Inward rectification in the inner segment of single retinal cone photoreceptors

1. Single cone photoreceptors were dissociated from the retina of a lizard with the aid of papain. The majority of the cells lost their outer segments but had well-preserved, large synaptic pedicles. Electrical properties of the cells were studied with tight-seal electrodes in the whole cell configuration. On the average, cone inner segments had a resting potential of -55 mV, and at this potential their input resistance was 2.6 G omega and their capacitance was 8 pF. 2. Under current clamp the cones exhibited a pronounced anomalous voltage rectification in response to hyperpolarizing currents. The voltage rectification was eliminated by external Cs+. 3. The Cs(+)-sensitive current underlying voltage rectification was isolated by blocking other currents present in the cone. Co2+ blocked a voltage-dependent Ca2+ current and a Ca2(+)-dependent Cl- current, and tetraethylammonium (TEA)+ blocked a delayed-rectifier K+ current. 4. The Cs(+)-sensitive current was activated by hyperpolarization to potentials more negative than -50 mV, and its current-voltage (I-V) relationship exhibited inward rectification. 5. The inward-rectifying current was selective for K+, but not exclusively. Increasing external K+ concentration 10-fold shifted the reversal potential by 13 mV. If Na ions also permeate through the inward-rectifying channels, the ratio of permeabilities (PK+/PNa+) in normal solution is approximately 3.9. 6. The kinetics of the inward-rectifying current were described by the sum of two exponentials, the amplitudes and time constants of which were voltage dependent. 7. The voltage dependence of the inward-rectifying current was described by Boltzmann's function, with half-maximum activation at -79 mV and a steepness parameter of 7.5 mV. 8. The voltage dependence and kinetics of the inward-rectifying current suggest that it is inactive in a cone photoreceptor in the dark. However, it becomes activated in the course of large hyperpolarizations generated by bright-light illumination. This activity will modify the waveform of the photovoltage--the current will generate a depolarizing component that opposes the light-generated hyperpolarization.