Fig. 5.

Fig. 5. Fluid secretion by rat lacrimal gland. Receptor activation stimulates K⁺ channels that extrude K⁺ across the basolateral membrane and Cl⁻ and K⁺ across the apical membrane into the lumen. Water then follows. Movement of Cl⁻ into the lumen drives cations from paracellular spaces into the lumen. Because of the decreased cellular concentrations of K⁺ and Cl⁻, the Na⁺/K⁺/2Cl⁻ cotransporter becomes active to replace intracellular K⁺ and Cl⁻. The Na⁺/H⁺ exchanger becomes active to extrude H⁺. The Cl⁻/HCO⁻3 exchanger drives Cl⁻ uptake. Na⁺-K⁺ ATPase becomes active to extrude Na⁺ and cause K⁺ re-uptake. (Reprinted from Hodges RR, Dartt DA: Regulatory pathways in lacrimal gland epithelium. Int Rev Cytol 231:129, 2003, with permission.)