Chloride secretion by bovine ciliary epithelium: A model of aqueous humor formation

PURPOSE. TO study the physiological mechanisms of the Cl transport across t he bovine ciliary body-epithelium (CBE). METHODs. Fresh isolated bovine CBE was mounted in an Ussing-type chamber. T he effects of ion substitution and transport inhibitors on electrical measu rements and Cl transport were investigated. RESULTS. The potential difference (PD) across the preparation was 0.55 +/- 0.04 mV and was consistently negative at the aqueous side. The short-circui t current (SCC) and tissue resistance (R-t were found to be 8.0 +/- 0.7 mu A/cm(2) and 72 +/- 3 Omega/cm(2), respectively. Both the PD and the SCC of the bovine CBE were found to depend primarily on the concentration of the C I bath and to a lesser extent on the Na or HCO, concentration. At 30 mM Cl, the polarity of the PD and the direction of the SCC were reversed. Reducin g the extracellular Na or Cl concentration abolished the net C1 transport i nto the eye under the short-circuited condition. Bilateral bumetanide (0.1 mM), but not 4,4 '-diisothiocyanatostilbene-2-2 '-disulfonic acid (DIDS; 0. 1 mM), greatly inhibited the CI transport. Bumetanide, when applied to eith er side, inhibited the CI transport. The effect, however, was three times g reater on the stromal side than on the aqueous surface. Bilateral heptanol (3.5 mM) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB; 0.1 mM) on the aqueous side also inhibited the Ci transport by 80% and 92%, respectiv ely. CONCLUSIONS. The results suggest that the major C1 influx pathway at the ba solateral border in pigmented epithelial cells is through the Na-K-2Cl cotr ansporter, but not the Cl/HCO3 and Na/H double exchangers. Intercellular ga p junctions between the two cell layers and the NPPB-sensitive CI channels at the basolateral surface in nonpigmented epithelial cells also play a cru cial role in regulating the C1 movement across the functional syncytium.