Light - dependent Ion Influx into Toad Photoreceptors

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-Ca2+ (10-8 M), low-ClRinger'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 Na' 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 109 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 .