Okadaic acid, an inhibitor of protein phosphatase 1 in Paramecium, causes sustained Ca²⁺-dependent backward swimming in response to depolarizing stimuli

Backward swimming is a Stereotypic behavioural response of Paramecium. It is triggered by depolarizing stimuli, which open calcium channels in the excitable ciliary membrane. The influx of Ca²⁺ causes the reversal of ciliary beat and initiates backward swimming. Here, we demonstrate that the protein phosphatase inhibitor okadaic acid does not affect the normal forward swimming pattern of Paramecium, but greatly extends the duration of backward swimming as initiated by depolarization caused by a rise in extracellular K⁺. Chelation of external Ca²⁺ results in an immediate Resumption of forward swimming. The results suggest that the voltage-operated calcium channel is inactivated by a dephosphorylation event, and that okadaic acid blocks this dephosphorylation without any effect on the motile apparatus of the cilia. In addition, Paramecium is unique among eukaryotic cells, in that okadaic acid inhibits just one protein phosphatase, namely a type 1 enzyme, 75% of which is tightly associated with the excitable ciliary membrane. The type 2A protein phosphatases in Paramecium are unaffected by okadaic acid. The results indicate that protein phosphatase 1 is the enzyme responsible for the dephosphorylation and closure of the calcium channel in Paramecium.