Retinal rods and cones respond to light with a membrane hyperpolarization. This hyperpolarization is mediated by an ionic conductance (the light-regulated conductance) that is kept open in darkness by cyclic GMP acting as a ligand, and which closes in the light as a result of an increase in cGMP hydrolysis triggered by illumination. Calcium ions appear to have a role in this phototransduction process: they provide negative feedback between the conductance, which is permeable to Ca2+ (refs 4, 5), and the concentration of cGMP, which is sensitive to Ca2+ (refs 6-8). This feedback down-regulates the sensitivity to light of a photoreceptor and probably contributes to the important phenomenon of light adaptation in vision. It is still not clear, however, how much of the light adaptation is actually attributable to this Ca2+ feedback. We have examined the responses of amphibian rods and cones to light with the Ca2+ feedback removed. Normally, the response of a cell to a step of light rises transiently to a peak, but rapidly relaxes to a lower level, indicative of light adaptation. When the feedback is removed, however, the relaxation of the response is completely absent; furthermore, the steady response levels at different light-step intensities are well predicted by a statistical superposition of invariant single-photon responses. We therefore conclude that the Ca2+ feedback underlies essentially all light adaptation in rods and cones.

Previous work has suggested that a Na-Ca exchanger may have a key role in visual transduction in retinal rods. This exchanger is thought to maintain a low internal free Ca2+ concentration in darkness and to contribute to the rod's recovery after light by removing any internally released Ca2+. Little else is known about this transport mechanism in rods. We describe here an inward membrane current recorded from single isolated rods which appears to be associated with such external Na+-dependent Ca2+ efflux activity. External Na+, but not Li+, could generate this current; high external K+ inhibited it while small amounts of La3+ (10 microM) completely abolished it. The exchanger can also transport Sr2+, but not Ba2+ or other divalent cations. The exchange ratio was estimated to be 3Na+:1Ca2+. As well as demonstrating clearly the Na-Ca exchanger in the rod outer segment, our experiments also cast serious doubt on the commonly held view that light simply releases internal Ca2+ to bind to and block the light-sensitive conductance.

The response of retinal rod photoreceptors to light consists of a membrane hyperpolarization resulting from the decrease of a light-sensitive conductance in the outer segment. According to the calcium hypothesis, this conductance is blocked by a rise in intracellular free Ca triggered by light, a notion supported by the findings that an induced rise in internal Ca leads to blockage of the light-sensitive conductance and that light triggers a net Ca efflux from the outer segment via a Na-Ca exchanger, suggesting a rise in internal free Ca in the light. We have now measured both Ca influx and efflux through the outer segment plasma membrane and find that, contrary to the calcium hypothesis, light seems to decrease rather than increase the free Ca concentration in the rod outer segment. This result implies that Ca does not mediate visual excitation but it probably has a role in light adaptation.

1. Membrane current was recorded from an isolated, dark‐adapted toad rod by sucking either its inner segment or outer segment into a tight‐fitting glass pipette containing Ringer solution. The remainder of the cell was exposed to bath solution which could be changed rapidly. 2. In normal Ringer solution the current response of a cell to a saturating flash or step of light showed a small secondary rise at its initial peak. The profile of this secondary rise (i.e. amplitude and time course) was independent of both the intensity and the duration of illumination once the light response had reached a plateau level. 3. This secondary rise disappeared when external Na+ around the outer segment was replaced by Li+ or guanidinium, suggesting that it represented an electrogenic Na+‐dependent Ca2+ efflux which was declining after the onset of light. 4. This Na+‐Ca2+ exchange activity showed a roughly exponential decline, with a time constant of about 0.5 s. Exponential extrapolation of the exchange current to the time at half‐height of the light response gave an initial amplitude of about 2 pA. Using La3+ as a blocker, we did not detect any steady exchange current after the initial exponential decline. 5. An intense flash superposed on a just‐saturating steady background light failed to produce any incremental exchange current transient. 6. Our interpretation of the above results is that in darkness there are counterbalancing levels of Ca2+ influx (through the light‐sensitive conductance) and efflux (through the Na+‐Ca2+ exchange) across the plasma membrane of the rod outer segment. The exchange current transient at the onset of light merely represents the unidirectional Ca2+ efflux which becomes revealed as a result of the stoppage of the Ca2+ influx, rather than a de novo Ca2+ efflux triggered by light. 7. Consistent with this interpretation, a test light delivered soon after a saturating, conditioning light elicited little exchange current, which then gradually recovered to control value with a time course parallel to the restoration of the dark current. Conversely, when the dark current was increased above its physiological level by IBMX (isobutylmethylxanthine) the exchange current transient became larger than control.(ABSTRACT TRUNCATED AT 400 WORDS)