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 ⁴⁵Ca, 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.