Cyclic GMP, calcium and photoreceptor sensitivity in mice heterozygous for the rod dysplasia gene designated "rd".

The rise in photoreceptor cGMP, induced by less than 1.0 nM extracellular calcium, is delayed in retinas of mice heterozygous for the rod dysplasia gene (+/rd). The calcium ionophore A23187 reduces the delay, suggesting that +/rd outer segments contain more calcium than normal. In turn, this might explain the increased photosensitivity of the +/rd retina. During the response to low calcium there is no correlation in +/rd retinas between the total concentration of cGMP and the photoresponse amplitude and its time to peak. The observations imply that either free cGMP is abnormally independent of the bound pool in the +/rd photoreceptor outer segment or that factors other than cGMP and its phosphodiesterase are modulating the rising phase of the response. The time-to-peak of PIII in a +/rd retina, incubated in a standard medium and stimulated with dim light, is abnormally delayed. Reduction of extracellular calcium induces an abnormal delay as well in responses to higher light levels. In addition to this, a second delay manifests slowly in both the normal and the +/rd retina. More studies are needed to explain these observations.