Modulation of the digestive lysosomal system in Paramecium caudatum. II. Physiological effects of cytochalasin B, colchicine and trifluoperazine.

Abstract

The heterophagic pathway of the digestive lysosomal system in Paramecium caudatum includes at least four steps: digestive vacuole (DV) formation, acidification-condensation, lysosomal fusion-digestion, and defecation. The second and the third require about 20 min, during which DVs are not egested. Because these steps occur at predictable intervals, the mechanism for each can be explored by exposing labeled DVs to different drugs prior to each step. In this study the effects of cytochalasin B (CB), colchicine and the calmodulin antagonist, trifluoperazine (TFP), were studied. All three drugs inhibited DV formation in a dose-dependent manner when cells were pulsed with latex beads and a drug simultaneously. TFP was cytotoxic above 5 microM. Vacuole formation was completely shut down when cells were pre-exposed to 5 microM TFP for 13 min. At this level, the duration of the acidification step was lengthened, and the rate of defecation decreased with increasing exposure. These results suggest that these inhibitory effects may be more related to TFP's cytotoxicity than to its action on calmodulin-mediated process. Colchicine at 1 mM had no effect on the third or fourth step, but inhibited the acidification step so that DVs were egested later and at a slower rate. Exerting a differential effect on all four steps, CB inhibited DV release from the cytopharynx, egestion of defecation-competent DVs at the cytoproct and lengthened the duration but did not block the lysosomal fusion-digestion step of the acidic DVs; it was most potent in blocking acidification, which prevented both lysosomal fusion with the labeled DVs as well as DV egestion, the latter for more than 50 min.(ABSTRACT TRUNCATED AT 250 WORDS)