Voltage-dependence of ciliary activity in the ciliate Didinium nasutum

In the gymnostome ciliate Didinium nasutum, swimming behaviour depends upon the cyclic activity of about 3000 cilia. The normal beating mode, resulting in forward swimming of the cell, is characterized by a posteriad effective beat (18 left of the longitudinal axis) at a frequency of approximately 15 Hz. Activation of depolarization-sensitive ciliary Ca2+ channels leads to an increase in intracellular Ca2+ concentration and a change in the beating mode. Following rapid reorientation, the effective stroke is anteriad (24 ° right of the longitudinal axis) and the beating frequency is about 26 Hz, resulting in fast backward swimming of the cell. In response to minor depolarizations, and hence small increases in cytoplasmic Ca2+ concentration, the cilia inactivate. Frequency increase and reversal in beat orientation share a single threshold level of membrane potential, since both changes of the beating mode occur simultaneously.