Ionic currents of solitary horizontal cells isolated from goldfish retina.

Solitary horizontal cells, dissociated from papain‐treated goldfish retinas, produce action potentials and show a non‐linear current‐voltage relationship. Underlying ion‐conductance mechanisms were analysed by a single‐micro‐electrode voltage‐clamp technique. Pharmacological and ion‐substitution experiments revealed that ionic currents could be separated into at least four voltage‐dependent currents: a Ca current and three types of K currents. The Ca current was activated by membrane depolarization beyond ‐45 mV, reached a maximal value near 0 mV, and became smaller at more positive potentials. By extrapolation, the reversal potential was estimated to be approximately +50 mV. The Ca current was inactivated by accumulation of intracellular Ca ions but not by membrane depolarization. Co ions (4mM) blocked this current. The first type of K current showed anomalous (inward‐going) rectification near the resting potential (congruent to ‐60 mV). Hyperpolarization from the resting level produced a large, almost steady inward current, while depolarization evoked only a small, steady outward current. The current‐voltage relationship revealed a shallow negative resistance region at membrane potentials beyond ‐50 mV. The current was blocked by Cs (10 mM) or Ba (1 mM) ions. The second type of K current (the transient outward current) was activated by membrane depolarization beyond ‐25 mV. The peak amplitude increased almost exponentially as the membrane was depolarized. During steady depolarization this current decayed exponentially (time constant congruent to 500 ms at +20 mV). The current was inactivated by conditioning depolarization (greater than 10 s) beyond ‐30 mV and blocked by 4‐aminopyridine (10 mM). The third type of K current was the maintained outward current which was activated by membrane depolarization beyond ‐20 mV, increased to a steady level in a few hundred milliseconds, and showed little inactivation. The amplitude increased as the membrane was depolarized. The current was blocked by tetraethylammonium ions (20 mM). A Ca‐mediated K current was not detected. Action potentials and the non‐linear current‐voltage relationship of solitary horizontal cells can be explained qualitatively by the combination of the four ionic currents.