Ion transport across the murine retinal pigment epithelium

Abstract

The retinal pigment epithelium (RPE) is important for the normal transport of ions and water from the subretinal space. Due to the small size of mice, ion transport across fresh murine RPE has not been studied in vitro. Sheets of RPE with the retina attached from normal mice were mounted in EasyMount Ussing chambers with an aperture of 0.031 cm2 (Physiological Instruments Inc) with aireated 38°C Krebs. The short‐circuit current, ISC, and the transepithelial resistance, TER, was recorded with a voltage clamp apparatus. Initial ISC (n=7) was −17,6 ±4,5 μA/cm2, with apical side negative, and TER was 82,8 ±7,6 Ohm*cm2. The following substances were tested (30 min each, both apical and basolateral sides): ATP (100 μM) had no effect on the ISC. Epinephrine (100 μM) increased the ISC by 5%. Bumetanide (100 μM) inhibited the ISC from −16.1 ± 5.2 to −14.5 ± 5.4 μA/cm2. Ouabain (1 mM) induced a biphasic response, a rapid 6 min increase to −17.4 ± 2.6 μA/cm2, and a slower 30 min decrease to 36% of pre‐addition ISC. The Isc was reduced by low Cl− Krebs. Reducing the K+ concentration on the apical side decreased the Isc, which was inhibited by 1 mM BaCl2. The mouse RPE preparation was stable for 2–3 hours. The results indicate that adrenergic but not purinergic receptors affect murine RPE ion transport as measured by the ISC. The ion transport is likely due to Cl− transport with some K+ involvement, with the NaK‐ATPase serving as an important driving force.