Light-induced changes in GTP and ATP in frog rod photoreceptors. Comparison with recovery of dark current and light sensitivity during dark adaptation.

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Abstract
Light decreases GTP and ATP levels in purified suspensions of physiologically active frog rod outer segments still attached to their inner segment ellipsoids (OS-IS). The GTP decrease is slower in OS-IS (t1/2 = 40 s) than in isolated outer segments (t1/2 = 7 s), which suggests there is more effective buffering in OS-IS. The GTP decrease becomes detectable only at intensities greater than those required to saturate the photoresponse. As the intensity of a continuous light is increased over 4 log units, GTP levels decrease linearly with log intensity by as much as 60%. GTP is reduced to steady intermediate levels during extended illumination of intermediate intensity. At levels of illumination bleaching > 0.003% of the rhodopsin, a decrease in ATP levels becomes detectable. Following a flash, GTP levels fall and then rise with a recovery time dependent on the intensity of the flash. After both 0.2 and 2% flash bleaches, the recovery of GTP levels parallels the recovery of light sensitivity, which is slower than the recovery of the dark current. This raises the possibility of a link between GTP levels and light sensitivity.