title: Light responses in rods of vitamin A-deprived Xenopus.
abstract: OBJECTIVE
  Accumulation of free opsin by mutations in rhodopsin or insufficiencies in the visual cycle can lead to retinal degeneration. Free opsin activates phototransduction; however, the link between constitutive activation and retinal degeneration is unclear. In this study, the photoresponses of Xenopus rods rendered constitutively active by vitamin A deprivation were examined. Unlike their mammalian counterparts, Xenopus rods do not degenerate. Contrasting phototransduction in vitamin A-deprived Xenopus rods with phototransduction in constitutively active mammalian rods may provide new understanding of the mechanisms that lead to retinal degeneration.
  
  
  METHODS
  The photocurrents of Xenopus tadpole rods were measured with suction electrode recordings, and guanylate cyclase activity was measured with the IBMX (3-isobutyl-1-methylxanthine) jump technique. The amount of rhodopsin in rods was determined by microspectrophotometry.
  
  
  RESULTS
  The vitamin A-deprived rod outer segments were 60% to 70% the length and diameter of the rods in age-matched animals. Approximately 90% of its opsin content was in the free or unbound form. Analogous to bleaching adaptation, the photoresponses were desensitized (10- to 20-fold) and faster. Unlike bleaching adaptation, the vitamin A-deprived rods maintained near normal saturating (dark) current densities by developing abnormally high rates of cGMP synthesis. Their rate of cGMP synthesis in the dark (15 seconds(-1)) was twofold greater than the maximum levels attainable by control rods ( approximately 7 seconds(-1)).
  
  
  CONCLUSIONS
  Preserving circulating current density and response range appears to be an important goal for rod homeostasis. However, the compensatory changes associated with vitamin A deprivation in Xenopus rods come at the high metabolic cost of a 15-fold increase in basal ATP consumption.
authors.affiliation: Department of Ophthalmology, SUNY Upstate Medical University, Center for Vision Research, Syracuse, New York 13210, USA. solessie@upstate.edu
authors.forename: Eduardo
authors.initials: E
authors.lastname: Solessio
authors.forename: Yumiko
authors.initials: Y
authors.lastname: Umino
authors.forename: David A
authors.initials: DA
authors.lastname: Cameron
authors.forename: Ellis
authors.initials: E
authors.lastname: Loew
authors.forename: Gustav A
authors.initials: GA
authors.lastname: Engbretson
authors.forename: Barry E
authors.initials: BE
authors.lastname: Knox
authors.forename: Robert B
authors.initials: RB
authors.lastname: Barlow
mesh_terms: D000818:Animals; D064026:Calbindins; D006152:Cyclic GMP / Q000378:metabolism; D003623:Dark Adaptation; D004594:Electrophysiology; D019084:Fluorescent Antibody Technique, Indirect; D006162:Guanylate Cyclase / Q000378:metabolism; D006868:Hydrolysis; D008027:Light; D015591:Microspectrophotometry; D010775:Photic Stimulation; D012162:Retinal Degeneration / Q000378:metabolism / Q000503:physiopathology*; D017948:Retinal Rod Photoreceptor Cells / Q000502:physiology*; D012243:Rhodopsin / Q000378:metabolism; D064030:S100 Calcium Binding Protein G / Q000378:metabolism; D014785:Vision, Ocular / Q000502:physiology* / Q000528:radiation effects; D014802:Vitamin A Deficiency / Q000378:metabolism / Q000503:physiopathology*; D014982:Xenopus laevis
chemical_list: D064026:Calbindins; D064030:S100 Calcium Binding Protein G; D012243:Rhodopsin; D006162:Guanylate Cyclase; D006152:Cyclic GMP