Effects of a high-galactose diet on the interphotoreceptor matrix of two strains of rat.

PURPOSE The authors previously reported that a diet containing 30% galactose retards the development of the late-onset photoreceptor dystrophy in the spontaneously hypertensive (SHR) rat. It was suggested that the dystrophy might result from faulty galactosylation of a critical glycoprotein or glycosaminoglycan (GAG) in the interphotoreceptor matrix (IPM). In the current study, this hypothesis was tested by studying IPM protein and GAG composition in SHR and normotensive Wistar-Kyoto (WKy) control strain rats fed a standard or a high-galactose (30%) diet. METHODS The authors performed biochemical analyses of the IPM of SHR and of WKy control rats fed either the basal diet or a 30% galactose diet for 14 mo and labeled at the termination of the experiment with 3H-glucosamine and 35S-sulfate. Analyses included high-performance liquid chromatography and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and isoelectric focusing (IEF) of extracted proteoglycans with identification of the GAG by selective enzymatic degradation and immunoblotting of SDS-PAGE gels for interphotoreceptor retinol-binding protein (IRBP), with slot blots for additional quantitation. RESULTS Although several differences were detected in GAG radiolabeling between the two strains, only one, a decreased overall synthesis of 3H-glucosamine-labeled GAG in the WKy rats, was altered by the high-galactose diet. There was no apparent difference in the protein patterns of the IPM as evaluated by two-dimensional SDS-PAGE and IEF. However, slot blots of that portion of the IPM extracted from the neural retinas showed less reactivity per microgram of protein for IRBP in the galactose-fed animals of both strains than for the rats fed a basal diet. This was statistically significant, however, only in the WKy rats. CONCLUSIONS The composition of the IPM of these two strains of rat appears similar to the composition of the IPM of other species that have been studied. Whether the quantitative alteration in IRBP that appears to be produced by galactose feeding has functional significance, with particular relevance to retarding the development of the photoreceptor cell dystrophy of the SHR rat, is unclear at this time.