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 pos teriad effective beat (18 degrees left of the longitudinal axis) at a frequency of approximately 15 Hz. Activation of depolarization-sensiti ve ciliary Ca2+ channels leads to an increase in intracellular Ca2+ co ncentration and a change in the beating mode, Following rapid reorient ation, the effective stroke is anteriad (24 degrees right of the longi tudinal axis) and the beating frequency is about 26 Hz, resulting in f ast backward swimming of the cell, In response to minor depolarization s, and hence small increases in cytoplasmic Ca2+ concentration, the ci lia inactivate. Frequency increase and reversal in beat orientation sh are a single threshold level of membrane potential, since both changes of the beating mode occur simultaneously.