Calcium couples flagellar reversal to photostimulation in Chlamydomonas reinhardtii

Abstract

SEVERAL recent findings^1–3 suggest that the intracellular concentration of free calcium, [Ca]_i, has an important role in the regulation of ciliary and flagellar beating. For example, Paramecium extracted with Triton X-100 and reactivated with Mg²⁺ and ATP swim backwards when the Ca²⁺ concentration of the test medium, [Ca]₀, is raised above 10⁻⁶M (ref. 1). Similarly, extracted cells of the typanosomid Crithidia show a reversal in wave propagation that is controlled by Ca ions. In studies on the intact flagellar apparatus isolated from a wall-less mutant of Chlamydomonas reinhardtii, the flagella have been shown to reverse direction of the effective stroke when external [Ca]_o is raised above 10⁻⁶ M (ref. 3). Electro-physiological evidence indicates that, in Paramecium, reversal of ciliary beating is caused by Ca which enters the cell when the Ca permeability of the membrane is increased^4,5. We have investigated whether Ca is involved in the phototactic response and more specifically whether this Ca influx is regulated by the cell membrane. Electrophysiological recording was impractical because of the small size of these cells (< 10 µm), and so the approach was to photograph the light-induced motor responses of cells swimming in solutions containing different concentrations of Ca or Ca-blocking agents. The results demonstrate that the reversed beating response of the flagella depends on Ca. Inhibition of the response by Ca-blocking agents supports the view that reversed beating response of the flagella is coupled to photostimulation by an influx of Ca ions.