Electrical properties of the light-sensitive conductance of rods of the salamander Ambystoma tigrinum

Abstract

The light-sensitive conductance of isolated rods from the retina of the tiger salamander was studied using a voltage-clamp method. The membrane current of the outer segment was collected with a suction electrode while the internal voltage was measured and controlled with a pair of intracellular electrodes. Saturating light blocked the outer segment current at all potentials, the residual conductance usually becoming less than 20 pS. This suggests that light-sensitive channels comprise the main ionic conductance in the surface membrane of the outer segment. Current-voltage relations determined 10-40 ms after changing the voltage showed outward-going rectification, the outward current increasing e-fold for a depolarization of 11-14 mV. The reversal potential of the light-sensitive current was estimated as 5 +/- 4 mV. This is consistent with other evidence indicating that the channel is not exclusively permeable to Na. Applying steady light, lowering external Ca, or changing the intracellular voltage to a new steady level scaled the light-sensitive current without altering the reversal potential or the form of the rectification. This suggests that all three manipulations change the number of channels in the conducting state without changing the ionic concentration gradients or the mechanism of permeation through an 'open' channel. Hyperpolarizing voltage steps slowly increased the light-sensitive current and depolarizing steps reduced it. A gating variable Y expressing the fractional activation of the light-sensitive conductance in the steady state was derived from the ratio of the instantaneous and steady-state currents. Y declined at voltages positive to -100 mV and usually reached a minimum near 0 mV, with a secondary rise positive to 0 mV. Around the dark voltage Y changed e-fold in roughly 25 mV. The voltage-dependent gating in (6). appeared to involve two delays similar in magnitude to those of the four principal delays in the rod's response to a dim flash. Steady background light shortened the time-scale of gating and flash responses to a similar degree. Clamping the voltage at the dark level had little effect on the photocurrent evoked by a flash. The small, delayed effect actually observed is explained by the slow voltage-dependent gating of the light-sensitive conductance. Hyperpolarization had little effect on the kinetics of the response to a flash, but depolarization slowed the response, causing it to reach a larger, later peak. Depolarization also prolonged the blockage of the light-sensitive current after a saturating flash.(ABSTRACT TRUNCATED AT 400 WORDS)