Conversion of beating mode in Chlamydomonas flagella induced by electric stimulation

Electric stimulation of a single Chlamydomonas cell by means of a suction electrode induced a temporary conversion of flagellar waveform from an asymmetric forward mode to a symmetric reverse mode. The reverse mode continued for about 0.5 seconds, after which the forward mode was resumed. Anodic stimulation (current passing outward through the membrane outside the suction pipette) was more effective in inducing the flagellar response than cathodic stimulation. No flagellar response was induced in the absence of free Ca²⁺ or in the presence of calcium channel inhibitors, pimozide (5 μM) and diltyazem (0.3 mM). These findings indicate that the flagellar response is mediated by membrane depolarization followed by a Ca²⁺ influx through voltage dependent calcium channels. This experimental system allowed us to quantitatively analyze the behavior of flagella during the waveform conversion. The flagellar bending pattern quickly changed from the forward mode to the reverse mode and, thereafter, gradually resumed the forward mode through two discrete phases: changes during reverse mode beating (phase I) and a distinct transitional phase (phase II). Recovery in curvature and sliding velocity of principal bends occurred mostly in phase I. Almost all of the recovery of reverse bends, returning the curvature to the low values characteristic of asymmetric forward mode beating, occurred in phase II. Beat frequency recovered through both phases. Phase II was often interrupted by a temporary stoppage of beating. These findings indicate that the bending pattern is converted through multiple steps that are controlled by Ca²⁺.