Zinc uptake in vitro by human retinal pigment epithelium

Zinc, an essential trace element, is present in unusually high concentrations in the chorioretinal complex relative to most other tissues. Because little has been known about the interactions between the retinal pigment epithelium and free or protein-associated zinc, we studied 65Zn uptake by human retinal pigment epithelium in vitro. When monolayers were exposed to differing concentrations from 0 to 30 microM 65Zn in Dulbecco's modified Eagle's medium with 5.4 gm/l glucose at 37 degrees C and 4 degrees C, we observed a temperature-dependent saturable accumulation of the radiolabel. With 15 microM 65Zn, we saw a biphasic pattern of uptake with a rapid first phase and a slower second phase over 120 min. Uptake of 65Zn was inhibited by iodacetate and cold, and reduced approximately 50% by the addition of 2% albumin to the labelling medium. Neither ouabain nor 2-deoxyglucose inhibited uptake. Cells previously exposed to 65Zn retained approximately 70% of accumulated 65Zn 60 min after being changed to radiolabel-free medium. Following removal of cells from the extracellular matrix adherent to the dish bottom, a variable amount of nonspecific binding of 65Zn to the residual matrix was demonstrated. These observations are consistent with a facilitated type of transport and demonstrate the ability of human retinal pigment epithelium in vitro to accumulate and retain zinc.