Fig. 2. Effects of inhibitors on active ammonia excretion and transepithelial potential difference (PD_te) across isolated perfused gills of the shore crab Carcinus maenas. The rate of disappearance of total ammonia from the internal perfusion medium was measured with symmetrical NH₄Cl concentrations (100 µmol l^-1) in the external and internal baths (A,B,D) or with 200 µmol l^-1 NH₄Cl in the internal bath and none initially in the external bath (C). (A) Symmetrical application of bafilomycin A₁ (Baf.) (1 µmol l^-1) followed by basolateral addition of ouabain (Ouab.) (5 mmol l^-1) (N=4). (B) Basolateral addition of colchicine (Colch.) (0.2 mmol l^-1) (N=6). (C) Basolateral application of colchicine (0.2 mmol l^-1) with an initial outwardly directed NH₄⁺ gradient of 200 µmol l^-1 (N=5). (D) Basolateral addition of cytochalasin D (Cytoch.) (5 µmol l^-1) (N=5). Data represent means + S.E.M.