The properties of single cones isolated from the tiger salamander retina

Abstract

The properties of isolated single cones were studied using the voltage-clamp technique, with 2 micro-electrodes inserted under visual control. Single cones had input resistances, when impaled with 2 electrodes, of up to 270 M.OMEGA.. This is probably lower than the true membrane resistance, because of damage by the impaling electrodes. The cone capacitance was .apprx. 85 pF [picoFaraday]. The cone membrane contains a time-dependent current, IB, controlled by voltage, and a separate photosensitive current. The gated current, IB, is an inward current with a reversal potential .apprx. -25 mV. It is activated by hyperpolarization over the range -30 to -80 mV, and at constant voltage obeys 1st order (exponential) kinetics. The gating time constant is typically 50 ms at the resting potential of -45 mV, rises to 170 ms at -70 mV and decreases for further hyperpolarization. The spectral sensitivity curve of the cone light repsonse peaks at 620 nm wave-length and is narrower than the nomogram for vitamin A2-based pigments. The light responses of isolated cones are spectrally univariant. Voltage-clamped photocurrents were recorded at various membrane potentials for light steps of various intensities. The photocurrent reversed at .apprx. -8 mV. The time course of the photocurrent, for a given intensity, was approximately independent of voltage (although its madnitude was voltage-dependent). The shape of the peak current-voltage relation of the light-sensitive current was independent of light intensity (although its magnitude was intensity-dependent). These results can be explained if light simply changes the number of photosensitive channels open, without altering the properties to an open channel, and if the reactions controlling the production of internal transmitter, the binding of internal transmitter to the photosensitive channels and the closing and opening of the channels are unaffected by the electric field in the cone membrane, even though at least some of these reactions take place in the membrane. IB plays only a small role in shaping the cone voltage response to light.