[en] Using isolated rod outer segment discs from bovine retina the authors have studied the effect of pH on the release of ⁴⁵Ca. ⁴⁵Ca was loaded into the discs using the ATP-dependent calcium uptake activity of the disc membrane. Measurements were made at 1 or 5 min. 250 μM cGMP released 12.9 +/- 7% of the ⁴⁵Ca over the pH range 6.9-7.9 and 3.1 +/- 2.8% at pH 6.4 (⁴⁵Ca is only a portion of the total disc calcium; actual release is greater). 25 μM 8-BrcGMP (more potent than cGMP at releasing disc calcium) released 10.6 +/- 2.0% of the ⁴⁵Ca over the 6.9-7.9 pH range and 1.4 +/- 1.5% at pH 6.4. 250 μM cAMP released 1.0 +/- 1.3% of the ⁴⁵Ca at pH 6.9. The external calcium level was submicromolar in the above experiments. When the external calcium was micromolar or above there was a release of ⁴⁵Ca from discs incubated at pH 7.9 as compared to those at pH 6.9 of 11.2 +/- 1.9%. The release was apparently due to pH alone as no cyclic nucleotide was present in the incubation buffers. This release at pH 7.9 was not seen when the external calcium was submicromolar. This data indicates that the ⁴⁵Ca release being studied is specific to cGMP (as cAMP at the same conc. caused minimal release) and that the ability of cGMP to release calcium from ROS discs is attenuated at pH 6.4 as compared to the release at pH 6.9-7.9 (submicromolar external free calcium). It also indicates that high pH alone can cause calcium release from discs when the external free calcium level is micromolar or above