Ca2+-dependent changes in cyclic GMP levels are not correlated with opening and closing of the light-dependent permeability of toad photoreceptors.

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Abstract
The levels of cGMP in isolated retinas from the toad were measured to investigate their correlation to the opening and closing of the light-dependent permeability of photoreceptors. When Ca2+-induced changes in cGMP concentration are compared with the Ca2+-induced changes in the permeability of photoreceptor light-dependent channels, 4 quantitative dissimilarities are noted. 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 cGMP levels are not significantly changed. When [Ca2+]o is increased from 1.8 to 20 mM, the light-dependent permeability is suppressed, but cGMP levels are decreased by only 10-15%, .apprx. 1/4 the decrease that can be obtained with bright illumination. When [Ca2+]o is increased from 10-8 M to 20 mM, the light-dependent permeability is closed rapidly, but the cGMP decrease is slow. When [Ca+2+]o is lowered to 10-8 M, the sensitivity of the light-dependent permeability to steady illumination is decreased by 3-4 orders of magnitude, but the sensitivity of the light-dependent decrease in cGMP is not significantly affected. Apparently, there is no simple correlation between cGMP levels and the permeability of the light-dependent channels, and Ca2+ can affect the conductance in the absence of changes in cGMP content.
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