ABSTRACT
E. histolytica takes up close to 30 per cent of its own volume per hour by fluid-phase pinocytosis (Serrano and Reeves, 1975; Aley et al., 1984; Bakker-Grunwald et al., 1985). This process is constitutive, i.e. it proceeds without any apparent need for an external trigger. As a consequence, all solutes present in the medium will be taken up through the pinocytic route. The rate by which this occurs will (at least in the absence of binding) depend linearly on the solute concentration; thus, without additional data this mode of transport is kinetically indistinguishable from free
diffusion across the plasma membrane. It should be kept in mind that, after pinocytic uptake, substrates are not yet available to the cytoplasmic enzymes: for this, they would still have to cross a membrane barrier.1 Pinocytic uptake depends on acidification of the pinocytic compartment (Löhden-Bendinger and BakkerGrunwald, 1990) and, presumably, on active cytoskeletal rearrangements; in this sense it is an energy-requiring process. As in higher eukaryotic cells, both pinocytic uptake and exocytic secretion are inhibited by cytochalasin B and by weak permeant bases (Bakker-Grunwald et al., 1985, 1986).
