Rhodopsin Regeneration, Calcium, and the Control of the Dark Current in Vertebrate Rods

Abstract

Abstract— The dark current of retinal rods is suppressed for an extended period when their rhodopsin is bleached. An 8% bleach completely suppresses the current for 8 min and after 35 min it is fully recovered. The dark current can recover fully from a bleaching flash without any rhodopsin being regenerated. Moreover the recovery can be hastened either by lowering the activity of calcium ions surrounding the rods or by regenerating the rhodopsin. The recovery of the dark current following these bleaches showed zero order kinetics, regardless of whether the recovery was hastened by low calcium, 11‐cis retinaldehyde or not. If all the rhodopsin is bleached in the retina, the dark current does not recover; the addition of 11‐cis retinaldehyde, but not all‐trans retinaldehyde, to the bleached retina causes the dark current to begin its recovery as early as 10 min after the addition with zero order kinetics (1.3% min‐1). In two of three similar experiments, the dark current recovered 100%. When the recovery rate of the dark current from the retina showing the earliest response is compared with the rate of the regeneration of rhodopsin in the plasma and disc membranes, the dark current begins its recovery after the plasma membrane rhodopsin is fully regenerated and the disc rhodopsin is half regenerated. When the disc rhodopsin is fully regenerated, the dark current is recovered 75%, and 20 min later it is completely recovered.

Cellular mechanisms that underlie bleaching and background adaptation.

“…A variety of studies have demonstrated that the steady-state effects of bleaching adaptation which are observed in isolated retinas can be reversed by exposing the tissue to solutions containing l l-c/s retinal (Pepperberg et al, 1978;Albani et al, 1980;Perlman et al, 1982). The experiments described in the present study demonstrate that these effects produced by exogenous application of retinoids result from events occurring within the rod outer segment and shed some light on the mechanisms regulating these changes.…”

“…6). This is somewhat faster than rates observed for rod responses in skate (-20 min for total recovery, see Pepperberg et al, 1978), bullfrog (-50 min, see Perlman et al, 1982), and rat (~40 min, see Albani et al, 1980), but different methods of application and different concentrations of 11-cis retinal were used in these studies. Though no systematic effort was made to examine the dose dependency of the resensitizing effects of 11-c/s retinal, the rates of recovery of sensitivity were lower when vesicles containing less retinoid were used.…”

“…The final steady-state level of desensitization that is achieved persists indefinitely in the absence of pigment regeneration (Weinstein et al, 1967;Grabowski and Pak, 1975;Brin and Ripps, 1977;Pepperberg et al, 1978;Leibovic et al, 1987). The total recovery of sensitivity requires the regeneration of rhodopsin through the application of exogenous 11-c/s retinal (Pepperberg et al, 1978;Albani et al, 1980;Perlman et al, 1982). The steady-state component of bleaching adaptation has been shown to be spatially localized to the bleached region of the rod (Cornwall et al, 1983).…”

Utilization of retinoids in the bullfrog retina.

“…Recent studies of several isolated retina preparations suggested the workability of an electrophysiological assay for this process. These studies indicated that the formation of rhodopsin, induced by incubation of the previously bleached retina with 11-cis retinal, promotes a substantial increase in the photic sensitivity of the rods (Pepperberg et al, 1976(Pepperberg et al, , 1978Pepperberg and Masland, 1978;Albani et al, 1980). We reasoned that the measurement Of receptor sensitivity could similarly afford the analysis, within demonstrably functional photoreceptors, of the activities of putative precursors of 11-cis retinal.…”

Effects of Ca2+ on the decay of rhodopsin photoproducts and photoreceptor adaptation in the isolated bullfrog retina.

“…TOBA and HANAWA (1985) also indicated that changes in the extracellular Ca2+ concentration altered the adaptive state of the red rods. A reduction in the extracellular Ca2+ concentration leads to a remarkable acceleration in the recovery of rod sensitivity after a flash bleaching of a substantial amount of rhodopsin (LIPTON et al, 1977;ALBANI et al, 1980). Consequently, it would seem that if the rod sensitivity is influenced by rhodopsin intermediates, changes in the extracellular Ca2 + concentration should affect the decay rate of the intermediates.…”