|
|
|
|
|
|
|
||
Original Article |
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
3,000-fold. Although the fast adaptation is likely to originate from the well characterized Ca2+-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.
Key Words: photoreceptors • phototransduction • adaptation • calcium • cGMP
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
This article has been cited by other articles:
|
|
 |
|
  L. A. Astakhova, M. L. Firsov, and V. I. Govardovskii Kinetics of Turn-offs of Frog Rod Phototransduction Cascade J. Gen. Physiol., November 1, 2008; 132(5): 587 - 604. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 F. S. Soo, P. B. Detwiler, and F. Rieke Light Adaptation in Salamander L-Cone Photoreceptors J. Neurosci., February 6, 2008; 28(6): 1331 - 1342. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. M. Krispel, M. Sokolov, Y.-M. Chen, H. Song, R. Herrmann, V. Y. Arshavsky, and M. E. Burns Phosducin Regulates the Expression of Transducin {beta}{gamma} Subunits in Rod Photoreceptors and Does Not Contribute to Phototransduction Adaptation J. Gen. Physiol., August 27, 2007; 130(3): 303 - 312. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. C. Pentia, S. Hosier, and R. H. Cote The Glutamic Acid-rich Protein-2 (GARP2) Is a High Affinity Rod Photoreceptor Phosphodiesterase (PDE6)-binding Protein That Modulates Its Catalytic Properties J. Biol. Chem., March 3, 2006; 281(9): 5500 - 5505. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S Nymark, H Heikkinen, C Haldin, K Donner, and A Koskelainen Light responses and light adaptation in rat retinal rods at different temperatures J. Physiol., September 15, 2005; 567(3): 923 - 938. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. Solessio, S. S. Mani, N. Cuenca, G. A. Engbretson, R. B. Barlow, and B. E. Knox Developmental regulation of calcium-dependent feedback in Xenopus rods J. Gen. Physiol., October 25, 2004; 124(5): 569 - 585. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
F. He, M. Mao, and T. G. Wensel Enhancement of Phototransduction G Protein-Effector Interactions by Phosphoinositides J. Biol. Chem., March 5, 2004; 279(10): 8986 - 8990. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. M. Krispel, C.-K. Chen, M. I. Simon, and M. E. Burns Prolonged Photoresponses and Defective Adaptation in Rods of G{beta}5-/- Mice J. Neurosci., August 6, 2003; 23(18): 6965 - 6971. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. Hu, Z. Zhang, and T. G. Wensel Activation of RGS9-1GTPase Acceleration by Its Membrane Anchor, R9AP J. Biol. Chem., April 11, 2003; 278(16): 14550 - 14554. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. E. Brockerhoff, F. Rieke, H. R. Matthews, M. R. Taylor, B. Kennedy, I. Ankoudinova, G. A. Niemi, C. L. Tucker, M. Xiao, M. C. Cilluffo, et al. Light Stimulates a Transducin-Independent Increase of Cytoplasmic Ca2+ and Suppression of Current in Cones from the Zebrafish Mutant nof J. Neurosci., January 15, 2003; 23(2): 470 - 480. [Abstract] [Full Text] [PDF]
|
|
|
|
