1. Rod photoreceptors were isolated from the retinae of tiger salamanders and voltage clamped using the whole-cell patch-clamp technique. 2. Hyperpolarizing the cell to potentials more negative than -50 mV evoked an inward current termed Ih. 3. Ih did not turn on immediately following a hyperpolarizing step but showed a marked delay. The activation time course of Ih could be described by the sum of two exponential components of opposite polarity. 4. The steady-state chord-conductance was half activated at -67 mV. 5. The reversal potential of Ih was close to -30 mV in normal standard salt solution. Increasing the external potassium concentration tenfold shifted the reversal potential by +17 mV. 6. The conductance-voltage relation and the kinetic parameters were not affected by changes in the external potassium concentration. 7. When fully activated, the zero-current conductance underlying Ih depended on the square root of the concentration of external potassium. 8. The permeability ratio PNa/PK depended on the external potassium concentration. It was 0.2 at an external potassium concentration of 2.0 mM and 0.3 at an external potassium concentration of 10.0 mM. The interaction of potassium with Ih suggests that Ih is a multi-ion pore. 9. It is concluded that Ih differs from the inward rectifier that is found in egg cells, frog muscle and heart muscle. 10. The kinetics and voltage sensitivity of Ih suggest that it does not play a role in the dark resting state or in the response to dim flashes of light. Its properties indicate that it may have a major role in the response to bright flashes.
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