Synchronous exocytosis in Paramecium cells. V. Ultrastructural adaptation phenomena during re-insertion of secretory organelles.

We used aminoethyldextran (AED) as a secretagogue for the simultaneous exocytosis of a great proportion of secretory organelles (trichocysts) from Paramecium tetraurelia cells and we applied freeze-fracturing and ultrathin sectioning for a quantitative analysis of the re-arrangement of ultrastructural details within the cell membrane during re-insertion of new trichocysts. Characteristic arrangements of membrane-intercalated particles (MIPs) occur over trichocyst docking sites: 300 nm large double rings of MIPs and a MIP rosette in the middle. Empty sites, displaying a compressed ring ("parenthesis") without a rosette, are expanded to full size rings when a trichocyst is docked. We obtained a first hint on the possible existence of a short lived (approximately 5 min) adaptation stage, represented by rings without a rosette. This could mean that docking of a trichocyst would induce the assembly of rosette MIPs over a newly docked trichocyst. The reformation of rosettes is paralleled by an increasing number of extrudable trichocysts which underscores the causal role of rosette MIPs in exocytosis performance. New trichocysts are inserted at the old predetermined sites after removal of ghosts and formation of a "plug" as a receptor-type structure. The number of non-docked, free trichocysts in the cytoplasm is only slightly changed, indicating a continuous synthesis and docking rate (approximately 2-3 organelles per min). Since in strain 7S ghosts are removed within approximately 10 min and docking goes on over 9 h, there occur many empty docking sites in the time period in between, with a maximum of approximately 1 h after AED triggering, thus providing a unique situation for further experiments.