Magnitude of Increase in Retinal CGMP Metabolic Flux Determined by 18O Incorporation into Nucleotide Alpha-phosphoryls Corresponds with Intensity of Photic Stimulation

The property of cyclic nucleotide phosphodiesterases to catalyze 3'-P--O bond cleavage and the insertion of a single nonexchangeable atom of 18O from [18O]water into the phosphoryl of the 5'-nucleotide product has been utilized as a means for measuring the hydrolytic flux of cGMP and cAMP in isolated dark-adapted intact rabbit retinas. Without illumination 18O labeling of guanine nucleotide (GTP and GDP) alpha-phosphoryls proceeds linearly for at least 80 s at a rate of 3.3 nmol of 18O/s.g of retina (wet weight). This rate is estimated to be approximately 8 times greater in the rod outer segment layer where over 90% of retinal cGMP metabolic components reside. Photic stimulation during a 20-s incubation was provided by intermittent flashes of light representing 800 ms of total illumination. Light stimuli over a range of intensities of greater than 3 log units commencing with a minimally detectable intensity produce graded increments in the rate of 18O incorporation into guanine nucleotide alpha-phosphoryls to a maximum increase of 5-fold. On the basis of only the 800-ms period of illumination this maximum increase is 125-fold. Steady state levels of retinal cGMP are not altered appreciably over this greater than 3 log range of light intensities but a light stimulus exceeding this intensity range causes an approximate 50% decrease in retinal cGMP concentration and a relative decline in the maximal rate of 18O labeling of guanine nucleotide alpha-phosphoryls. No light-related increases were detected in 18O incorporation into adenine nucleotide alpha-phosphoryls nor the gamma-phosphoryls of GTP or ATP or Pi. These observations indicate that light stimuli over greater than 3 log of light intensity produce incremental increases in cGMP metabolic flux that result from comparable increases in the rates of both cGMP generation and cGMP hydrolysis. It is postulated that increases in cGMP metabolic flux rather than changes in cGMP steady state levels are integral to phototransduction by a mechanism that involves the coupling of cGMP synthesis and/or hydrolysis to either the release of calcium from disc membranes or the inhibition of Na+ conductance by the photoreceptor membrane. This is suggested to occur by an energy-linked process and/or the generation of protons.