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Two temporal phases of light adaptation in retinal rods.

Calvert PD, Govardovskii VI, Arshavsky VY, Makino CL - J. Gen. Physiol. (2002)

Bottom Line: We examined the time course of the onset of light adaptation in bullfrog rods and compared it with the projected onset of feedback reactions thought to underlie light adaptation on the molecular level.We tested the hypothesis that the slow adaptation phase is mediated by cGMP dissociation from noncatalytic binding sites on the cGMP phosphodiesterase, which has been shown to reduce the lifetime of activated phosphodiesterase in vitro.Other possible mechanisms are discussed.

View Article: PubMed Central - PubMed

Affiliation: Department of Ophthalmology, Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA. pdcalvert@meei.harvard.edu

ABSTRACT
Vertebrate rod photoreceptors adjust their sensitivity as they adapt during exposure to steady light. Light adaptation prevents the rod from saturating and significantly extends its dynamic range. We examined the time course of the onset of light adaptation in bullfrog rods and compared it with the projected onset of feedback reactions thought to underlie light adaptation on the molecular level. We found that adaptation developed in two distinct temporal phases: (1) a fast phase that operated within seconds after the onset of illumination, which is consistent with most previous reports of a 1-2-s time constant for the onset of adaptation; and (2) a slow phase that engaged over tens of seconds of continuous illumination. The fast phase desensitized the rods as much as 80-fold, and was observed at every light intensity tested. The slow phase was observed only at light intensities that suppressed more than half of the dark current. It provided an additional sensitivity loss of up to 40-fold before the rod saturated. Thus, rods achieved a total degree of adaptation of approximately 3,000-fold. Although the fast adaptation is likely to originate from the well characterized Ca(2+)-dependent feedback mechanisms regulating the activities of several phototransduction cascade components, the molecular mechanism underlying slow adaptation is unclear. We tested the hypothesis that the slow adaptation phase is mediated by cGMP dissociation from noncatalytic binding sites on the cGMP phosphodiesterase, which has been shown to reduce the lifetime of activated phosphodiesterase in vitro. Although cGMP dissociated from the noncatalytic binding sites in intact rods with kinetics approximating that for the slow adaptation phase, this hypothesis was ruled out because the intensity of light required for cGMP dissociation far exceeded that required to evoke the slow phase. Other possible mechanisms are discussed.

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cGMP dissociation from the noncatalytic binding sites on PDE. The cGMP levels of cryodissected rod outer segments were determined in dark-adapted samples or in samples exposed to steady illumination at three different intensities (given on the right) as described in materials and methods. Each data point is the average of at least four determinations. Error bars represent the SEM. The line through the triangles is a linear regression; the slope did not differ significantly from zero. The line through the squares is a fitting with a single exponential, τ = 62.5 s. The line through the circles is a fitting with the sum of two exponentials: a = 0.34, τa = 13 s; b = 0.66, τb = 532 s.
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fig5: cGMP dissociation from the noncatalytic binding sites on PDE. The cGMP levels of cryodissected rod outer segments were determined in dark-adapted samples or in samples exposed to steady illumination at three different intensities (given on the right) as described in materials and methods. Each data point is the average of at least four determinations. Error bars represent the SEM. The line through the triangles is a linear regression; the slope did not differ significantly from zero. The line through the squares is a fitting with a single exponential, τ = 62.5 s. The line through the circles is a fitting with the sum of two exponentials: a = 0.34, τa = 13 s; b = 0.66, τb = 532 s.

Mentions: About 95% of cGMP in the outer segment of a dark-adapted rod is bound to the noncatalytic sites on PDE (Cote and Brunnock, 1993). So nearly the entire light-induced decline in cGMP is due to its dissociation from these sites and its subsequent hydrolysis. Fig. 5 shows the time course of light-dependent cGMP decline in ROS of physiologically active retinas exposed to constant light producing 580, 46,500, or 465,000 R* rod−1 s−1. For the brightest intensity, 30% of the cGMP dissociated with a time constant of 13 s, whereas the remainder dissociated with a time constant of 532 s. The kinetics of cGMP decline at the brightest light level was similar to that reported for transducin-activated PDE in isolated ROS (Cote et al., 1994; Yamazaki et al., 1996; Calvert et al., 1998). These results contrast with those reported by Cohen and Blazynski (1988)(1993) and Blazynski and Cohen (1986) who used an approach similar to ours; the basis for the difference is not known.


Two temporal phases of light adaptation in retinal rods.

