Ionic Currents Activated by Voltage and by Intracellular Calcium: A Voltage Clamp Study in Solitary Vertebrate Photoreceptors

Solitary rod inner segments were obtained by enzymatic dissociation of the tiger salamander retina. Individual membrane currents were studied with the single-electrode voltage-clamp technique and with pharmacological agents. Extracellular TEA blocked an outward current which was activated, by depolarization and carried predominantly by potassium ions. Extracellular caesium blocked an inward current which was activated by hyperpolarization and carried by sodium and potassium ions. An inward calcium current was activated by depolarization and could be blocked by extracellular cobalt. Intracellular accumulation of calcium activates two other currents: one is blocked by intracellular caesium and is most likely a calcium- activated potassium current; the current remaining in the presence of intracellular caesium is carried, in part, by chloride. The five currents described can all be activated in the range of voltages in which rod photoreceptors normally operate.