Ionic movements through light‐sensitive channels of toad rods

Electrical photoresponses of rods in the isolated toad retina were recorded during ionic manipulations of the Na+‐free extracellular medium. In the presence of a concentration of external Ca2+ above 10(‐5) M, voltage photoresponses were observed only in the presence of external Na+ or Li+. When external Ca2+ was reduced below 10(‐6) M, voltage photoresponses of normal polarity could be detected even in the absence of Na+ or Li+, but in the presence of external Mg2+. In the presence of normal extracellular Ca2+ hyperpolarizing photoresponses were observed even in the absence of Na+ or Li+, provided small amounts of phosphodiesterase inhibitors (IBMX, RO 20‐1724, papaverine, caffeine, theophylline) were added to the perfusate. Responses obtained in low‐Na+ IBMX solutions required the presence of millimolar amounts of a variety of divalent cations, among which Mn2+ and Ba2+ were the most effective. When the concentration of both external Ca2+ and Mg2+ was reduced to micromolar amounts, depolarizing photoresponses were observed. In these conditions measurements with radioactive tracers showed a light‐modulated efflux of 42K+ or 86Rb+. The light‐modulated 42K+ or 86Rb+ efflux was halved by 2 X 6 mM‐external K+ and was completely blocked when K+ was raised above 10 mM. These results show that ionic movements through light‐sensitive channels are controlled by Ca2+ and Mg2+ and possibly also be the intracellular level of cyclic nucleotides. Moreover, the movement of ions through the light‐sensitive channel, does not obey the independence principle. © 1983 The Physiological Society