Cytoplasmic free Ca2+ in the marine alga Acetabularia: measurement with Ca2+-selective microelectrodes and kinetic analysis
Neutral carrier-based Ca2+-selective microelectrodes have been examined for application in concentrated multi-ion solutions. Calculations with data from the literature and our calibration series with Ca2+-EGTA buffers (a convenient algorithm for their calculation is given) provide the physico-chemic...
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Published in | Journal of experimental botany Vol. 43; no. 252; pp. 875 - 885 |
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Main Authors | , , |
Format | Journal Article |
Language | English |
Published |
01.07.1992
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Subjects | |
Online Access | Get more information |
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Summary: | Neutral carrier-based Ca2+-selective microelectrodes have been examined for application in concentrated multi-ion solutions. Calculations with data from the literature and our calibration series with Ca2+-EGTA buffers (a convenient algorithm for their calculation is given) provide the physico-chemical conditions for determination of submicromolar concentrations of free Ca2+ in the cytoplasm (with about 400 mM K+ and 70 mM Na+) of the marine alga Acetabularia acetabulum. The experimental results give a cytoplasmic concentration of 560 nM free Ca2 corresponding to 140 nM activity. Recordings of cytoplasmic Ca2+ upon removal and re-addition of external (10 mM) Ca2+ show steady-state changes by about 50 nM (following the direction of external Ca2+) which are preceded by transient overshoots. These kinetics are better described by damped oscillations of a feedback control system than by two superimposed exponentials. Using the maximum rate of decrease of cytoplasmic Ca2+ upon removal of external Ca2+, a unidirectional Ca2+ efflux of greater than or equal to 0.3 micromole m-2 s-1 is determined which is considered to mark the steady-state turnover of Ca2+ at the plasmalemma. This high rate and the high electrochemical driving force for Ca 2+ (about -580 mV) across the plasmalemma at a resting voltage of about -170 mV, point to a powerful Ca2+ transport system which cannot sufficiently be fuelled by ATP-hydrolysis but requires additional energy. |
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ISSN: | 0022-0957 1460-2431 |
DOI: | 10.1093/jxb/43.7.875 |