AMPLITUDE, KINETICS, AND REVERSIBILITY OF A LIGHT-INDUCED DECREASE IN GUANOSINE 3',5'-CYCLIC-MONOPHOSPHATE IN FROG PHOTORECEPTOR-MEMBRANES

The concentration of guanosine 3′,5′-cyclic monophosphate (cyclic GMP) has been examined in suspensions of freshly isolated frog rod outer segments using conditions which previously have been shown to maintain the ability of outer segments to perform a light-induced permeability change (presence of calf serum, anti-oxidant, and low calcium concentration). Illumination causes a rapid decrease in cyclic GMP levels which has a half-time ~125 ms. With light exposures that bleach less than 100 rhodopsin molecules in each rod outer segment, at least 104-105 molecules of cyclic GMP are hydrolyzed for each rhodopsin molecule bleached. Half of the total cyclic GMP in each outer segment, ~2 × 107 molecules, is contained in the light-sensitive pool. If outer segments are exposed to continuous illumination, using intensities which bleach between 5.0 × 101 and 5.0 × 104 rhodopsin molecules/outer segment per second, cyclic GMP levels fall to a value characteristic for the intensity used. This suggests that a balance between synthesis and degradation of cyclic GMP is established. This constant level appears to be regulated by the rate of bleaching rhodopsin molecules (by the intensity of illumination), not the absolute number of rhodopsin molecules bleached. After brief exposure to light of varying intensities, cyclic GMP concentration returns to near the value observed in unilluminated outer segments within 30-60 s. The recovery is most rapid after dim illumination. Decreases in cyclic GMP levels induced by light superimposed on background illumination are reduced by an amount proportional to the intensity of the background light. Background light appears to have no effect on the sensitivity of the light-dependent cyclic GMP decrease. Calcium ions lower the level of cyclic GMP without influencing either the final level to which cyclic GMP is reduced by illumination or the light sensitivity of the reduction. These data, together with those of a previous paper presenting several correlations between cyclic GMP levels and light suppressible ionic permeability of isolated frog rod outer segments (Woodruff et al., 1977. J. Gen. Physiol. 69: 667-679), suggest that cyclic GMP is involved in visual transduction, perhaps mediating between photon absorption and the permeability decrease. © 1979, Rockefeller University Press., All rights reserved.