In vitro aging of bovine and human retinal pigment epithelium: Number and activity of the NA/K ATPase pump

We have previously observed that the density of Na/K ATPase pumps is lower in RPE cells in the posterior pole of both bovine and human eyes. The posterior pole in human eyes includes the macula, a region which is predisposed to aging pathology. In this study we examined the effect of age on the sodium pump using cultures of bovine (bRPE) and human (hRPE) RPE cells that were aged in vitro by repeated passage. The cultures were assayed for cell number, protein, pump density (specific binding of [³H]ouabain) and pump activity (specific uptake of ⁸⁶Rb) at confluency at each passage. In culture, bRPE had more pumps per cell (3·2 × 10⁶) than hRPE (1·2 × 10⁶), but the bRPE pumps were less active so the pumping capacity per cell was nearly equal. Bovine RPE aged more rapidly in vitro (survived fewer passages) than hRPE. With aging, RPE cells from both species showed declines in cell number at confluency. Pump number and pump activity per cell remained constant. Because cell number declined, the pumping capacity per unit area of confluent epithelium was diminished with culture aging. RPE cell number is known to decline with age in situ, especially in the macula. If Na/K ATPase pump number and activity per RPE cell remain constant with aging in vivo as shown here in vitro, the effective pumping capacity of the RPE per unit area of ‗monolayer‘ would decline in aged eyes. Diminished pumping capacity with aging could affect the ability of the RPE to transport ions and other metabolites especially in the aging macula where pump density is lower even in normal young eyes.