Light responses in rods of vitamin A-deprived Xenopus.
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.
Solessio
Eduardo
E
Department of Ophthalmology, SUNY Upstate Medical University, Center for Vision Research, Syracuse, New York 13210, USA. solessie@upstate.edu
Umino
Yumiko
Y
Cameron
David A
DA
Loew
Ellis
E
Engbretson
Gustav A
GA
Knox
Barry E
BE
Barlow
Robert B
RB
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
D064026:Calbindins; D064030:S100 Calcium Binding Protein G; D012243:Rhodopsin; D006162:Guanylate Cyclase; D006152:Cyclic GMP