THE DEPOLARIZING MECHANORECEPTOR POTENTIAL AND CA MG RECEPTOR-CURRENTOF THE MARINE CILIATE EUPLOTES VANNUS/

The anterior depolarizing mechanoreceptor potential and the correlated receptor currents have been studied in the marine ciliate Euplotes va nnus. Mechanical stimuli that mimicked cell-cell collisions depolarize d the resting potential of about - 25 mV to maximally - 5 mV, with a s peed of 1.2 mV/ms, a delay to the stimulus of about 15 ms, and a repol arization within 30 to 300 ms. The power-stroke direction of the cirri -beat reversed from backward to forward during this response. The rece ptor 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 t ime constant between 7 and 9 ms. Similar current-reversal potentials, after substitution of extracellular Ca2+ by Mg2+ and vice versa, indic ate that the mechanically activated conductance is identical for Ca2or Mg2+. The current can be carried by Ba2+ as well, but not by K+ or Na+. Decirriation experiments have shown that the mechanosensitivity i s located within the soma membrane.