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Epinephrine-Induced Increases in [Ca²⁺]_in and KCl-Coupled Fluid Absorption in Bovine RPE

² From the School of Optometry and the ¹ Department of Molecular and Cell Biology, University of California, Berkeley.

PURPOSE. To define the ionic basis for the apical epinephrine-induced increase of fluid absorption (J_V) across isolated bovine RPE–choroid.

METHODS. Epinephrine-induced changes in RPE [Ca²⁺]_in levels were monitored with the ratioing dye fura-2. Transepithelial potential, resistance, and unidirectional fluxes of ³⁶Cl, ⁸⁶Rb (K substitute), and ²²Na were simultaneously determined in paired tissues from the same eye mounted in modified Üssing flux chambers. Radioisotopes (5–7 µCi) were added to the apical bath of one tissue and the basal bath of the other, and the appearance of label in the opposite bath was measured.

RESULTS. Apical epinephrine (100 nM) transiently increased [Ca²⁺]_in by 153 ± 78 nM. This increase was inhibited by the ₁-adrenoreceptor antagonist prazosin (1 µM) and blocked by CPA(5 µM), an inhibitor of endoplasmic reticulum Ca²⁺-adenosine triphosphatases (ATPases). Apical epinephrine (100 nM) more than doubled the net Cl absorption rate, increased net K (⁸⁶Rb) absorption by fivefold, and tripled net fluid absorption (J_V), as predicted by isotonic coupling between ion and fluid transport. The epinephrine-induced increases in ion and fluid transport were completely inhibited by apical bumetanide (100 µM).

CONCLUSIONS. Epinephrine increased fluid absorption across bovine RPE by activating apical membrane ₁-adrenergic receptors, increasing [Ca²⁺]_in, and stimulating bumetanide-sensitive Na,K,2Cl uptake at the apical membrane and KCl efflux at the basolateral membrane.

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