Mechanisms of effects of small hyperosmotic gradients on the chick RPE

This paper presents electrophysiological findings of the effects of small trans-tissue osmotic gradients on the chick retinal pigment epithelium (RPE). These gradients are similar to those produced in the human "hyperosmolarity response," a clinical test of RPE integrity. Effects of 25 mOsm osmotic gradients (mannitol) were observed on the electrical parameters of the tissue and on light-evoked responses in a preparation of chick neural retina-RPE-choroid. Making the retinal perfusate hyperosmolar to the choroidal side, (retinal hyperosmolarity), depolarized the RPE basal membrane and increased the amplitude of the light-evoked c-wave of the ERG. Retinal hyperosmolarity also decreased RPE basal membrane resistance as estimated from measurements of resistance parameters (trans-tissue resistance and a, the RPE membrane resistance ratio), and of RPE membrane polarizations during the c-wave. Choroidal hyperosmolarity led to a hyperpolarization of the basal membrane and a decrease in the amplitude of the light-evoked c-wave. Measurements of resistance parameters indicated an increase in basal membrane resistance. In addition, hyperosmolar loads of either direction decreased the amplitude of the light peak of the DC-ERG. The effects on the light-evoked c-wave and light-peak responses did not occur with bilateral hyperosmolarity, indicating that a trans-epithelial osmotic gradient is necessary for the effects on the RPE. We conclude that the basal membrane of the RPE is the principal site of the effects of small hyperosmotic loads of either direction, and that the ERG c-wave is a sensitive measure of the effects on RPE basal membrane resistance.