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 stimulates the melanotrope cells in the pars intermedia of the pituitary gland to release proopiomelanocortin (POMC)-derived peptides, including alpha-MSH and N-acetyl-beta-endorphin. In this study three types of secretory granules, electron-dense (approximately 130 nm phi), moderately electron-dense (approximately 160 nm phi) and electron-lucent (approximately 180 nm phi), have been identified in these cells. Apparently, only dark granules are formed by the Golgi apparatus and lucent granules release their contents via exocytosis. Immuno-electron microscopy (immunogold double labelling) of glutaraldehyde-fixed and freeze-substituted material shows that desacetyl-alpha-MSH and N-acetyl-beta-endorphin coexist in all three granule types. Quantification of immunostaining revealed that immunoreactivities to these peptides are lowest in the dark granules and highest in the light ones. It is proposed that intragranular processing of POMC to immunoreactive desacetyl-alpha-MSH and N-acetyl-beta-endorphin involves an increase in granule size and a decrease in granule electron density. Black background-induced activation of the melanotrope cell is reflected by an increase in immunoreactivity of the secretory granules to each of the antisera. This suggests that cell activation stimulates the formation of peptides by intragranular processing of POMC and/or of intermediate POMC-processing products. In addition, cell activation evoked an increase in the percentage of the granule population that reacts with anti-N-acetyl-beta-endorphin, probably by stimulating intragranular acetylation of beta-endorphin. 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 exocytosis.