Effect of Extracellular pH on Motility and K + ‐Induced Ciliary Reversal in Paramecium caudatum 1

.Paramecium caudatum , reared on bacterized hay infusions at pH 6.5 to 6.9, were washed into various buffered solutions containing 0.016 mM CaCl 2 and a pH of 3.5 to 10.4. Solutions of pH 4.5 to 9.5 support strong swimming of the cells for at least 24 h. At pH values acid to the culture medium, cells show an increasing frequency of spontaneous ciliary reversal episodes (“avoiding reactions”). Uninterrupted forward swimming is usually observed over the pH range of 7.1 to 8.5, and above pH 8.5, forward motion is interrupted by circular swimming. For all pH values tested, transfer of cells to a more acidic test solution than the solutions into which they were washed (adaptation solution) usually induced short duration, periodic ciliary reversal behavior. With transfer to a more alkaline test solution than the adaptation solution, the cells shift from forward left spiralling motion to forward right spiralling motion. With decreasing pH, the cells show progressively less sensitivity to KC1 stimulation, and at pH values of less than 5.0, cells fail to show significant ciliary reversal response to any KC1 concentration tested (1 ‐ 128 mM). At alkaline pH values and higher KC1 concentrations, the cells show very pronounced ciliary reversal behaviors but usually fail to regain forward swimming behavior.