Protoplasmic pH modifies water and solute transfers in Beta vulgaris root vacuoles

AMODEO G, SUTKA M, DORR R, PARISI M

JOURNAL OF MEMBRANE BIOLOGY

Springer

Lugar: New York; Año: 2002 vol. 187 p. 175 - 184

0022-2631

Volume changes were studied in Beta vulgaris storage root vacuoles, using video microscopy, when exposed to hypotonic conditions. The osmotic gradient was either step applied or progressively imposed in perfusion experiments. Pre-incubation at low pH (6.6) or with HgCl2 strongly reduced the vacuoles water permeability, measured in step experiments. Furthermore, the volumetric response was depending on the rate with which the aniso-osmotic condition was established. In perfusion experiments a “plateau value” (osmotic equilibrium or steady state volume value) was observed, which was significantly lower that the theoretically expected one. Furthermore if vacuoles were pre-incubated in presence of HgCl2 or at low pH and then the hypo-osmotic challenge was applied in perfusion experiments, a still lower “plateau value” was observed. This reduction was concentration-dependent and completely reversible. In these conditions, when HgCl2 concentration was 300 mM or medium pH was 6.6 the volume change was abolished. In other experiments when urea iso-osmotically replaced mannitol a reversible, pH dependent volumetric response was observed. These results can be interpreted accepting that 1) Mercury sensitive water channels, present in the studied structure, were blocked by low pH during the hypo-osmotic challenge. 2) Modification of water permeability prevents excessive swelling during the osmotic shock. 3) The effectiveness of this last mechanism was depending on the osmotic challenge rate; and 4) Additionally, urea reflection coefficients was also modified by reduced medium pH.