Cyclic GMP and the permeability of the disks of the frog photoreceptors.

1. The diffusion of sodium, potassium and rubidium (not chloride) ions across the disk membrane is increased by cyclic guanosine monophosphate (cyclic GMP). 2. The increase is greater for sodium than for rubidium in the 0.01-0.1 mM concentration range. 3. Cyclic adenosine monophosphate (cyclic AMP) is less efficient than cyclic GMP; GMP and guanosine triphosphate are without effect. 4. The effect is present with either 1.8 mM calcium ions or 4 mM-EGTA in the perfusion fluid. 5. The presence of the cyclic GMP phosphodiesterase on the disk membranes is not needed for this effect. 6. The effect is present in both unbleached and fully bleached membranes.