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Relationship between the membrane potential of the contractile vacuole complex and its osmoregulatory activity in Paramecium multimicronucleatum
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
Author for correspondence (e-mail:
naitoh{at}pbrc.hawaii.edu)
Accepted 18 July 2002
The electric potential of the contractile vacuole (CV) of Paramecium multimicronucleatum was measured in situ using microelectrodes, one placed in the CV and the other (reference electrode) in the cytosol of a living cell. The CV potential in a mechanically compressed cell increased in a stepwise manner to a maximal value (approximately 80 mV) early in the fluid-filling phase. This stepwise change was caused by the consecutive reattachment to the CV of the radial arms, where the electrogenic sites are located. The current generated by a single arm was approximately 1.3x10-10 A. When cells adapted to a hypotonic solution were exposed to a hypertonic solution, the rate of fluid segregation, RCVC, in the contractile vacuole complex (CVC) diminished at the same time as immunological labelling for V-ATPase disappeared from the radial arms. When the cells were re-exposed to the previous hypotonic solution, the CV potential, which had presumably dropped to near zero after the cell's exposure to the hypertonic solution, gradually returned to its maximum level. This increase in the CV potential occurred in parallel with the recovery of immunological labelling for V-ATPase in the radial arm and the resumption of RCVC or fluid segregation. Concanamycin B, a potent V-ATPase inhibitor, brought about significant decreases in both the CV potential and RCVC. We confirm that (i) the electrogenic site of the radial arm is situated in the decorated spongiome, and (ii) the V-ATPase in the decorated spongiome is electrogenic and is necessary for fluid segregation in the CVC. The CV potential remained at a constant high level (approximately 80 mV), whereas RCVC varied between cells depending on the osmolarity of the adaptation solution. Moreover, the CV potential did not change even though RCVC increased when cells adapted to one osmolarity were exposed to a lower osmolarity, implying that RCVC is not directly correlated with the number of functional V-ATPase complexes present in the CVC.
Key words: contractile vacuole complex, membrane potential, V-ATPase, osmoregulation, fluid segregation activity, exocytotic cycle, Paramecium multimicronucleatum
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