Interstitial retinol-binding protein (IRBP) in the RCS rat: effect of dark-rearing

The effect of light- and dark-rearing on the amounts of rhodopsin and interstitial retinol-binding protein (IRBP) in RCS rats and their congenic controls (RCS-rdy+) was determined. Rhodopsin was measured spectroscopically and IRBP by dot-blot enzyme immunoassay utilizing rabbit antibovine IRBP IgG. After P15-20, dark-reared RCS and RCS-rdy+ rats always had more rhodopsin than their light-reared, age-matched counterparts. The rhodopsin in the light-reared RCS rats peaked at about 2 nmol/eye at P20-25. The rhodopsin in the dark-reared RCS rats peaked at about 4 nmol/eye at P60-70. Maintenance of RCS-rdy+ rats in darkness had no effect on their IRBP content, which continued to increase up to P80-110. In both groups of RCS rats, the amount of IRBP reached a peak at P22. In RCS rats maintained in darkness, the amount of IRBP attained at this peak was about twice that in the corresponding light-reared group and in RCS-rdy+ animals at this age. The decline of IRBP after P22 in RCS rats was slowed in darkness by approximately 10 days. This slowed decline of IRBP is associated with a decreased rate of photoreceptor degeneration, and the results are therefore consistent with the hypothesis that the photoreceptors synthesize and secrete IRBP. The layer of membranous debris would restrict the diffusion of IRBP in the subretinal space and could partially exclude this retinol transport protein from access to the zone adjacent to the apical surface of the retinal pigment epithelium (RPE).