Freeze-fracture and deep-etching studies on zymogen-granule membranes of the rat pancreas.

Whole pancreatic zymogen granules or their membrane fraction were examined by freeze-fracture or deep-etching under different experimental conditions. The granules were fixed for different time periods, or not fixed, and were cryoprotected with glycerol or DMSO; 3% glutaraldehyde followed by 30% glycerol were finally chosen for giving the best resolution and the highest density of intramembrane particles (IMP). IMP are present on the PF and EF leaflets. Their number decreases with the duration of the fixation. Several granules exhibit IMP-free blebs. Incubation of the granules with protamine sulfate causes an aggregation of IMP and of the rough-textured background on the EF leaflet. A second fracture plane can be formed and has been shown by deep-etching to be intercalated between PF and EF. Deep-etching has also shown that particles attached to the perimeter of the granules and of the blebs are, in fact, large nodules on the PS face which partially extend onto the blebs and do not aggregate with the IMP after protamine treatment. Fusion is also indicated between membrane vesicles. Freeze-fracture of the purified membrane fraction seems to indicate the formation of an IMP cap during the lysis of the granule. Moreover, large nodules remain present on the PS face on these membrane fractions but the majority disappear after washing at pH 11.2 with Na2CO3 and EDTA.