No abstract

SUMMARY1. The mechanism of light adaptation was investigated by recording intracellularly from single rods in the isolated, superfused retina of the toad, Bufo marines. Steady background lights produce decreases in rod sensitivity and changes in response wave form similar to those previously observed in the toad eyecup.2. The sensitivity of a dark-adapted rod is halved by a background light which bleaches about 4 rhodopsins per rod per second. Since a toad rod contains over 2000 disks, a rhodopsin bleached in one disk must alter the effectiveness of rhodopsins bleached in others. This could occur if the state of adaptation in the rod were regulated by the concentration of some diffusable substance.3. This diffusable substance cannot be Ca2+. Increases in intracellular Ca2+, produced experimentally either by increasing extracellular Ca2+ or by facilitating Ca2+ permeability into the rod with the ionophore X537A, cause a hyperpolarization of membrane potential and a decrease in response amplitude; but they do not produce changes in sensitivity and response wave form like those produced by background light.4. Either Ca2+ is not the internal transmitter released from the disks during excitation, or the disks release or otherwise alter the concentration of a second diffusable substance, in addition to Ca2 , which regulates the state of adaptation.

1. We have examined the effects of decreases in extracellular Ca2+ concentration on the intracellularly recorded light responses of rods from the toad, Bufo marinus. In agreement with previous results (Brown & Pinto, 1974; Lipton, Ostroy & Dowling, 1977), Ca2+ concentrations below 10−6 M produced a depolarization of rod resting membrane potential of approximately 30‐40 mV and a corresponding increase in the maximum amplitude of the rod's light responses, so that saturating flashes in normal and low Ca2+ Ringer produced hyperpolarizations to approximately the same membrane potential. 2. The rod's sensitivity was reduced in low Ca2+ Ringer by an amount dependent upon the extracellular Ca2+ concentration. At 10−6 M‐Ca2+, sensitivity was approximately 0·6 log units below normal. Thereafter, it dropped nearly linearly with [Ca2+]o to a value approximately 4·0 log units below normal at 10−9 M‐Ca2+. Most of the decline occurred within 1‐2 min after the solution change as the membrane potential depolarized, but sensitivity continued to fall slowly with time at the lowest Ca2+ concentrations. Exposure to low Ca2+ solutions altered the kinetics of the receptor response to brief flashes, delaying response onset and time‐to‐peak but affecting the time course of decay very little. 3. The sensitivity of the rod to maintained steps of light was also reduced in low Ca2+. Furthermore, the changes in sensitivity produced by background illumination were very much smaller in low Ca2+ than in normal Ringer. In some cases backgrounds actually increased sensitivity. 4. In 10−8 M‐Ca2+, backgrounds which themselves produced no response in the rod and no changes in rod sensitivity produced large decreases in response latency for responses of all amplitudes, and pronounced changes in time‐to‐peak and time‐to‐decay for moderate and large amplitude responses. 5. Since the effects of background light and low Ca2+ on the wave form of the rod are distinct and in some cases antagonistic, and since the changes in receptor sensitivity produced by backgrounds and low Ca2+ are not additive, the decreases in sensitivity produced by exposure to low Ca2+ appear to be caused by a mechanism distinct from normal light adaptation. We suggest that they are caused by an increase in the buffering capacity of the receptor cytosol for Ca2+ and that Ca2+ is the excitatory messenger or ‘internal transmitter’, as originally suggested by Yoshikami & Hagins (1971).

SUMMARY1. We have investigated the effects of Na+ substitution on the membrane potential and light responses of rods in the superfused retina of the toad, Bufo marinus.2. When all of the Na+ in the Ringer was replaced with Li+, the effects on the rods depended upon the external free Ca2+ concentration ([Ca2+] 5. Substitution of Na+ with K+ in low Ca2+ produced a complete suppression of the responses. However, it was still possible to measure large light-induced changes in rod input resistance. 6. Substitution of Na+ with tetramethylammonium, tetraethylammonium, Tris, or choline in low Ca2+ produced a large hyperpolarization of the membrane potential and a diminution of response amplitude. However, we were unable to observe a complete suppression of the responses for these cations.7. Substitution ofNa+ with tetrapropylammonium or with an uncharged substance (glucose or urea) in low Ca2+ produced a large hyperpolarization of membrane potential and a considerable decrease in the light responses. In about half our attempts, the responses were observed to decline reversibly to less than 20 % of their peak amplitude in Na+.8. Results with tetrapropylammonium were indistinguishable from those of glucose or urea, indicating that the light-dependent conductance probably is not permeable to TPA. The resistance changes measured with K+ substitution and the responses observed in the presence of the organic ions TMA, TEA, Tris and choline suggest that these species may be permeable, but we are unable to discount alternative explanations.

To measure the influx of Na' and other ions through the lightdependent permeability of photoreceptors, we superfused the isolated retina of the toad, Bufo marinus, with a low-Ca 2+ (10-8 M), low-Cl-Ringer's solution containing 0.5 mM ouabain . Under these conditions, the membrane potential of the rod is near zero and there is no light-induced potential change either in the rod or in more proximal neurons . The photoreceptors, however, continue to show a light-dependent increase in membrane resistance, which indicates that the light-sensitive channels still close with illumination . Dark-adapted retinas show a larger 22