc ytosis.