Study of the ionic basis of visual transduction in vertebrate retinal rods.

The experiments described here have clarified some issues about visual transduction in rods. Thus, for instance, the long puzzling reversal potential for the light response is now readily explained by the ability of both Na and K to go through the light-sensitive conductance. The experiments also underscored the dual action of Na in transduction, namely, its roles as a current carrier and as a keeper for the open state of the conductance. The rather low ionic selectivity of the conductance in itself is also interesting and surprising, especially in view of some recent evidence that its unit conductance may be as much as a thousand times less than those of other conductances (Detwiler et al. 1982). The movement of Ca through the light-sensitive conductance and its regulation by the Na-Ca exchanger as described earlier should also be taken into account for any understanding of the role played by Ca in transduction. There is little doubt now that intracellular Ca somehow contributes to the control of the open and closed states of the light-sensitive conductance, but additional experiments are still necessary to find out exactly where and how it acts. Fig. 1 summarizes some of the findings described here.