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.
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000818:Animals; D002023:Bufo bufo; D002118:Calcium / Q000378:metabolism; D002412:Cations / Q000378:metabolism*; D007473:Ion Channels / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D010775:Photic Stimulation; D010786:Photoreceptor Cells / Q000378:metabolism / Q000502:physiology*; D011188:Potassium / Q000378:metabolism; D012413:Rubidium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism
D002412:Cations; D007473:Ion Channels; D012964:Sodium; D008274:Magnesium; D012413:Rubidium; D011188:Potassium; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine