Electrically induced Ca2+ transport across the membrane of Paramecium caudatum measured by means of flow-through technique

Transmembrane calcium fluxes related to excitation were studied in Paramecium caudatum. Radioactive (⁴⁵Ca²⁺) or inactive solution was flowed through a dense suspension of unlabelled or labelled cells, and radioactivity was monitored in the solution. The organisms were electrically stimulated by means of extracellular electrodes. As a result of stimulation Ca²⁺ uptake and release was measured. The uptake response dropped with increasing number of successive stimulation periods and increased with growing stimulus amplitude and duration. Maximum uptake was obtained at 20 V/cm and at least 60 s duration and for temperatures between 10 and 15°C. A Ca²⁺ influx of 700 pmol/1000 cells upon 1 min stimulation was measured at 15°C. This corresponds to an increment of the intraciliary [Ca²⁺] of about $\text{5 · 10}{}^{-4}\text{M}$. Ca²⁺ release was dependent on the stimulus amplitude in a similar manner as was Ca²⁺ uptake. Photographic recordings of the swimming behaviour of the organisms were used to interpret the flux data. At temperatures up to 15°C the cells swam backward perpendicular to the applied electric field of 10 to 20 V/cm. At 25°C they showed forward spiralling movement. For the first time evidence was brought for stimulated Ca²⁺ influx in Paramecium at physiological temperatures. It is concluded from the results that a strong active Ca²⁺ extrusion from the intraciliary space counteracts the influx. The Ca²⁺ pump rate must be at least $\text{8 · 10}{}^{12}$ calcium ions per s per cm² ciliary surface.