Ca2+-dependent changes in cyclic GMP levels are not correlated with opening and closing of the light-dependent permeability of toad photoreceptors.
We have measured the levels of 3',5'-guanosine monophosphate (cyclic GMP) in isolated retinas from toad to investigate their correlation to the opening and closing of the light-dependent permeability of photoreceptors. When Ca2+-induced changes in cyclic GMP concentration are compared with the Ca2+-induced changes in the permeability of photoreceptor light-dependent channel, four quantitative dissimilarities are noted. First, when extracellular Ca2+ ([Ca2+]o) is reduced from normal physiological levels to between 10(-6) and 10(-7) M, the light-dependent permeability is increased, but cyclic GMP levels are not significantly changed. Second, when [Ca2+]o is increased from 1.8 to 20 mM, the light-dependent permeability is suppressed, but cyclic GMP levels are decreased by only 10-15%, about one-quarter the decrease that can be obtained with bright illumination. Third, when [Ca2+]o is increased from 10(-8) M to 20 mM, the light-dependent permeability is closed rapidly, but the cyclic GMP decrease is slow. Fourth, when [Ca2+]o is lowered to 10(-8) M, the sensitivity of the light-dependent permeability to steady illumination is decreased by three to four orders of magnitude, but the sensitivity of the light-dependent decrease in cyclic GMP is not significantly affected. These observations indicate that there is no simple correlation between cyclic GMP levels and the permeability of the light-dependent channels and that Ca2+ can affect the conductance in the absence of changes in cyclic GMP content.
D000818:Animals; D002024:Bufo marinus; D002118:Calcium / Q000494:pharmacology*; D006152:Cyclic GMP / Q000378:metabolism*; D007700:Kinetics; D008027:Light; D010539:Permeability; D010786:Photoreceptor Cells / Q000502:physiology* / Q000528:radiation effects
D006152:Cyclic GMP; D002118:Calcium