Ionic currents of solitary horizontal cells isolated from goldfish retina.

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Abstract
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 analyzed by a single microelectrode voltage-clamp technique. Pharmacological and ion-substitution experiments revealed that ionic currents could be separated into at least 4 voltage-dependent currents: a Ca current and 3 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 .apprx. +50 mV. The Ca current was inactivated by accumulation of intracellular Ca ions but not by membrane depolarization. Co ions (4 mM) blocked this current. The 1st type of K current showed anomalous (inward-going) rectification near the resting potential (.simeq. -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 Ba2+ (1 mM). The 2nd 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 .simeq. 500 ms at +20 mV). The current was inactivated by conditioning depolarization (> 10 s) beyond -30 mV and blocked by 4-aminopyridine (10 mM). The 3rd 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 ms, 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 4 ionic currents.

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