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Ionic movements through light‐sensitive channels of toad rods.
Author(s) -
Capovilla M,
Caretta A,
Cervetto L,
Torre V
Publication year - 1983
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1983.sp014893
Subject(s) - biophysics , ibmx , extracellular , chemistry , toad , depolarization , divalent , calcium , sodium , biochemistry , forskolin , biology , endocrinology , receptor , organic chemistry
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.