THE UTILIZATION OF EXOGENOUS RETINOIDS BY VERTEBRATE PHOTORECEPTORS (BULLARD)
Abstract
The capacity of the retina to synthesize 11-cis retinal from retinoids arising naturally in the eye was examined in the isolated retina of the bullfrog, Rana catesbeiana. Retinoids, cosuspended with phosphatidyl choline, were applied topically to the photoreceptor surface of the retina following substantial bleaching of the visual pigment initially present in the red rods. The formation of rhodopsin, employed as an assay for the appearance of 11-cis retinal in the photoreceptors, was measured spectrophotometrically; in separate experiments, the increase in phototreceptor sensitivity associated with the formation of rhodopsin was analyzed by extracellular measurement of the photoreceptor potential. Treatment with 11-cis retinal and 11-cis retinol induced substantial increases in both the rhodopsin content and photic sensitivity of previously bleached receptors; the formation of rhodopsin and the electrophysiological sensitization induced by 11-cis retinol typically were slower and/or less extensive than the corresponding processes induced by 11-cis retinal. The all-trans isomers of retinyl palmitate, retinol, and retinal, as well as the 11-cis isomer of retinyl palmitate, were inactive by both the electrophysiological and spectrophotometric criteria for the generation of rhodopsin. Treatment with any one of the "inactive" retinoids did not abolish the capacity of (subsequently applied) 11-cis retinal or 11-cis retinol to promote the formation of rhodopsin. Incubation of the isolated retina with all-trans retinal resulted in the formation of a hydroxylamine-sensitive retinal-Schiff base complex ((lamda)(,max)=470 nm); this material formed equally well in both bleached and unbleached retinas. Under these conditions, irradiation with light of appropriate wavelength resulted in the photo-induced formation of rhodopsin. These results are discussed in relation to the mechanism(s) of rhodopsin regeneration in vivo.
Degree
Ph.D.
Subject Area
Biology
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