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

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Abstract
The diffusion of Na, K and Rb (not Cl-) ions across the disk membrane is increased by cyclic [c]GMP. The increase is greater for Na than for Rb in the 0.01-0.1 mM concentration range. Cyclic AMP is less efficient than cGMP; GMP and GTP are without effect. The effect is present with 1.8 mM Ca ions or 4 mM-EGTA [ethylene glycol-bis-(.beta.-aminoethyl ether) N,N,N'',N''-tetraacetic acid] in the perfusion fluid. The presence of the cGMP phosphodiesterase on the disc membranes is not needed for this effect. The effect is present in unbleached and fully bleached membranes.