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Journal of Experimental Biology, Vol 204, Issue 2 291-304, Copyright © 2001 by Company of Biologists
JOURNAL ARTICLES |
C Stock, RD Allen and Y Naitoh
Pacific Biomedical Research Center, Snyder Hall 306, University of Hawaii at Manoa, Honolulu, HI 96822, USA.
The rate of fluid expulsion, R(CVC), from the contractile vacuole complex (CVC) of Paramecium multimicronucleatum was estimated from the volume of the contractile vacuoles (CVs) immediately before the start of fluid discharge and from the time elapsing between discharges. The R(CVC) increased when the cell was exposed to a strongly hypotonic solution and decreased in a weakly hypotonic solution. When the cell was exposed to an isotonic or a hypertonic solution, R(CVC) fell to zero. The time constant, tau, used to describe the change in R(CVC) in response to a change in external osmolarity shortened after a short-term exposure to a strongly hypotonic solution and lengthened after a short-term exposure to a less hypotonic solution. A remarkable lengthening of tau occurred after a short-term exposure to isotonic or hypertonic solution. Under natural conditions, mechanisms for controlling R(CVC) are effective in maintaining the cytosolic osmolarity hypertonic within a narrow concentration range despite changes in the external osmolarity, which is normally hypotonic to the cytosol. Cells exposed to an isotonic or hypertonic solution resumed CV activity when left in the solution for 12 h. The cytosolic osmolarity was found to increase and to remain hypertonic to the external solution. This will permit cells to continue to acquire water. The increase in the cytosolic osmolarity occurred in a stepwise fashion, rather than linearly, as the external osmolarity increased. That is, the cytosolic osmolarity first remained more-or-less constant at an increased level until the external osmolarity exceeded this level. Thereupon, the cytosolic osmolarity increased to a new higher level in 12 h, so that the cytosol again became hypertonic to the external solution and the cells resumed CV activity. These results imply that the cell needs to maintain water segregation activity even after it has been exposed to an isotonic or hypertonic environment. This supports the idea that the CVC might be involved not only in the elimination of excess cytosolic water but also in the excretion of some metabolic waste substances.
This article has been cited by other articles:
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  K. Sugino, T. Tominaga, R. D. Allen, and Y. Naitoh Electrical properties and fusion dynamics of in vitro membrane vesicles derived from separate parts of the contractile vacuole complex of Paramecium multimicronucleatum J. Exp. Biol., October 15, 2005; 208(20): 3957 - 3969. [Abstract] [Full Text] [PDF]
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M. Iwamoto, K. Sugino, R. D. Allen, and Y. Naitoh Cell volume control in Paramecium: factors that activate the control mechanisms J. Exp. Biol., February 1, 2005; 208(3): 523 - 537. [Abstract] [Full Text] [PDF]
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 B. Becker and A. Hickisch Inhibition of Contractile Vacuole Function by Brefeldin A Plant Cell Physiol., January 15, 2005; 46(1): 201 - 212. [Abstract] [Full Text] [PDF]
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M. Iwamoto, R. D. Allen, and Y. Naitoh Hypo-osmotic or Ca2+-rich external conditions trigger extra contractile vacuole complex generation in Paramecium multimicronucleatum J. Exp. Biol., December 15, 2003; 206(24): 4467 - 4473. [Abstract] [Full Text] [PDF]
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H. K. Gronlien, C. Stock, M. S. Aihara, R. D. Allen, and Y. Naitoh Relationship between the membrane potential of the contractile vacuole complex and its osmoregulatory activity in Paramecium multimicronucleatum J. Exp. Biol., October 15, 2002; 205(20): 3261 - 3270. [Abstract] [Full Text] [PDF]
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 C. Stock, H. K. Gronlien, R. D. Allen, and Y. Naitoh Osmoregulation in Paramecium: in situ ion gradients permit water to cascade through the cytosol to the contractile vacuole J. Cell Sci., January 6, 2002; 115(11): 2339 - 2348. [Abstract] [Full Text] [PDF]
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