Local trichocyst exocytosis provides an efficient escape mechanism for Paramecium cells.

More than 1000 secretory organelles (trichocysts) are docked at the plasma membrane of Paramecium cells. After stimulation, the trichocyst contents are expelled as needle-like structures in an exocytotic response. Neither the function nor the natural stimulus of trichocyst exocytosis are known in this well established system. Several hypotheses have been put forward during the last 100 years, but as yet, none has withstood critical experimental testing. Using video-aided light microscopic evaluation of the explosive trichocyst exocytosis we have obtained conclusive evidence for a defensive mechanism. When stimulated locally by a non-cytotoxic chemical secretagogue, cells were rapidly propelled in the opposite direction by vigorous local trichocyst discharge. The same phenomenon was observed during encounter with a predatory ciliate, Dileptus. Whereas exocytosis-competent paramecia escaped by rapid propulsion away from the attacking predator, cells non-competent for exocytosis were paralysed and engulfed. Thus, oriented locomotion by locally stimulated trichocyst exocytosis serves as a rapid escape mechanism of Paramecium.