Mechanisms of Ion Transport Across the Mouse Retinal Pigment Epithelium Measured In Vitro.

Purpose: To examine ion transport across the mouse retinal pigment epithelium (RPE), measured by the short-circuit current (ISC) and transepithelial resistance (TER). Methods: Sheets of RPE from mice (C57BL6/J) with retina, choroid, and sclera attached were mounted in Ussing chambers (0.031-cm2 aperture) and Krebs solution. The ISC and TER were recorded with voltage clamps. Receptors implicated in ion transport were blocked or stimulated by ligands applied to both sides. Results: The mean initial ISC was -12.0 ± 3.9 µA/cm2 (basolateral negative), and mean TER was 67.1 ± 8.0 ohm·cm2. RPE preparations remained stable for 3 hours, with ISC decreasing by 0.078 ± 0,033 µA/cm2/hr. Adenosine triphosphate (100 µM) increased ISC by 2.22 ± 0.41 µA/cm2 (P = 0.003). Epinephrine (100 µM) increased ISC by 1.14 ± 0.19 µA/cm2 (P = 0.011). Bumetanide (100 µM) reduced ISC by 1.72 ± 0.73 µA/cm2 (P = 0.027). Ouabain (1 mM) induced a biphasic response: an ISC increase from -7.9 ± 2.4 to -15.49 ± 2.12 µA/cm2 and then a decrease to -3.7 ± 2.2 µA/cm2. Ouabain increased TER by 15.3 ± 4.8 ohm·cm2. These compounds were added sequentially. Apical [K+]o at zero mM transiently increased ISC by 3.36 ± 1.06 µA/cm2. Ba++ decreased ISC from -10.4 ± 3.1 to -6.6 ± 1.8 µA/cm2 (P = 0.01). Ba++ reversed the K+-free response, with Isc decreasing further from -5.65 ± 1.24 to -3.37 ± 0.79 µA/cm2 (P = 0.029). Conclusions: The ISC and TER can be recorded from the mouse RPE for 3 hours. Adrenergic and purinergic receptors affect murine RPE ion transport. Sodium-potassium adenosine triphosphatase plays a role in net ion transport across mouse RPE, and Na-K-2Cl cotransporter activity partly accounts for transepithelial ion transport. Mimicking light-induced changes, low subretinal [K+]o increases ion transport transiently, dependent on K+ channels.