Voltage-gated calcium currents in isolated retinal ganglion cells of the cat

Voltage-gated calcium current (ICa) was recorded from retinal ganglion cells dissociated from the adult cat under the voltage-clamp condition using a patch pipette in the whole cell configuration. ICa was isolated from the voltage-dependent potassium and sodium currents by ion substitution and selective blockers. ICa was activated by depolarization of the cell from a holding potential (Vh) of -97 mV to more positive voltages than -57 mV. All recorded cells showed similar voltage dependence of ICa activation: 50% activation at about -23 mV. Current-voltage (I-V) relationship of ICa showed a symmetrical bell-shape with a single peak at around -7 mV. The I-V curve recorded with Vh of -57 mV was nearly identical to that obtained with Vh of -97 mV. During a depolarizing command, the amplitude of ICa gradually decreased. Inactivation of ICa depended on Ca influx into the cell. ICa became more sustained either when the extracellular Ca was replaced by Ba, or when the cell was loaded with 30 mM EGTA. Nifedipine (10(-4) M) inhibited ICa reversibly. Effects of Bay K 8644 were bimodal: augmentation at a low concentration (10(-8) M) and suppression at a high concentration (10(-4) M). All these characteristics are identical to the previous findings for the high-threshold (L-type) ICa. The type of ICa recorded from the retinal ganglion cells in the adult cat is different from those in newborn rats.