Effect of eye closures and openings on photostasis in albino rats.

PURPOSE: To determine the effects of eye closure and opening on photostasis, the regulation of light absorption by retinal rods in the albino rat.
METHODS: The approach was to measure the effect of eye closure and opening on rhodopsin bleaching in situ and to use those results to simulate what happens to rhodopsin when a living rat opens or closes its eyes during daylight exposure. Completely dark-adapted, dead albino rats, each with one eye closed or open, were exposed to a standard lighting situation. The rhodopsin bleaching rate in closed versus open eyes was measured. Rhodopsin bleached at a more reduced rate in closed eyes than in open eyes. This measured reduction of rate in closed eyes was applied to a simulation of rhodopsin bleaching in open and closed eyes. The simulation used idealized conditions to verify the simulation itself, and then it was applied to previously published photostasis results.
RESULTS: Rhodopsin in closed eyes bleaches at half the rate found in open eyes. The absorption spectrum of rat red blood cells was compared with the rate rhodopsin absorption spectrum, and the comparison showed that blood does not absorb the main-band wavelengths of rhodopsin. Simulating rhodopsin bleaching with eyes closed (half intensity) and open (full intensity) during daylight hours showed a slight effect on the total number of photons absorbed in an entire day. The simulation set limits to the maximal effect of eyes open all day versus eyes closed all day. At a habitat intensity of 200 lux, for example this maximal effect (eyes always open versus always closed) was calculated to be +/- 9%. At the lowest intensity, 3 lux, this maximal effect was +/- 28%, but it is only 1% at the highest intensity, 400 lux.
CONCLUSIONS: Eye closures and openings have a slight effect on photostasis in albino rats. There are two reasons for this: The eyelids reduce the effective bleaching intensity by half. Moreover, during the "dim-out" of closure, rhodopsin continues to regenerate and approaches a new, higher value. This accumulation of rhodopsin enhances the rate of photon absorption because the rate is proportional to the product (rhodopsin x intensity). Thus, the increased rhodopsin concentration in the rods partially compensates for the reduced intensity of lid closure, and the photon absorption rates, with eyes closed, do not decrease by the full factor of 2 implied by the intensity reduction. In addition, when the eyes are subsequently opened after such a dim-out, the retina is suddenly exposed again to the full intensity of the environment. At this time, photon absorption rate, rhodopsin x intensity, is transiently higher than just before eye opening. Thus, the compensatory interplay between bleaching and regeneration in closed and open eyes results in the near compensation of light absorption and maintenance of the stasis close to 10(16) photons per eye per day.