Rod outer segment lipid-opsin ratios in the developing normal and retinal dystrophic rat

Abstract

The outer segment membrane lipid and opsin contents were determined in photoreceptor cell rods isolated from the eyes of developing normal rats reared in cyclic light or dark environments and dark-reared dystrophic rats. In cyclic light-reared normals rhodopsin/eye increased 49% during the period 20–60 days. Total ROS lipid content, a measure of ROS length, increased 50% while the polyunsaturated fatty acid docosahexaenoate increased from 42–51 mol % during the same period. The phospholipid/opsin ratio of cyclic light reared rat ROS membranes was 67 mol/mol at 20 days and 68 mol/mol at 60 days. In young dark-reared normals the phospholipid/opsin ratio was the same as for cyclic light-reared rats. Although 60 day-old dark-reared normals had 30% more rhodopsin/eye than their cyclic light-reared counterparts, non-significant changes in ROS length (14% longer) and in the phospholipid/opsin ratio (8% lower) were measured in these rats. In addition, light deprivation had no significant effect on the concentrations of polyunsaturated fatty acids or the lipid composition of the isolated ROS. The eyes of dark-reared rats with retinal dystrophy accumulated two times more rhodopsin than dark-reared normals during the 20–60-day period. The phospholipid/opsin ratio of mutant rat ROS was only 7% lower than dark normal at 20 days and 13% lower at 34 days. However, by 60 days of age, the phospholipid/opsin ratio in dystrophic rat ROS was significantly lower than in ROS from either cyclic light-or dark-reared normals. Docosahexaenoic acid in mutant rat ROS lipids averaged 40 mol% during the developmental period. These levels were significantly lower than the levels of docosahexaenoate measured in dark normals at both the 34- and 64-day periods. The glycerophospholipid composition of dystrophic rat ROS was the same as normal at all ages but the cholesterol/phospholipid ratio was higher than in normals. The data show: (1) that the retina accomodates changes in rhodopsin content induced by environmental light, age and genetic differences by alterations in ROS opsin density and length: (2) that the content of ROS membrane polyunsaturated fatty acids (fluidity) increases during development in normals but not in dystrophic rats. The data also suggest that basal membrane synthesis and/or post sythetic membrane modification of ROS lipid are impaired as a function of age in dystrophic rats.