Effect of Metabolic Inhibitors on Pinocytosis of Uranyl lons by Radish Root Cells: Probable Mechanisms of Pinocytosis

Pinocytotic uptake of uranyl ions by root cells of the radishRaphanus sativus L. cv. Red Giant and effects of monoiodacetate(MIA), 2,4-dinitrophenol (DNP), 2-mercaptoethanol (ME) and cytochalasinB (CB) on this process were studied. The number of invaginationsand pinocytotic vesicles in cells incubated with 0.15 mM uranylacctate (UA) is 15–20 times greater than in control cells,as estimated by counting the structures (plasmalemma derivatives)in serial sections. Monoiodacetate (0.05 mM) slightly stimulatedand ME (1.5 mM) completely inhibited pinocytosis. DNP (0.1 mM)inhibits UA pinocytosis by 45 per cent: DNP combined with MIAdiminishes pinocytotic activity by nearly 70 per cent. In thepresence of CB (4 µM) and UA quite large invaginations(exceeding 1 µm) have been observed. Cells treated withUA and UA $ MIA contain considerably more secretory vesicles.The results provide evidence for two types of pinocytosis inradish root cells; one independent of metabolic energy (I) andthe other essentially dependent on energy from respiration (II).Experiments with liposomes which show a pinocytosis-like behaviourin the presence of the pinocytosis inductors (including UA)indicate that type I is due solely to the effect exerted bythe inductor on the membrane. Type II is probably directly dependenton metabolism and may be regarded as a combination of uptakeand of sui generis removal of any excess plasmalemma increasedby exocytotic secretion. Raphanus sativus, pinocytosis, exocytosis, metabolic inhibitors, liposomes, electrostatic interaction, lipid peroxidation