Short Communication Distribution of Ion Channels in the Membrane of the Dinoflagellate Noctiluca Miliaris

Noctiluca miliaris, a marine dinoflagellate, exhibits spontaneous flexion of its tentacle. The movement of the tentacle is always accompanied by perturbations of membrane potential, termed the 'tentacle regulating potentials' (TRPs) (Eckert and Sibaoka, 1967). Though the waveform of the TRPs is liable to variation, it consists of four successive basic components: (1) a Na-dependent depolarizing (positive) spike, (2) a depolarizing plateau potential, (3) a Cl~-dependent hyperpolarizing (negative) spike, and (4) a long-lasting hyperpolarization (Eckert and Sibaoka, 1967; Oami et al. 1988). A slow flexion of the tentacle is seen during the plateau, and the flexion is suddenly accelerated when the negative spike occurs. The tentacle then extends during the long-lasting hyperpolarization. There is no correlation between the positive spike and the tentacular movement (Hisada, 1957; Oami et al. 1988). To investigate the control mechanism of the tentacle movement by the TRPs, we have determined the distribution of the ion channels responsible for the TRPs over the cell surface by examining the membrane impedance and the membrane electric current in various regions of the cell surface. We found that the depolarization-sensitive Na channels responsible for the positive spike were restricted to the cytostomal region, while the hyperpolarization-sensitive Cl~ channels were present anywhere in the cell surface membrane, though they were most densely clustered around the cytostome region. Specimens of Noctiluca miliaris were cultured in an artificial sea water (ASW) with their green algal food, Dunaliella, as previously described (Oami et al. 1988). A single specimen was held at the tip of a suction pipette, for either impedance or current recording, by lowering hydrostatic pressure inside the pipette. The TRPs were recorded using a conventional glass capillary microelectrode filled with 3moll" KC1 and inserted into the flotation vacuole of the specimen. For impedance recording, the suction pipette was 200 ftm in inner diameter, and served