When retinal sections were isolated from dark-adapted bullfrogs and placed in normal ringer’s solution, they contained 40.7 +/- 0.2 pmol cGMP/mg protein (mean +/- SEM, 30 samples). When isolated, dark-adapted retinal sections were removed from normal ringer’s solution and placed in calcium-deficient 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 min. 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 CaCl(2)), there was a slow but significant decrease in cGMP levels. After 20 min in high-calcium ringer’s solution the cGMP level was 0.58 +/- 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 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.
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