Light-induced changes of cyclic GMP content in frog retinal rod outer segments measured with rapid freezing and...

Cyclic GMP concentration was measured in the rod outer segments (ROS) of the isolated frog retinas. Retinas were quickly frozen in 0.5 s after the short light flash producing 90%-saturated late receptor potential (2,000 rhodopsins bleached per rod). ROS were obtained by microdissection, the cGMP levels were determined by radioimmunoassay method. No detectable changes in cGMP concentration was found in this stimulus condition. Dark-adapted ROS contained 46.3 +/- 2 pmol/mg. 3-s bright illumination (ca. 10(7) rhodopsins bleached per rod per second) led to approximately 30% drop in cGMP content. It is supposed that the main part of cGMP with the ROS is in the bound state and therefore fast light-induced changes in its minor free fraction may escape the detection.