Calvert PD, Govardovskii VI, Arshavsky VY, Makino CL - J. Gen. Physiol. (2002)

cGMP dissociation from the noncatalytic binding sites on PDE. The cGMP levels of cryodissected rod outer segments were determined in dark-adapted samples or in samples exposed to steady illumination at three different intensities (given on the right) as described in materials and methods. Each data point is the average of at least four determinations. Error bars represent the SEM. The line through the triangles is a linear regression; the slope did not differ significantly from zero. The line through the squares is a fitting with a single exponential, τ = 62.5 s. The line through the circles is a fitting with the sum of two exponentials: a = 0.34, τa = 13 s; b = 0.66, τb = 532 s.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2233805&req=5

fig5: cGMP dissociation from the noncatalytic binding sites on PDE. The cGMP levels of cryodissected rod outer segments were determined in dark-adapted samples or in samples exposed to steady illumination at three different intensities (given on the right) as described in materials and methods. Each data point is the average of at least four determinations. Error bars represent the SEM. The line through the triangles is a linear regression; the slope did not differ significantly from zero. The line through the squares is a fitting with a single exponential, τ = 62.5 s. The line through the circles is a fitting with the sum of two exponentials: a = 0.34, τa = 13 s; b = 0.66, τb = 532 s.
Mentions: About 95% of cGMP in the outer segment of a dark-adapted rod is bound to the noncatalytic sites on PDE (Cote and Brunnock, 1993). So nearly the entire light-induced decline in cGMP is due to its dissociation from these sites and its subsequent hydrolysis. Fig. 5 shows the time course of light-dependent cGMP decline in ROS of physiologically active retinas exposed to constant light producing 580, 46,500, or 465,000 R* rod−1 s−1. For the brightest intensity, 30% of the cGMP dissociated with a time constant of 13 s, whereas the remainder dissociated with a time constant of 532 s. The kinetics of cGMP decline at the brightest light level was similar to that reported for transducin-activated PDE in isolated ROS (Cote et al., 1994; Yamazaki et al., 1996; Calvert et al., 1998). These results contrast with those reported by Cohen and Blazynski (1988)(1993) and Blazynski and Cohen (1986) who used an approach similar to ours; the basis for the difference is not known.

Bottom Line: We examined the time course of the onset of light adaptation in bullfrog rods and compared it with the projected onset of feedback reactions thought to underlie light adaptation on the molecular level.We tested the hypothesis that the slow adaptation phase is mediated by cGMP dissociation from noncatalytic binding sites on the cGMP phosphodiesterase, which has been shown to reduce the lifetime of activated phosphodiesterase in vitro.Other possible mechanisms are discussed.

View Article: PubMed Central - PubMed

Affiliation: Department of Ophthalmology, Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA. pdcalvert@meei.harvard.edu

ABSTRACT
Vertebrate rod photoreceptors adjust their sensitivity as they adapt during exposure to steady light. Light adaptation prevents the rod from saturating and significantly extends its dynamic range. We examined the time course of the onset of light adaptation in bullfrog rods and compared it with the projected onset of feedback reactions thought to underlie light adaptation on the molecular level. We found that adaptation developed in two distinct temporal phases: (1) a fast phase that operated within seconds after the onset of illumination, which is consistent with most previous reports of a 1-2-s time constant for the onset of adaptation; and (2) a slow phase that engaged over tens of seconds of continuous illumination. The fast phase desensitized the rods as much as 80-fold, and was observed at every light intensity tested. The slow phase was observed only at light intensities that suppressed more than half of the dark current. It provided an additional sensitivity loss of up to 40-fold before the rod saturated. Thus, rods achieved a total degree of adaptation of approximately 3,000-fold. Although the fast adaptation is likely to originate from the well characterized Ca(2+)-dependent feedback mechanisms regulating the activities of several phototransduction cascade components, the molecular mechanism underlying slow adaptation is unclear. We tested the hypothesis that the slow adaptation phase is mediated by cGMP dissociation from noncatalytic binding sites on the cGMP phosphodiesterase, which has been shown to reduce the lifetime of activated phosphodiesterase in vitro. Although cGMP dissociated from the noncatalytic binding sites in intact rods with kinetics approximating that for the slow adaptation phase, this hypothesis was ruled out because the intensity of light required for cGMP dissociation far exceeded that required to evoke the slow phase. Other possible mechanisms are discussed.

Show MeSH
Related in: MedlinePlus