Uptake of imidazole and its effects on the intracelluar pH and ionic relations of Chara corallina
1988
Smith, F.A. | Whittington, J.
Ammonia (pKa 9.25) and methylamine (pKa 10.65) increase cytoplasmic pH and stimulate Cl- influx in Chara corallina, these effects being associated with influx of the amine cations on a specific porter. The weak base imidazole (pKa 6.96) has similar effects but diffuses passively into the cell both as an unionized base and as a cation. When the external pH is greater than 6.0 influx of the unionized species predominates. Imidazole accumulates to high concentrations in the vacuole, where it is protonated. Cytoplasmic pH and vacuolar pH rise by only 0.2-0.3 units, suggesting a large balancing proton influx across the plasma membrane. Balance of electric charge is partially maintained by net efflux of K+ and net influx of Cl-. Calculation of vacuolar concentrations of imidazole (from [14C] imidazole uptake, assuming that there is no metabolism) plus K+ and Na+ indicates an excess of cations over inorganic anions (Cl-). However, although the osmotic potential of the cells increases, also indicating increased solute concentrations, the increase is less than that predicted by the calculated ionic concentrations. This discrepancy remains to be resolved. Because the osmotic potential also increases when imidazole is absorbed from Cl- -free solutions it is likely that maintenance of charge-balance can also involve synthesis and vacuolar storage of organic or amino acids.
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