The regeneration of visual pigments and the change of rod hypersensitivity after irradiation by bleaching light in frog retina

Breaking the covalent bond--a pigment property that contributes to desensitization in cones

Retinal rod and cone pigments consist of an apoprotein, opsin, covalently linked to a chromophore, 11-cis retinal. Here we demonstrate that the formation of the covalent bond between opsin and 11-cis retinal is reversible in darkness in amphibian red cones, but essentially irreversible in red rods. This dissociation, apparently a general property of cone pigments, results in a surprisingly large amount of free opsin--about 10% of total opsin--in dark-adapted red cones. We attribute this significant level of free opsin to the low concentration of intracellular free 11-cis retinal, estimated to be only a tiny fraction (approximately 0.1 %) of the pigment content in red cones. With its constitutive transducin-stimulating activity, the free cone opsin produces an approximately 2-fold desensitization in red cones, equivalent to that produced by a steady light causing 500 photoisomerizations s-1. Cone pigment dissociation therefore contributes to the sensitivity difference between rods and cones.

The effect of lead on photoreceptor response amplitude--influence of the light stimulus

The mass receptor potential of the excised, superfused retina of the bullfrog was studied. Photoreceptor responses were isolated by addition of sodium aspartate to the Ringer solutions. Responses of the cones were monitored independently from responses of the rods by employing a two-flash method of stimulation which took advantage of the very different rates of rapid dark adaptation of rods and of cones. Stimulation with paired flashes of white light at regular intervals caused enhancement of rod response amplitude in that the response grew larger with subsequent flashes until reaching a stable plateau. The degree of enhancement was directly proportional to the amount of light exposure and increased with either increasing stimulus intensity or decreasing stimulus interval. Only the rod response was affected; the cone response was not enhanced by continued stimulation. The effects of 12.5 microM PbCl2 on rod response amplitude were complex. There was a small (less than 10%) but consistent depression of rod response amplitude even when the rods were in the unenhanced state. However, the most striking effect of lead was on the enhanced response, where treatment with 12.5 microM PbCl2 led to a depression of about 33%. When added prior to light stimulation, lead significantly decreased the degree to which the rod response could be enhanced, but never prevented enhancement entirely. Removal of lead resulted in a very large increase in the degree to which the rod response was enhanced by light, even when compared to the first, lead-free control. The cone response was unaffected by 12.5 microM PbCl2.

The action of phosphodiesterase inhibitors on the hypersensitivity of frog photoreceptor

The increase in sensitivity of frog photoreceptors after illumination, "hypersensitivity", was studied by recording fast-PIII responses of the isolated bull frog retina superfused with physiological saline containing 5 mM sodium aspartate. The hypersensitivity is mot marked after illumination which bleaches only 0.01% of the rhodopsin initially present. The wavelength of light, which is optimum to induce the hypersensitivity, is about 500 nm. Several kinds of phosphodiesterase inhibitors, isobutylmethylxanthine (IBMX), papaverine, theophylline and caffeine act to induce the hypersensitivity, as does lowered extracellular concentrations of calcium ion, [Ca2+]out. Such action of phosphodiesterase inhibitor can be antagonized by an increase in [Ca2+]out. The relation between effects of Ca2+ and cyclic nucleotides on photoreceptor is discussed.