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.) |