Membrane properties of solitary horizontal cells isolated from goldfish retina.

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Abstract
Solitary horizontal cells were obtained by dissociating the adult goldfish retina using the enzyme papain. The cells were identified on morphological grounds and could be kept in culture for over a week. Solitary horizontal cells, penetrated with microelectrodes, had resting potentials of about -75 mV in normal solution. When external K+ concentration was changed, the membrane potential varied with EK calculated from the Nernst equation. All solitary horizontal cells tested showed an action potential in response to superthreshold depolarizing current pulses. The action potential had an overshoot of about +20 mV and a plateau potential lasting for several seconds. The action potential appeared to be Ca-dependent for the following reasons: TTX [tetrodotoxin] or low [Na+] did not affect the action potential, Sr2+, Ba2+ or high [Ca2+] enhanced the action potential, while Co2+ or high [Mg2+] blocked it. No regenerative activity was observed in horizontal cells in the retina but it is possible that the regenerative mechanism is suppressed normally. A role for K+ was indicated by an increase in the duration and amplitude of the action potential on the application of tetraethylammonium. The steady-state current-voltage (I-V) curve, measured by applying constant current pulses, was S-shaped (current on the abscissa) and composed of inward- and outward-going rectifying regions and a transitional region between them. A similar non-linear I-V relationship was reported in vivo. The transitional region was characterized by a sudden potential jump and hysteresis, suggesting the presence of a negative resistance. This potential jump appeared not to be produced by the Ca-conductance mechanism mentioned above, since similar jumps were observed in the presence of Co2+.

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