Transport Systems of Ventricaria ventricosa: I/V Analysis of Both Membranes in Series as a Function of [K+]
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Authors: | MJ Beilby MA Bisson |
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Institution: | (1) Biophysics Department, School of Physics, University of New South Wales, Sydney 2052, Australia, AU;(2) Department of Biological Sciences, State University of New York at Buffalo, Cooke Hall 109, Box 1300, Buffalo, NY 14260-1300, USA, US |
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Abstract: | The current-voltage (I/V) profiles of Ventricaria (formerly Valonia) membranes were measured at a range of external potassium concentrations, K+]
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, from 0.1 to 100 mm. The conductance-voltage (G/V) characteristics were computed to facilitate better resolution of the profile change with time after exposure to different
K+]
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. The resistance-voltage (R/V) characteristics were computed to attempt resolution of plasmalemma and tonoplast. Four basic electrophysiological stages
emerged: (1) Uniform low resistance between −60 and +60 mV after the cell impalement. (2) High resistance between +50 and
+150 for K+]
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from 0.1 to 1.0 mm and hypotonic media. (3) High resistance between −150 and −20 mV for K+]
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of 10 mm (close to natural seawater) and hypertonic media. (4) High resistance between −150 and +170 mV at K+]
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of 100 mm.
The changes between these states were slow, requiring minutes to hours and sometimes exhibiting spontaneous oscillations of
the membrane p.d. (potential difference). Our analysis of the I/V data supports a previous hypothesis, that Ventricaria tonoplast is the more resistive membrane containing a pump, which transports K+ into the vacuole to regulate turgor. We associate state (1) with the plasmalemma conductance being dominant and the K+ pump at the tonoplast short-circuited probably by a K+ channel, state (2) with the K+ pump ``off' or short-circuited at p.d.s more negative than +50 mV, state (3) with the K+ pump ``on,' and state (4) with the pump dominant, but affected by high K+. A model for the Ventricaria membrane system is proposed.
Received: 5 November 1998/Revised: 11 May 1999 |
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Keywords: | :Ventricaria (Valonia) — I/V analysis — Potassium dependence — Electrogenic pump — Turgor regulation |
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