Epinephrine-induced increases in [Ca2+](in) and KCl-coupled fluid absorption in bovine RPE.

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 (36)Cl, (86)Rb (K substitute), and (22)Na were simultaneously determined in paired tissues from the same eye mounted in modified Ussing flux chambers. Radioisotopes (5-7 microCi) 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 alpha(1)-adrenoreceptor antagonist prazosin (1 microM) and blocked by CPA(5 microM), an inhibitor of endoplasmic reticulum Ca2+-adenosine triphosphatases (ATPases). Apical epinephrine (100 nM) more than doubled the net Cl absorption rate, increased net K ((86)Rb) 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 microM). CONCLUSIONS Epinephrine increased fluid absorption across bovine RPE by activating apical membrane alpha(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.