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.
D015761:4-Aminopyridine; D000200:Action Potentials / Q000187:drug effects; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D001464:Barium / Q000502:physiology; D002118:Calcium / Q000502:physiology; D002478:Cells, Cultured; D003530:Cyprinidae / Q000502:physiology*; D004533:Egtazic Acid / Q000494:pharmacology; D006054:Goldfish / Q000502:physiology*; D007395:Interneurons / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects; D011188:Potassium / Q000502:physiology; D012160:Retina / Q000166:cytology*; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D000631:Aminopyridines; D013757:Tetraethylammonium Compounds; D001464:Barium; D004533:Egtazic Acid; D015761:4-Aminopyridine; D011188:Potassium; D002118:Calcium