ACIDIFICATION STIMULATES CHLORIDE AND FLUID ABSORPTION ACROSS FROG RETINAL-PIGMENT EPITHELIUM

Radioactive tracers and a modified capacitance-probe technique were used to characterize the mechanisms that mediate Cl and fluid absorption across the bullfrog retinal pigment epithelium (RPE)-choroid. In control (HCO3/CO2) Ringer solution, Cl-36 was actively absorbed (retina to choroid) at a mean rate of 0.34 mu eq.cm(-2).h(-1) (n = 34) and accounted for similar to 25% of the short-circuit current. Apical bumetanide (100 mu M) or basal 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS; 1 mM) inhibited active Cl transport by 70 and 62%, respectively. Active Cl absorption was doubled, either by removing HCO3 from the bathing media or by elevating CO2 from 5 to 13%, and the increased flux was inhibited by apical bumetanide or basal DIDS. Open-circuit measurements of fluid absorption rate (J(v)) and the net fluxes of Cl-36, Na-22, and Rb-86 (K substitute) indicated that CO2-induced acidification stimulated NaCl and fluid absorption across the RPE. During acidification, bumetanide produced a twofold larger inhibition of J(v) compared with control. Stimulation of net Cl absorption was most likely caused by inhibition of the the basolateral membrane intracellular pi-I-dependent Cl-HCO3 exchanger.