Aims: 

To investigate changes in the "dark" current noise of a rod photoreceptor cell as the absorption spectrum of the visual pigment is shifted to shorter wavelengths by switching chromophore from 11-cis-3,4-dehydroretinal (A2) to 11-cis-retinal (A1) (pigment l_max = 528 and 502 nm, respectively).

Methods: 

Dark noise, light-induced noise and responses to brief flashes of light were recorded with the suction electrode technique in the membrane current of isolated red rods from the retina of larval tiger salamander: (i) in the "native" state, with mainly A2 pigment and (ii) in the "final" state, after bleaching most of the native pigment and regenerating with A1. The proportions of A1 and A2 pigment (A1/A2 ratios) in the native and final states were determined by measurements of spectral sensitivity and absorbance. The rates of spontaneous activations of visual pigment were estimated from power spectra of dark current noise.

Results: 

The estimated rate of spontaneous pigment activations per rod changed from 0.238 ± 0.026 rod^-1s^-1 to 0.030 ± 0.006 rod^-1s^-1 (mean ± SEM), as the A1/A2 ratio changed from 0.2/0.8 (native state) to 0.9/0.1 (final state). Extrapolation to pure A1 and pure A2 pigment indicates that the A1 pigment is at least 30 times more stable than its A2 pair.

Conclusion: 

The results support the hypothesis (Barlow H.B.: Purkinje shift and retinal noise, Nature 179:255-256, 1957) that spectral tuning of visual pigments towards shorter wavelengths is coupled with a decreased rate of spontaneous (thermal) activations.