Distribution of filipin-sterol complexes in villus goblet cell membranes of rat small intestine

Abstract

The polyene antibiotic, filipin, was used as a morphologic probe to determine the distribution of cholesterol in plasma and intracellular membranes in freeze fracture replicas of villus goblet cells from rat small intestine. Endoplasmic reticular and cis-face Golgi membranes displayed only rare filipin-sterol complexes. Membranes of the Golgi trans-face and adjacent nascent secretory granules revealed a moderate number of complexes. Membranes of mature secretory granules contained numerous filipin-sterol complexes, especially in sites where adjacent granules abutted. Complexes were so abundant on the apical plasma membrane that they approached confluence. The areal density of filipin-sterol complexes on the lateral plasma membrane was less than on the apical membrane but varied from goblet cell to goblet cell. The number of complexes on the lateral membrane of cells that appeared to be actively secreting mucus at the time of fixation was greater than on cells that showed no evidence of active mucus secretion. Thus, in villus goblet cells of rat small intestine (a) mucous granule membranes and the apical plasma membrane with which they fuse during secretion appear strikingly enriched with cholesterol, (b) cholesterol appears to be incorporated into mucous granule membranes during and shortly after their formation at the Golgi trans-face, and (c) the cholesterol content of the lateral plasma membrane of secreting goblet cells appears to be greater than that of nonsecreting cells.