Relationship of cholesterol content to spatial distribution and age of disc membranes in retinal rod outer segments

The initial events of visual transduction occur on disc membranes which are sequestered within the photoreceptor outer segment. In rod cells, the discs are stacked in the outer segment. Discs are formed at the base of the rod outer segment (ROS) from evaginations of the plasma membrane. As new discs form, older discs move toward the apical tip of the rod, from which they are eventually shed and subsequently phagocytosed by the adjacent pigment epithelium. Thus, disc membranes within a given rod cell are not of uniform age. We have recently shown that disc membranes are not homogeneous with respect to cholesterol content (Boesze-Battaglia, K., Hennessey, T., and Albert, A. D. (1989) J. Biol. Chem. 264, 8151-8155). In the present study, freshly isolated bovine retinas were incubated with [³H]leucine for 4 h in order to allow sufficient time for the radiolabeled proteins to become incorporated into the basal-most (newest) discs. Osmotically intact discs were then isolated. After the addition of digitonin, the discs were fractionated based on cholesterol content, and radioactivity (indicative of newly synthesized protein) was measured. Discs which exhibited high cholesterol content also exhibited high radioactivity. These results demonstrate that the cholesterol heterogeneity of ROS disc membranes is related to the age, and thus the position, of the discs in the ROS.