The anterior depolarizing mechanoreceptor potential and the correlated receptor currents have been studied in the marine ciliate Euplotes vannus. Mechanical stimuli that mimicked cell-cell collisions depolarized the resting potential of about -25 mV to maximally -5 mV, with a speed of 1.2 mV/ms, a delay to the stimulus of about 15 ms, and a repolarization within 30 to 300 ms. The power-stroke direction of the cirri-beat reversed from backward to forward during this response. The receptor current rose to an average amplitude of 1.4 nA with a speed of 0.1-0.3 nA/ms and decayed with a single exponential time course with a time constant between 7 and 9 ms. Similar currentreversal potentials, after substitution of extracellular Ca 2 + by Mg 2+ and vice versa, indicate that the mechanically activated conductance is identical for Ca 2 + or Mg 2 +. The current can be carried by Ba 2 + as well, but not by K + or Na +. Decirriation experiments have shown that the mechanosensitivity is located within the soma membrane.
Key words Mechanoreception -Calcium current 9 Magnesium current -Euplotes vannus
Abbreviations EASW artificial sea-water adapted for electrophysiology 9 EGTA ethylene glycol-O, O'-bis(2-aminoethyl)-N',N,N',N'-tetraacetic acid 9 NMDG N-methyl-D-glucamine 9 TEA tetraethyl ammonium. V,, membrane potential 9 gx conductance for the ion X 9 D600 Methoxyverapamil T. Krfippel (lyz~).