Calcium channels in plants - Measurements of Ca2+ fluxes in intact plant cells

Owing to the central role of Ca2+ in signal transduction processes, it is important to measure membrane fluxes of Ca2+ in cells which are as undisturbed as possible, particularly when studying the control of these fluxes. To this end, techniques have been developed to measure Ca2+ fluxes in intact, turgid plant cells. The measurements are principally of influx across the plasma membrane where Ca2+ transport is likely to occur through cation-selective channels. The most direct method measures tracer fluxes of Ca2+, but special procedures are required to distinguish between influx and extracellular binding of Ca2+. Unfortunately, such techniques are currently only applicable to giant cells where surgical separation of the intracellular contents from the cell wall is possible. The influx of Ca2+ into normal, resting cells of the green alga Chara corallina is usually about 0.3 nmol m-2 s-1 (at an external Ca2+ concentration of 0.5 mol m-3). This flux is up to 5 times higher in actively growing cells, 20 times higher in cells depolarized by 20 mol m-3 K+ and 1000 times higher during an action potential. Reducing cell turgor by a wide range of solutes increases Ca2+ influx, especially near plasmolysis. Ca2+ influx is sensitive to alterations in both external and cytosolic pH, but is inhibited by complete darkness and by low concentrations of La3+. Various organic Ca2+ channel antagonists had mixed effects on Ca2+ influx into Chara. The work described in this paper should enable further study of the control of Ca2+ fluxes into intact, turgid plant cells, and their role in signal transduction and the control of cellular activities.