Low temperature blocks fluid-phase pinocytosis and receptor-mediated endocytosis in Trypanosoma cruzi epimastigotes

Gold-labeled albumin and transferrin were used to follow at the ultrastruct ural level the early events and the effect of low temperature on protein up take by Trypanosoma cruzi epimastigotes. In parasites incubated for 5 min a t 28 degrees C with protein-gold complexes, extracellular markers were foun d only at the cytostome and/ or the flagellar pocket regions, whereas intra cellular gold particles were detected inside small uncoated vesicles locate d nearby. Within 10 min, labeling was also observed in uncoated vesicles cl ose to the nucleus. Only after 30 min could the tracers be detected in the reservosomes. Weak labeling in the cytostome and flagellar pecker of parasi tes incubated at 4 degrees C with the albumingold solution indicated that a lbumin uptake occurred by fluid-phase pinocytosis. On the other hand, inten se labeling at the cytostome was observed in parasites incubated at 4 degre es C with gold-labeled transferrin, showing that receptor-mediated endocyto sis occurs mainly at this site. Both proteins were absent from the cells at 4 degrees C and 12 degrees C. Raising the temperature from 12 degrees C to 28 degrees C led to transferrin labeling in intracellular vesicles dispers ed throughout the cytoplasm, but not in reservosomes. Our results suggest t hat low temperatures affect the transport and pinching of endocytic vesicle s as well as the rate of delivery of transferrin to reservosomes.