Trifluoperazine‐induced changes in swimming behavior of paramecium: Evidence for two sites of drug action

Trifluoperazine (TFP), a drug that binds to Ca²⁺‐calmodulin (CaM) complexes, altered swimming behavior not only in living paramecia, but also in reactivated, Triton‐extracted “models” of the ciliate. By comparing the responses of living cells and models, we have ascertained that two sites of drug action exist in paramecium cilia. Swimming movements were recorded in darkfield stroboscopic flash photomicrographs; this permitted accurate quantitation of velocities and body‐shape parameters. When living paramecia were incubated in a standard buffer containing 10 μM TFP, their speed of forward swimming fell over several minutes and their bodies shortened. Untreated paramecia backed up repeatedly and frequently upon transfer to a solution containing barium ions (the “barium dance”), but cells preincubated in TFP did not “dance.” Instead they swam forward slowly for long periods of time without reversing and occasionally then exhibited abnormally prolonged reversals. W7 effects on swimming mimicked low doses of TFP, and the analog W5 did not visibly alter normal swimming patterns. These results suggest that TFP induces a decrease in the intracellular pCa of living paramecia, perhaps by reducing the efficiency of a calmodulin‐activated calcium pump in the cell membrane. Paramecia extracted with Triton X‐100 and reactivated to swim forward (7 ≥ pCa ≥ 6) were not affected by addition of up to 40 μM TFP to the reactivation medium. We conclude that the main drug effect in living cells is probably not at the axoneme. However, at low pCa, TFP directly affected the ciliary axoneme to shift its behavior to one characteristic of a higher pCa: TFP inhibited backward swimming in models reactivated at pCa < 6; instead they swam forward or rocked in place. The mechanism of ciliary reversal in paramecium may therefore depend on an axonemal Ca⁺‐sensor, possibly bound CaM, which is affected by TFP only at low pCa, as has been postulated for other types of cilia.