Ionic Control of the Reversal Response of Cilia in Paramecium caudatum

Abstract

The duration of ciliary reversal of P. caudatum in response to changes in external ionic factors was determined with various ionic compositions of both equilibration and stimulation media. The reversal response was found to occur when Ca ions bound by an inferred cellular cation exchange system were liberated in exchange for externally applied cations other than Ca. Factors which affect the duration of the response were initial amount of Ca bound by the cation exchange system, final amount of Ca bound by the system after equilibration with the stimulation medium, and concentration of Ca ions in the stimulation medium. An empirical equation is presented which relates the duration of the response to these 3 factors. On the basis of these and previously published data, the following hypothesis is proposed for the mechanism underlying ciliary reversal in response to cationic stimulation: Ca++ liberated from the cellular cation exchange system activates a contractile system which is energized by ATP. Contraction of this component results in the reversal of effective beat direction of cilia by a mechanism not yet understood. The duration of reversal in live paramecia is related to the time course of bound Ca release.