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Synthesis and Release of Docosahexaenoic Acid by the RPE Cells of prcd-Affected Dogs

From the Departments of ¹ Ophthalmology, ⁵ Cell Biology, and ⁶ Biochemistry and Molecular Biology, and ² Oklahoma Center for Neuroscience, University of Oklahoma Health Sciences Center, Oklahoma City; and ³ Dean A. McGee Eye Institute, Oklahoma City, Oklahoma; and ⁴ James A. Baker Institute for Animal Health, College of Veterinary Medicine, Cornell University, Ithaca, New York.

Abstract

PURPOSE. Dogs affected with progressive rod-cone degeneration (prcd) have reduced levels of docosahexaenoic acid (DHA, 22:6n-3) in their plasma and rod photoreceptor outer segments (ROS). Dietary supplementation of DHA has failed to increase the ROS DHA levels to that of unaffected control dogs. The present study was undertaken to test the hypothesis that prcd-affected dogs have a reduced capacity for the synthesis and/or release of DHA in retinal pigment epithelial (RPE) cells.

METHODS. RPE cells (first passage cultures) from prcd-affected and normal dogs were incubated with [³H]eicosapentaenoic acid (EPA, 20:5n-3) for 24 and 72 hours. After incubation, the radiolabeled fatty acids in the cells and media were analyzed.

RESULTS. DHA and all its metabolic intermediates were detected in RPE cells from prcd-affected and normal dogs. No significant difference was found in the amount of products (including DHA) synthesized between normal and affected RPE cells at either time point. In the culture media, RPE cells from prcd-affected dogs released significantly more DHA than cells from normal dogs after 72-hour incubation, but not after 24-hour incubation.

CONCLUSIONS. RPE cells from prcd-affected dogs can synthesize and release DHA at least as efficiently as cells from normal dogs. Therefore, synthesis of DHA from its precursor and its release from RPE cells does not appear to contribute to the reduction in ROS DHA levels found in prcd-affected animals.

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