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Modulation of the digestive lysosomal system in Paramecium caudatum. III. Morphological effects of cytochalasin B.

Morphological explanations for the effects of cytochalasin B (CB) on the digestive vacuole system of Paramecium caudatum were sought using electron-microscopic studies. Cytochalasin B (0.3 mM) essentially stopped vacuole release without stopping vacuole growth. Acidosome fusion with the new vacuoles was inhibited and was accompanied by the formation of discoidal vesicle-lined clumps of microfilament-like material next to these vacuoles. Vacuole membrane retrieval at the open cytoproct was inhibited and was accompanied by the loss of the microfilament network typical of this region. At 0.15 mM CB vacuole release and fusion of the acidosomes and lysosomes with the vacuoles were not stopped, but membrane retrieval at the cytoproct was affected. Cytochalasin B at the higher concentration also affected the movement of the horseradish peroxidase (HRP)-labeled vesicles away from the nascent vacuole. Membrane retrieval by the formation of tubules of 35 to 50 nm diameter from membrane of early and late vacuole stages was also affected by CB. It is conceivable that all of these effects were the result of the disruption of membrane associated actin networks. By contrast, several highly organized microfilamentous systems in the cortex and oral region of these cells appeared to be unaffected by CB or dimethyl sulfoxide (DMSO) at the concentrations used. These results show that CB-susceptible structures assume a key role in the normal functioning of the digestive vacuole system and in vacuole membrane retrieval and reutilization in Paramecium.