Cyclic GMP releases calcium from disc membranes of vertebrate photoreceptors.

Physiological concentrations of cyclic guanosine 3',6'-monophosphate (cGMP) inhibit 45Ca uptake and increase 45Ca release from vertebrate photoreceptor rod outer segment disc membranes. These effects are specific for cGMP. Several facts, including the independence of these effects from added triphosphates, suggest that cGMP diminishes the Ca-binding capacity of the disc membranes. Preliminary data show that the apparent affinity constant of the cGMP-dependent Ca-binding sites of the disc membranes is of the same (or even higher) order of magnitude as that of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. As expected, the observed cGMP effects are not dependent on the light or dark conditions of the disc membranes.