Temporal Relationship between Cytosolic Free Ca2+ and Membrane Potential during Hypotonic Turgor Regulation in a Brackish Water Charophyte Lamprothamnium succinctum

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Abstract
Internodal cells of a brackish water charophyte, Lamprothamnium succinctum, regulate turgor pressure in response to changes in external osmotic pressure by modifying vacuolar concentrations of KCl. An increase in cytosolic concentration of free Ca2+ ([Ca2+]c) is necessary for the progress of turgor regulation induced by hypotonic treatment. Initial changes in membrane potential and [Ca2+]c upon hypotonic treatment were measured to examine the temporal relationship between the two parameters. Fura-dextran (potassium salt, Mr 10,000, anionic) that had been injected into the cytosol was used to measure [Ca2+]c. Membrane potential and membrane conductance under a current-clamp condition were also measured. Decrease in external osmotic pressure by 0.16 Osm induced a simultaneous increase in [Ca2+]c with both depolarization of the membrane and increase in the membrane conductance. Decrease in external osmotic pressure by 0.05 Osm induced a simultaneous increase in [Ca2+]c with membrane depolarization but the increase in membrane conductance started later than the other two processes. There was a close temporal relationship between the increase in [Ca2+]c and membrane depolarization on the initial response of turgor regulation induced by hypotonic treatment.

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