Epithelial action potentials inOikopleura (Tunicata: Larvacea)

Summary The passive electrical properties and initiation of action potentials have been examined in the external epithelium of oikopleurid larvacean tunicates. The epithelial cells are electrically coupled, and are polygons up to 200 m across and up to 1.4–2.8 m thick. Membrane constants determined by a 2-electrode study were forO. dioica:: 922 m; R_m: 4.3 kcm₂; R_i: 82.7 cm. Corresponding values for the largerO. longicauda were: 3350 m; 35.6 kcm²; and 104.5 cm. Mean resting potentials in both species were around 80 mV. Mechanical stimulation evokes over-shooting action potentials propagated (at 18 °C) at some 40 cm/s. These are rapid events, repolarisation being almost complete in 5 ms. There is no undershoot.When the recording electrode penetrates the epithelial cell from its inner surface distant mechanical stimulation may evoke similar action potentials arising from the stimulus site, but more often evokes graded small depolarisations which give rise to action potentials with increasing strength of mechanical stimulation. Reasons are given for considering these to be generator potentials resulting from deformation of the outer epithelial cell membrane by the tip of the recording electrode. The effects of epithelial action potentials upon the potentials recorded from the caudal muscle cells are briefly described.