Freeze- XXXX study of the zymogen granule membrane of pancreas: two novel types of intramembrane particles.

@entity15129 @entity174 @entity1579
The ultrastructure of the zymogen granule (ZG) membrane has been observed in vitro by rapid freezing and freeze- @entity174 techniques. Unidirectional shadowing of the plasmic @entity174 (PF) leaflet of the intact granule reveals a relatively smooth surface uniformly studded by intramembrane particles ( @entity15129 ; 360 microns2) their diameters ranging from 5 to 18 nm (mean = 10.2 nm) but does not allow a clear visualization of the particles on the external @entity174 (EF) leaflet. Indeed, rotary shadowing reveals that the EF leaflet presents a highly textured subparticle background with a significantly lower frequency of @entity15129 (44 microns2) showing diameters from 9 to 18 nm and a shift to larger @entity15129 (mean = 12.3 nm). Two hitherto undescribed types of @entity15129 are found on both leaflets of the membrane: first a population of 13-nm particles with an electron-lucent center or "pore", the most frequent type on the EF face (26%), is a second population of large @entity15129 (15 nm) characterized by a large "pore" (5.0 nm diameter) subdivided by a delicate cross-shaped structure. In alkaline conditions, pH 8.2, ZG lysis occurs rapidly and membrane ghosts thus obtained were rapidly frozen or suspended in dextran and filtered immediately. @entity1579 ( @entity1579 ) shows many opened ghosts with adhering amorphous material and numerous small vesicles near or still attached to openings in the ghosts. Freeze- @entity174 preparations show that granule lysis is accompanied by major alterations of membrane ultrastructure; the subparticle background on the EF leaflet is now visible only as a cap or linear crest at one pole of the ghosts. These two newly formed zones are demarcated by a row of 13-nm particles, whereas the other @entity15129 are confined to the subparticle background. Some images suggest that the subparticle background and 13-nm @entity15129 necklace give rise to vesicles, some of them occasionally attached to the ghosts. The subparticle background on the EF leaflet shows a complementary imprint on the PF leaflet which is similarly modified. This study shows the presence of hitherto undescribed types of @entity15129 and also demonstrates alterations of certain domains of zymogen granule membranes that occur at the moment of lysis, associated with a redistribution of different particle populations.
