When retinas from dark-adapted C57BL/6 mice were incubated in the dark for 5 rain at 37~ in Earle's medium, they contained 80-120 pmol/mg protein of cGMP and about 13 pmol/mg protein of cAMP. When the incubation in darkness was in calcium-deficient Earle's medium with 3 mM EGTA, a 10-20-fold increase occurred in the cGMP level, peaking at 2-3 min, but no change occurred in cAMP. This elevated level fell in 3 min to normal dark levels on return to normal Earle's medium, but was still about three times that of control levels after 15 rain in EGTA-containing solution. Bright light after 2 min of dark incubation of darkadapted retinas resulted in a 40-50% fall in cGMP, and bright light sharply reduced the elevated dark cGMP level of retinas in calcium-deficient media with 3 mM EDTA. However, no depression of normal dark levels of cGMP has thus far been obtained by increasing external calcium levels, even in the presence of the ionophore A23187. All the above phenomena involving dark cGMP levels and calcium are similar in Earle's medium with 100 mM of K + substituted for Na +. Congenic rodless (rd/rd) mouse retinas have <5% of control cGMP and show only traces of calcium sensitivity. Thus, the above phenomena in controls are likely to be largely occurring in rods. The data suggest a dependency of the dark cGMP level on the calcium level, but that the light-induced fall in cGMP may largely be calcium insensitive.

Abstract— The content of cyclic AMP and cyclic GMP was measured in whole eyes and in normal retinas from C57BL(6)J mice, in receptorless retinas from congenic mice homozygous for the receptor dystrophy gene (rd/rd), and in retinas from mice treated postnatally with monosodium glutamate. Normal retinas contain approx 320 μg of protein: dystrophic (rd/rd) retinas contain approx 110μg of protein, lack rods but possess some surviving cone somata and terminals: glutamate‐modified retinas contain approx 200 μg of protein and have both a reduced area and thickness with a marked deficiency of ganglion cells and amacrine cells. In normal mice, more than 90% of the cyclic GMP, but only 607, of the cyclic AMP of the whole eye was in the retina. In normal dark‐adapted retinas isolated under dim red light cyclic AMP and cyclic GMP content was 4.1 and 20.2pmol/retina, respectively. The content of both cyclic AMP and cyclic GMP was 40% less, 2.5 and 11.5pmol/retina, respectively, in light‐adapted retinas. In dark‐adapted retinas isolated under infra‐red light, cyclic AMP content was 40%, higher than that in retinas isolated under dim red light; cyclic GMP content was the same under these two conditions. Receptorless retinas contained approx 50% as much cyclic AMP and only 1‐2% as much cyclic GMP as normal retinas. Although glutamate‐modified retinas also had approx 50% as much cyclic AMP, they contained 60‐85%, as much cyclic GMP as normal retinas. Light decreased by 30‐50% levels of both cyclic AMP and cyclic GMP in glutamate‐modified retinas, but only reduced cyclic nucleotide levels in receptorless retinas by 20%. These data indicate that 95% or more of the cyclic GMP is in photoreceptor cells, whereas cyclic AMP is more evenly distributed throughout the retina. In addition, both cyclic AMP and cyclic GMP levels are influenced by light‐ and dark‐adaptation.

Guanylate cyclase activity is present in both soluble and particulate fractions of homogenates of mouse cerebellum and retina. Soluble guanylate cyclases in cerebellum and retina have an apparent Km for GTP of approx 40 and 70 μM, respectively; are stimulated by Ca2+ and Mg2+ in the presence of low Mn2+; and do not respond to NaN3, NH2OH or detergent. The particulate guanylate cyclase found in brain has an apparent Km GTP of 237 7mu;M, is not stimulated by Ca2+ or Mg2+ in the presence of low Mn2+, but is stimulated by NaN3, NH2OH, and detergent. In particulate fractions of normal retina, guanylate cyclase has two apparent Km GTP values (42 and 225 μM); has higher activity at low concentrations of Mn2+ (0.5 mM) than at high concentrations (5.0 mM); is inhibited by Ca2+; and does not respond to NaN3, NH2OH, or detergent. Retinas essentially devoid of photoreceptor cells (from mice with photoreceptor dystrophy) have soluble guanylate cyclase activity which is similar to that in normal retina, but have only 4% as much particulate guanylate cyclase activity. This residual particulate guanylate cyclase has an apparent Km GTP value of 392 μM and other properties similar to particulate guanylate cyclase from brain. These data indicate the presence of three distinguishable guanylate cyclases in CNS: (1) a soluble enzyme present in both brain and retina: (2) a particulate enzyme which is also present in brain and in the inner or neural retina: and (3) another particulate enzyme which is apparently unique and confined to retinal photoreceptor cells.

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