Light-dependent ion influx into toad photoreceptors.
To measure the influx of Na+ and other ions through the light-dependent permeability of photoreceptors, we superfused the isolated retina of the toad, Bufo marinus, with a low-Ca2+ (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 22Na+ accumulation than do light-adapted retinas. The extra accumulation of 22Na+ into dark-adapted retinas can be removed if the retinas are washed in darkness with low-Ca2+ Ringer's solutions, or if the ionophore gramicidin D is present in the perfusate. The additional accumulation in dark retinas corresponds to a flux of at least 10(9) Na+ per receptor per second, which is close to the value of the photoreceptor dark current. The light-dependent uptake of 22Na+ can be prevented by exposing the retinas to Ca2+ during the incubation period, but is restored if the phosphodiesterase inhibitor IBMX is added to the perfusate. A significant light-dependent ion accumulation can be observed for the cations K+, Rb+, Cs+, and Tl+, in addition to Na+, but not for methylamine, choline, or tetraethylammonium.
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000818:Animals; D002024:Bufo marinus; D002118:Calcium / Q000494:pharmacology; D002414:Cations, Monovalent; D002712:Chlorides / Q000494:pharmacology; D004553:Electric Conductivity; D005260:Female; D008027:Light; D010786:Photoreceptor Cells / Q000187:drug effects / Q000502:physiology*; D012160:Retina / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*
D002414:Cations, Monovalent; D002712:Chlorides; D012964:Sodium; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine