Uptake studies with ²²Na and ³⁶C1 were performed in cultured bovine pigmented ciliary epithelial cells (PE) to investigate interdependence of Na⁺ and CI⁻ transport.

(1) ²²Na uptake into NaCl depleted cells was stimulated by CI⁻. This stimulation was abolished by the simultaneous application of amiloride (1 mM) and bumetanide (0.1 mM), indicating two independent mechanism for CI⁻ stimulated Na⁺ uptake: loop diuretic sensitive Na⁺/Cl⁻ symport and an indirect stimulation of Na⁺/H⁺ exchange by CI⁻. The latter component of CI⁻ stimulated Na⁺ uptake was HCO3⁻ dependent.

(2) ³⁶C1 uptake was increased by extracellular Na⁺. Na⁺-stimulated CI⁻ uptake also consisted of two components. One was bumetanide sensitive and the other was blockable by amiloride and partly inhibited by the carbonic anhydrase (CA) inhibitor methazolamide (0.1 mM).

(3) Homogenized PE cells were tested for biochemical CA activity using an electrometric method. The cytoplasmic as well as the membrane fraction contained specific CA activity.

(4) A model is presented for Na⁺ and Cl⁻ transport into PE: in addition to Na⁺/Cl⁻ symport, Na⁺/H⁺ and CI-/HCO³⁻ double exchange may operate in the ciliary epithelium. The latter mechanism provides NaCl uptake into the cell in exchange for H⁺ and HCO3-, which recycle as CO2 across the membrane. This recycling of CO2 and HCO3-/H⁺ (and hence indirectly NaCl uptake) is facilitated by the cooperation between membrane bound and cytoplasmic CA.