Permeability Modulating Membrane Coats

Many protozoans and some other organisms possess contractile vacuoles (CVs), membranous organelles which help maintain cellular water/ electrolyte balance by extracting a dilute solution from the cytoplasmic colloid and expelling it from the cell (35, 37). Although the similarity of CV function in the various organisms has long been recognized, the expected similarity of morphological elements responsible for generating CV fluid has eluded detection (21). The initial step in this process, termed "fluid segregation" by Pappas and Brandt (34), is carried out by systems of vesicles or tubules which communicate with the main vacuole. The fluid segregation organelles exhibit characteristic differences in shape and relationship to the vacuole in various organisms; however, recent comparative examination of CVs in ciliates, flagellates, amebae, and freshwater sponges has revealed that the fluid segregation organelles are all composed of membrane with a similar specialization or "coat" (26). As described below for the fluid segregation tubules of peritrich protozoans, the coat consists of electron-dense, peg-shaped elements attached to the cytoplasmic dense lamina of the trilaminar membrane. The coat is of considerable comparative interest because morphologically similar specialized membranes occur in several other cells where water and electrolyte transport is emphasized, including insect digestive and excretory cells, and fish chloride cells. For reasons presented below, these similar coats are considered to be involved in membrane permeability modulation (PM).