VOLTAGE-DEPENDENT IONIC CURRENTS IN SOLITARY HORIZONTAL CELLS ISOLATED FROM CAT RETINA

1. Horizontal cells of the cat retina were isolated by enzymatic dissociation. Two types of horizontal cells were identified: the axonless (A-type) horizontal cell having four to six thick, long (approximately 100 mum) dendrites, and the short-axon (B-type) horizontal cell having many (>5) fine, short (approximately 30 mum) dendrites. 2. Membrane properties of isolated horizontal cells were analyzed under current-clamp and voltage-clamp conditions. In the A-type cell, the average resting potential was -55 mV and the mean membrane capacitance was 110 pF, whereas values in the B-type cell were -58 mV and 40 pF, respectively. The A-type cell showed long-lasting Ca spikes, but B-type cells had no Ca spikes. 3. Five types of voltage-dependent ionic currents were recorded: a sodium current (I(Na)), a calcium current (I(Ca)), and three types of potassium currents. Potassium currents consisted of potassium current through the inward rectifier (I(anomal)), transient outward potassium current (I(A)), and potassium current through the delayed rectifier (I(K(v)). I(Na) was recorded only from A-type cells. Other currents were recorded from both types of cells. 4. I(Na) activated when cells were depolarized from a holding potential (V(h)) of -95 mV, and it was maximal at -25 mV. This current was blocked by tetrodotoxin. Approximately half of the A-type cells had I(Na), but no B-type cell had this current. 5. L-type I(Ca), an inward-going sustained current, was activated with depolarization more positive than -25 mV. Current amplitude reached a maximal value near 15 mV and became smaller with further depolarization. Little voltage-dependent inactivation was observed between Vh of -45 and -95 mV. I(Ca) was enhanced by Bay K 8644 and blocked by nifedipine. 6. I(anomal) was observed when cells were hyperpolarized below the equilibrium potential of potassium ion. Its amplitude became larger with further hyperpolarization. This current was time independent and was blocked by cesium or barium ions. 7. I(A) was activated by membrane depolarization to voltages more positive than -37 mV from Vh of -57 mV. The inactivation of the current was voltage-dependent and almost disappeared when held at V(h) of -17 mV. 4-aminopyridine blocked this current. 8. I(K(v) was activated by membrane depolarization to voltages more positive than -7 mV from V(h) of -67 mV, and amplitude increased with larger depolarization. A positive V(h) of -27 mV partially inactivated I(K(v)). This current was blocked by tetraethylammonium ions. 9. We conclude that mammalian horizontal cells (cat) have the same five voltage-dependent ionic currents as those of lower vertebrates. A major difference is that I(A) in mammalian horizontal cells is especially large relative to the amplitude of the other four currents.