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Relationship between the membrane potential of the contractile vacuole complex and its osmoregulatory activity in Paramecium multimicronucleatum

Heidi K. Grønlien^*, Christian Stock, Marilynn S. Aihara, Richard D. Allen and Yutaka Naitoh

Pacific Biomedical Research Center, Snyder Hall 306, University of Hawaii, 2538 The Mall, Honolulu, HI 96822, USA
^* Present address: Department of Biology, University of Oslo, PO Box 1051, Blindern, N-0316 Oslo, Norway
Present address: Physiological Institute, University of Würzburg, Röntgenring 9, D-97070 Würzburg, Germany

View larger version (30K): [in a new window]   Fig. 1. Representative traces of the electrical potential of the contractile vacuole (CV) fluid with reference to the cytosol in Paramecium multimicronucleatum (CV membrane potential) and the input resistance of the CV (A; filled circles). (A) A non-compressed (normal) cell. (B) A mechanically compressed cell. F, R and D above segments of potential traces correspond to the fluid-filling, rounding and fluid-discharging phases of exocytotic cycles of the CV, respectively. Double slashes on the potential trace in B indicate interruptions to the CV membrane potential recording when the electrode tip was no longer in an intact CV during the fluid-discharging phase. See the text for further details. Exocytic cycles are numbered.

View larger version (159K): [in a new window]   Fig. 2. Consecutive video images of the contractile vacuole in a mechanically compressed cell of Paramecium multimicronucleatum. The number at the upper left corner of each frame corresponds to the time (s) when the picture was taken. Time 0 corresponds to the beginning of the series. CV, contractile vacuole; RA, radial arm. Asterisks show swollen portions of the radial arms. The radial arms become thinner after reattachment to the CV. See text for details. Scale bar, 10 µm.

View larger version (64K): [in a new window]   Fig. 3. (A) The contractile vacuole (CV) potential (filled squares) and the rate of fluid segregation in the CV complex (RCVC; open circles) as a function of the osmolarity of adaptation solution in Paramecium multimicronucleatum. Values are means ± S.E.M. (N=4-10). (B) DS-1 labelling of the decorated spongiome, visualized by its immunological fluorescence image. The number at the top of each picture corresponds to the osmolarity (mosmoll-1) to which the cell was adapted. Scale bar, 20 µm.

View larger version (63K): [in a new window]   Fig. 4. (A) The contractile vacuole (CV) potential (filled squares), and the rate of fluid segregation in the CV complex (RCVC; open circles) of Paramecium multimicronucleatum as a function of the time of re-exposure of the cells to their original hypotonic adaptation solution (4 mosmoll-1; abscissa) after they had received a prior exposure to a hypertonic solution (124 mosmoll-1) for 30 min. Values are means ± S.E.M. (N=5-9). (B) DS-1 labelling of the decorated spongiome, visualized in immunological fluorescence images. The number at the top of each picture corresponds to the time (min) when the picture was taken following re-exposure of the cell to the hypotonic solution. The control cell was adapted to the hypotonic solution. Scale bar, 20 µm.

View larger version (30K): [in a new window]   Fig. 5. (A) A schematic representation of the contractile vacuole (CV) complex in Paramecium multimicronucleatum to show the pathways of the electric currents generated by V-ATPases in the radial arms. CV, contractile vacuole; RA1-RAN, the radial arms 1-N, where N is the total number of radial arms; iH+, the current generated in a single radial arm; iRA, the passive current across the radial arm membrane caused by iH+; iCV, the passive current across the CV membrane caused by iH+ of those radial arms attached to the CV. (B) An electric circuit equivalent to A. rCV, the input resistance of the CV; rRA, the input resistance of a radial arm; SW, a switch corresponding to the attachment of the radial arm to (on) or detachment of the RA from (off) the CV; eCV, CV potential. (C) Simulated stepwise changes in the eCV based on the equivalent circuit (B) as a total of eight radial arms attach to the CV one by one. Numbers to the left signify potential steps as the radial arms are attached to the CV one by one.