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

To study the physiological mechanisms of the Cl transport across the bovine ciliary body-epithelium (CBE).

Fresh isolated bovine CBE was mounted in an Ussing-type chamber. The effects of ion substitution and transport inhibitors on electrical measurements and Cl transport were investigated.

The potential difference (PD) across the preparation was 0.55 +/- 0.04 mV and was consistently negative at the aqueous side. The short-circuit current (SCC) and tissue resistance (Rt) were found to be 8.0 +/- 0.7 microA/cm2 and 72 +/- 3 omega/cm2, respectively. Both the PD and the SCC of the bovine CBE were found to depend primarily on the concentration of the Cl bath and to a lesser extent on the Na or HCO3 concentration. At 30 mM Cl, the polarity of the PD and the direction of the SCC were reversed. Reducing the extracellular Na or Cl concentration abolished the net Cl transport into 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 Cl transport. Bumetanide, when applied to either side, inhibited the Cl transport. The effect, however, was three times greater 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 Cl transport by 80% and 92%, respectively.

The results suggest that the major Cl influx pathway at the basolateral border in pigmented epithelial cells is through the Na-K-2Cl cotransporter, but not the Cl/HCO3 and Na/H double exchangers. Intercellular gap junctions between the two cell layers and the NPPB-sensitive Cl channels at the basolateral surface in nonpigmented epithelial cells also play a crucial role in regulating the Cl movement across the functional syncytium.