[Carbonic anhydrase and its function in ion transport in cultivated pigmented ciliary body epithelial cells]

The effect of inhibitors of the carbonic anhydrase on aqueous humor formation suggests that carbonic anhydrase has an important role in ion transport across the ciliary epithelium. We therefore investigated the role of carbonic anhydrase in Na+ and Cl- transport in cultured bovine pigmented ciliary epithelial cells (PE) using the radioactive isotopes 22Na and 36Cl. Our findings can be summarized as follows. (1) Na+ uptake into the cell occurs via an amiloride-sensitive Na+/H+ exchanger. (2) Cl- is transported into the cell in exchange for bicarbonate ions. (3) PE cells contain biochemical carbonic anhydrase activity. (4) Na+ uptake into NaCl-depleted cells is markedly stimulated by Cl-. This stimulation is HCO3- dependent and is completely blocked by amiloride and partly inhibited by the carbonic anhydrase inhibitor methazolamide. (5) A model is introduced for transport of NaCl in the ciliary epithelium: carbonic anhydrase is responsible for coupling of Na+/H+ exchange and Cl-/HCO3- countertransport. This model could explain the effect of carbonic anhydrase inhibitors on intraocular pressure.