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S. Nymark, H. Heikkinen, K. Donner, A. Koskelainen; The Specific Absorbance of Rat Rods . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2980.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To measure the specific absorbance of rat rhodopsin rods. Current estimates (0.01 µm–1, Penn & Hagins (1972), Biophys. J. 12, 1073–94; 0.0085 µm–1, Robinson et al. (1993), J. Physiol. 462, 465–81) appear as unrealistically low compared with values for both rods and cones of many other vertebrate species, including humans. Methods: The specific absorbance of the rod outer segment (ROS) was deduced from measurements of light transmission by whole, isolated retinas. The retina was flat–mounted on a transparent circular area (diam. 3 mm) at the bottom of a glass chamber. The chamber was filled with Ringer’s solution including 50 mM hydroxylamine. The test light was a 501 nm unpolarized full–field exposure perpendicular to the retinal plane (bleaching ca. 0.02 % of the visual pigment). The intensity of transmitted light was measured with a calibrated photodiode before and after a total bleach (a full–field exposure estimated to bleach ca. 99.7% of the visual pigment). Results: Measurements from four retinas (out of 16) were accepted on the basis of morphological integrity. The percentage of 501 nm light absorbed by the visual pigment in the dark–adaped retina was 50.2 ± 0.8 (mean ± SEM, n = 4). Considering the retina as a homogeneous absorbing layer 20 µm thick (the length of the ROS), we obtain for the specific absorbance of this layer 0.0151 ± 0.0004 µm–1. Assuming hexagonal packing of photoreceptors (rod diameter 1.7 µm) and neglecting the very sparse population of cones, the estimate of ROS specific absorbance becomes 0.0167 ± 0.0004 µm–1. Conclusions: Our estimate 0.0167 µm–1 for the specific absorbance of rat rods is considerably higher than previously reported and consistent with estimates from other vertebrate photoreceptors. With increasing measurement accuracy, these tend to converge on values in the range 0.012 – 0.018 µm–1.
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