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Exposure of the giant marine alga Valonia utricularis to acute hypo-osmotic shocks induces a transient increase in turgor pressure and subsequent back-regulation. Separate recording of the electrical properties of tonoplast and plasmalemma together with turgor pressure was performed by using a vacuolar perfusion assembly. Hypo-osmotic turgor pressure regulation was inhibited by external addition of 300 μM of the membrane-permeable ion channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). In the presence of 100 μM NPPB, regulation could only be inhibited by simultaneous external addition of 200 μM 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS), a membrane-impermeable inhibitor of Cl− transport. At concentrations of about 100 μM, NPPB seems to selectively inhibit Cl− transporters in the tonoplast and K+ transporters in the plasmalemma, whereas 300 μM NPPB inhibits K+ and Cl− transporters in both membranes. Evidence was achieved by measuring the tonoplast and plasmalemma conductances (G t and G p) in low-Cl− and K+-free artificial seawater. Inhibition of turgor pressure regulation by 300 μM NPPB was accompanied by about 85% reduction of G t and G p. Vacuolar addition of sulfate, an inhibitor of tonoplast Cl− transporters, together with external addition of DIDS and Ba2+ (an inhibitor of K+ transporters) also strongly reduced G p and G t but did not affect hypo-osmotic turgor pressure regulation. These and many other findings suggest that KCl efflux partly occurs via electrically silent transport systems. Candidates are vacuolar entities that are disconnected from the huge and many-folded central vacuole or that become disconnected upon disproportionate swelling of originally interconnected vacuolar entities upon acute hypo-osmotic challenge. |
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