VOLTAGE-GATED CALCIUM CURRENTS IN ISOLATED RETINAL GANGLION-CELLS OF THE CAT

Voltage-gated calcium current (I(Ca)) 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. I(Ca) was isolated from the voltage-dependent potassium and sodium currents by ion substitution and selective blockers. I(Ca) was activated by depolarization of the cell from a holding potential (V(h)) of -97 mV to more positive voltages than -57 mV. All recorded cells showed similar voltage dependence of I(Ca) activation: 50% activation at about -23 mV. Current-voltage (I-V) relationship of I(Ca) showed a symmetrical bell-shape with a single peak at around -7 mV. The I-V curve recorded with V(h) of -57 mV was nearly identical to that obtained with V(h) of -97 mV. During a depolarizing command, the amplitude of I(Ca) gradually decreased. Inactivation of I(Ca) depended on Ca influx into the cell. I(Ca) 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 I(Ca) 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) I(Ca). The type of I(Ca) recorded from the retinal ganglion cells in the adult cat is different from those in newborn rats.