Metabolic characteristics of endocytosis of ferritin by gills of a marine bivalve mollusc.

Abstract

Lamellibranch molluscs, such as the common mussel Mytilus edulis, are filter feeders. In addition to their respiratory function, the gills of these animals are responsible for the maintenance of a steady current of water through the mantle cavity, the filtration of particles from the water and sorting of food from detritus. They are also capable of absorbing amino acids and sugars, which occur in low concentrations in sea water, by active carrier-mediated absorption mechanisms (PCquingat, 1973 ; Bamford & Gingles, 1974; Bamford C Indian Ink, thorotrast, ferritin and peroxidase were absorbed, whereas acidic dyes were not. In a kinetic study we have also shown that particulate ferric hydroxide and ferritin are absorbed by pinocytosis in the gills of Mytilus edulis, whereas transferrin is not (George et al., 1976a,b). In the present communication we report some results of a quantitative study of pinocytosis of ferritin in uitro in isolated gills of Mytilus and the effect of metabolic inhibitors on the process. In each experiment, gill segments, consisting of two larnellae joined at the top by mantle tissue, were removed from the mussels and preincubated with the appropriate inhibitor, in a flask containing lOml of oxygenated filtered ( 0 . 4 5 ~ Millipore filter) sea water for 30min at 15C, in a shakingwater bath under an atmosphere of 02. 1z51-labelled ferritin (final concn. 0.1 mg/ml) was added (zero time) and further incubation was carried out for 60min. Control preincubated tissues without inhibitor were sampled at zero time and 60min. The gill segments were removed, blotted, weighed, and radioactivity was determined. Tissue was also examined by electron microscopy to confirm that the ferritin was absorbed and not bound to the cell surfaces of the gills. Control tissue absorbed 207,ug of 1251-labelled ferritin/h per g wet wt. of tissue at 15°C. Electron-microscopic examination of the gill tissue showed that the ferritin was present in endocytic vesicles within the gill epithelial cells and that very little was bound to the cell surfaces. The uptake of ferritin was only inhibited by about 50% in the presence of inhibitors or uncouplers of the respiratory chain such as CN-, oligomycin, antimycin A, rotenone or dinitrophenol (Table 1). Moreover, the same degree of