Heterogeneous distribution of phospholipids in membranes along the secretory pathway in pancreatic B‐cells

The membrane content in phospholipids along the secretory pathway in rat pancreatic B‐cells was studied in situ by high‐resolution cytochemistry, applying the recently introduced phospholipase A 2 ‐gold technique. The gold particles were mostly associated with cell membranes, and the various types of membranes were labeled to a different extent. Quantitation of the labeling over these membranes revealed a heterogenous distribution of the labeling across the secretory pathway. This heretogeneity occured mainly as a progressive, decreasing gradient in the first half of this pathway, between the rough endoplasmic reticulum and the mi‐cisternae of the Golgi apparatus. The labeling density remained at a lower level in the trans‐most Golgi cisternae and immature secretory granule membranes, to increase in the mature secretory granule membrane, where it reached the value found in the plasma membrane. These results provide evidence that the functional heterogeneity existing across the membrane forming the secretory pathway is parallelled by substantial changes in their phospholipid content.