Cytochemical demonstration of guanylate cyclase activity in retinal photoreceptors with special reference to changes under light and dark adaptation.

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Abstract
The localization of guanylate cyclase activity was studied cytochemically in the rod outer segments of rats light and dark-adapted for various intervals. A brown colored reaction was detected in rod outer segments under a light microscope. Under an EM, a significant change in the distribution of the reaction products under light and dark adaptation was observed. When rats were dark-adapted for > 90 min, an intense activity was found on the cytoplasmic side of the plasma membrane, and most of the disc membranes showed lower activity than in light with the exception of a few disc membranes near the pigment epithelium which showed rather intense activity. At 1 s illumination, no significant change was found in the distribution of the guanylate cyclase. The 1st significant change was detected at 5 s of illumination. After a 15 s light exposure, the reaction products of guanylate cyclase activity were observed on all the disc membranes regularly, and the activity on the plasma membrane was very weak. Apparently, light affects the guanylate cyclase activity and its distribution in the rod outer segments, suggesting that guanylate cyclase or cGMP plays an important role in the control of ion permeability in the rod outer segment plasma membrane and a possible role in the visual adaptation process.