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Osmoregulation in Paramecium: in situ ion gradients permit water to cascade through the cytosol to the contractile vacuole

¹ Pacific Biomedical Research Center, Snyder Hall 306, University of Hawaii at Manoa, 2538 The Mall, Honolulu, HI 96822, USA
² Department of Biology, University of Oslo, PO Box 1051, Blindern, N-0316 Oslo, Norway

View larger version (77K): [in a new window]   Fig. 1. Calibration and application of ion-selective electrodes. (A) K+-selective electrode. (B) Cl-selective electrode. (a) Representative traces of electrical potentials from each ion-selective electrode in each of three different calibration solutions of known ionic activities (labeled below each segment of the trace). (b) Representative traces of electrical potentials from each ion-selective electrode inserted in situ first into the cytosol and then into the CV. Ionic activities in the cytosol and the CV fluid were estimated from the calibration plots shown in c. (c) Plots of the electrical potentials estimated from the traces in (a) against their respective corresponding ion activities in the calibration solutions. (C) Photographs showing the CV and the double-barreled ion-selective electrode (stars) in the cytosol (left) and the CV (right, broken line).

View larger version (15K): [in a new window]   Fig. 2. The relationship of the K+ (A) or the Cl- (B) activity (y-axis) to the external solutions of different osmolarities (x-axis). The paramecium multimicronucleatum cells had been adapted for 18 hours to various sorbitol-adjusted extracellular osmolarities. Open circles show the ionic activities in the cytosol, closed circles show the ionic activities in the CV fluid. Each point and its vertical line is mean±s.d.

View larger version (21K): [in a new window]   Fig. 3. K+ (Ai, Bi, Ci) and Cl- activities (Aii, Bii, Cii) in the cytosol (white bars) and in the CV fluid (black bars) of P. multimicronucleatum cells adapted to choline-containing solutions (Ai, Aii), to Ca2+-containing solutions (Bi, Bii) and to K+-containing standard solutions that served as controls (Ci, Cii). The osmolarity of the adaptation solution was either 24 mosmol l-1 (left pair of bars) or 124 mosmol l-1 (right pair of bars). Vertical lines represent s.d. The number of measurements varies between 5 and 9; see text for details.

View larger version (15K): [in a new window]   Fig. 4. Ca2+ activities in the CV fluid of P. multimicronucleatum adapted to either 24 mosmol l-1 (left three bars) or 124 mosmol l-1 (right three bars). The adaptation solution contained either 2 mmol l-1 K+ (white bars) or 2 mmol l-1 choline (checked bars) or 1.25 mmol l-1 Ca2+ (black bars). Each column represents a mean value±s.d. for five or six contractile vacuoles, one per cell.

View larger version (17K): [in a new window]   Fig. 5. Effects of 1 mmol l-1 furosemide on the rate of fluid segregation, RCVC (A) and the cell volume (B) of P. multimicronucleatum. (A) Representative results obtained from a single CV. The data in B are mean values±s.d. (vertical lines) obtained from five cells.