COMPARISON OF REACTIONS OF HUMAN AND RABBIT FUNDI TO PHOTIC EXPOSURE

Rhodopsin flash photolysis in man

1. Human rhodopsin in vivo was flash bleached by a 600 musec xenon flash which could deliver to the retina up to 15 rod-equivalent quanta per rhodopsin molecule, and the fraction bleached measured by fundus reflexion densitometry. 2. The curve relating fraction rhodopsin bleached to intensity of flash saturates at 0.5 to 0.6. Thus, 40-50% of the rhodopsin is left photo-regenerated by the brightest flashes. 3. Three types of densitometry experiments confirm the saturation of the bleaching curve. 4. The kinetic constants required to account for the observed photo-regeneration were somewhat discrepant with in vitro and in situ estimates from infrahuman species. Specifically, (i) the quantum efficiency of the back reaction, metarhodopsin I hv leads to rhodopsin, was inferred to be nearly as high as that of the forward reaction; and (ii) the rate of the metarhodopsin I leads to metarhodopsin II dark reaction was inferred to be less than 500 sec(-1).

The bleaching and regeneration of rhodopsin in the cat

1. The processes of bleaching and regeneration were monitored by retinal densitometry in living cats.2. Neither bleaching nor regeneration of rhodopsin can be described by the simple kinetic equation (Alpern, 1971) found valid for man.3. After a strong 1 min bleach, the retina contains more unbleached rhodopsin than expected on the basis of the initial bleaching rate.4. During the first 9 min after a 1 min bleach, cats regenerate rhodopsin only slowly; density changes during this period are dominated by formation and decay of metarhodopsin III. Subsequently, rhodopsin regeneration accelerates to a rate of 50%/11 min.5. No such delay precedes recovery from a prolonged (20 min) bleach.

The density and photosensitivity of human rhodopsin in the living retina

1. The visual pigment in a 5 degrees circular patch of the living human retina 18 degrees temporal from the fovea was studied with the Rushton retinal densitometer. The measuring light (570 nm) was selected to obviate artifacts from colour photoproducts.2. The action spectrum of a 10% bleach agrees well with the action spectrum at absolute threshold for the same patch of retina. The quantized C.I.E. scotopic spectral sensitivity curve is a good description of both spectra. Therefore, the visual pigment studied must be human rhodopsin.3. Its density has been estimated in five different ways. The results are in reasonable agreement. The optical density of human rhodopsin in vivo is about 0.35 (common logarithmic units) at its gamma(max.)4. The photosensitivity of human rhodopsin in vivo was determined by studying its rate of bleaching in response to steps of monochromatic light exposed to the dark adapted eye, by measuring the amount bleached in the steady state by monochromatic lights as well as the amount bleached by 10 sec flashes of white light.5. The results obtained by the different methods are in good agreement with each other and with previous estimates made by others using white light.6. The photosensitivity of human rhodopsin in vivo [epsilongamma(max) = 62,000 to 120,000 l./cm mole] is much higher than expected from in vitro measurements.