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P. Ala-Laurila, K. Donner, R. K. Crouch, M. C. Cornwall; Chromophore Switch From 11-cis-dehydroretinal (A2) to 11-cis-retinal (A1) Decreases the Rate of Spontaneous Activations of Salamander Red Rod Visual Pigment. Invest. Ophthalmol. Vis. Sci. 2007;48(13):609.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate changes in the dark noise and the adaptation state of a rod photoreceptor, as the chromophore is switched from 11-cis-3,4-dehydroretinal (A2) to 11-cis-retinal (A1) in the same rod.
Dark noise, light-induced noise and responses to brief flashes of light were recorded by suction electrode in the membrane current of red rods isolated from the dark-adapted retinae of larval tiger salamanders (Ambystoma tigrinum). Current recordings were performed in the native state containing primarily A2 pigment and in the final state after bleaching (>90%) the native pigment and regenerating with 11-cis A1. Spectral sensitivity and absorbance measurements were carried out to characterize the A1/A2 pigment ratio in the native and final states. Power spectra of dark current traces were constructed to estimate the rates of spontaneous isomerizations of visual pigments.
The frequency of spontaneous isomerizations, as estimated from the low-frequency component of power spectra, changed from 0.238 ± 0.026 rod-1s-1 to 0.030 ± 0.006 rod-1s-1 (mean ± SEM), as the A1/A2 pigment ratio changed from 0.2/0.8 (native state) to 0.9/0.1 (final state). No change in the continuous component of dark noise was seen. Furthermore, the mean amplitude and the integration time of single photon response were increased in final state.
The observed 8-fold decrease in the rate of spontaneous activations between the native and final states shows that A1 chromophore forms a much more stable pigment with the red rod opsin than A2 chromophore. This observation is in line with a theory according to which spectral tuning towards shorter wavelengths is coupled with a decreased rate of spontaneous activations. Furthermore, the larger and slower single photon response in the final state is consistent with the idea that spontaneous activations that occur at a rate of nearly one per integration time in the native state can light-adapt rods.
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