Amine Transport in Riccia fluitans

Abstract

The cytoplasmic and vacuolar pH and changes thereof in the presence of ammonia (NH4Cl) and methylamine (CH3NH3Cl) were measured in rhizoid cells of R. fluitans by means of a pH-sensitive microelectrode. On addition of 7 .mu.M NH4Cl, the cytoplasmic pH of 7.2-7.4 drops by 0.1-0.2 pH units, but shifts to pH 7.8 in the presence of 50 .mu.M NH4Cl or 500 .mu.M CH3NH3Cl. The pH of the vacuole increases drastically from 4.5 to 5.7 with these latter concentrations. Since a NH4+/Ch3NH3+ uniporter was demonstrated in the plasmalemma of R. fluitans previously (Felle, 1983), the concentration-dependent shifts of cytoplasmic pH are interpreted as results of 2 processes: 1st, acidification through deprotonation of the actively transported NH4+; and 2nd, alkalinization through protonation of NH3 which is taken up to a significant extent from high external concentrations. The determination of intracellular pH by means of methylamine distribution is not a reliable method for eucaryotic systems.