Amine transport at the plasmalemma of Riccia fluitans

Green thallus cells of the aquatic liverwort, Riccia fluitans, are rapidly depolarized in the presence of 1--20 microM NH4Cl and 5--100 microM CH3NH3Cl, respectively. Simultaneously, the membrane conductance is increased from 0.41 to 1.2 S . m-2. Uptake of [14C]methylamine is stimulated by increasing [K+]o and inhibited by increasing [Na+]o or [H+]o, is highly voltage sensitive, and saturates at low amine concentations. Double-reciprocal plots of (a) maximal membrane depolarization and (b) methylamine uptake vs. external amine concentration give apparent Km values of 2 +/- 1 microM ammonia and 25--50 microM methylamine; Km values for changes in conductance and membrane current are greater and voltage dependent. Whereas the amine transport into the cell is strongly inhibited by CN-, the amine efflux is stimulated. The current-voltage characteristics of the ammonia transport are represented by a sigmoid curve with an equilibrium potential of --60 mV, and this is understood as a typical carrier curve with a saturation current of about 70 mA . m-2. It is further concluded that the evidently carrier-mediated transport is competitive for the two amines tested, and that ammonia and methylamine are transported in the protonated form as NH4+ and CH3NH3+ into the cytoplasm.