TY  - JOUR
PY  - 2009//
TI  - Light responses in rods of vitamin A-deprived Xenopus
JO  - Investigative ophthalmology and visual science
A1  - Solessio, Eduardo
A1  - Umino, Yumiko
A1  - Cameron, David A.
A1  - Loew, Ellis
A1  - Engbretson, Gustav A.
A1  - Knox, Barry E.
A1  - Barlow, Robert B.
SP  - 4477
EP  - 4486
VL  - 50
IS  - 9
N2  - PURPOSE: 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.<p /> <p>Language: en</p>
LA  - en
SN  - 0146-0404
UR  - http://dx.doi.org/10.1167/iovs.08-3186
ID  - ref1
ER  -