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

Abstract

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 current-reversal potentials, after substitution of extracellular Ca2+ by Mg2+ and vice versa, indicate that the mechanically activated conductance is identical for Ca2+ or Mg2+. The current can be carried by Ba2+ as well, but not by K+ or Na+. Decirriation experiments have shown that the mechanosensitivity is located within the soma membrane.