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Although there is good agreement that light reduces the amount of cyclic GMP (cGMP) in the retina, the exact time-course of this decrease is not well established. Bullfrog retinal sections were isolated under infrared light and quick-frozen with liquid nitrogen-cooled, metal hammers after exposure to various intensities of continuous illumination. This quick-freezing should stop the degradation of cGMP within 50-100 ms. The frozen retinal sections were then slowly warmed up in the presence of perchloric acid to denature enzymes involved in cGMP metabolism, cGMP was determined by radioimmunoassay and comparison was made between light-and dark-adapted retinal sections from the same animal. The average cGMP concentration was 44.3:1:0.7 pmol cGMP/mg protein or 170.9:1:3.2 pmol cGMP/retina. After 1 s of illumination no significant change in cGMP concentration was found even with the brightest light used (~ 7 • 10 7 rhodopsins bleached/second per rod. At this intensity the first significant decrease in cGMP from dark-adapted levels was detected 3-5 s after the initiation of illumination; cGMP decayed to 70-75% of the darkadapted value after ~ 30 s. With lower intensity illumination the eGMP levels recovered to dark-adapted levels after the initial decrease even though the bleaching light remained on.

No abstract

A BSTRAC T When retinal sections were isolated from dark-adapted bullfrogs and placed in normal Ringer's solution, they contained 40.7 :t: 0.2 pmol cGMP/ mg protein (mean :t: SEM, 30 samples). When isolated, dark-adapted retinal sections were removed from normal Ringer's solution and placed in calciumdeficient Ringer's solution with 3 mM EGTA, there was about a threefold rise in cyclic GMP (cGMP) levels by 1.5 min and about a 10-fold rise by 5 rain. The cGMP level remained high with no detectable decrease for at least 40 min (the longest time measured). When isolated, dark-adapted retinal sections were removed from normal Ringer's solution and placed in Ringer's solution which contained high-calcium (20 mM CaCI2), there was a slow but significant decrease in cGMP levels. After 20 rain in high-calcium Ringer's solution the cGMP level was 0.58 :t: 0.07 (mean + SEM, eight samples) of the cGMP level in normal Ringer's solution incubated for the same time. The rate at which 10-fold elevated cGMP levels in low calcium decreased upon illumination was examined using quick-freezing techniques on the retinal sections. The elevated cGMP level in retinal sections incubated in low-calcium Ringer's solution was found to decay about 15-fold faster than cGMP levels in retinal sections incubated in normal Ringer's solution. The cGMP level in low calcium was significantly different (P ---0.005) after 1 s illumination, whereas the cGMP level in normal calcium was not significantly different.

No abstract