Kinetic evidence for Na+-glucose co-transport in the pigmented rabbit conjunctiva

Abstract

PURPOSE. To obtain kinetic evidence for the existence of a Na + -coupled glucose co-transport process on the mucosal (tear) side of the pigmented rabbit conjunctiva. METHODS. The excised pigmented rabbit conjunctiva was mounted in the modified Ussing chamber for measurement of 22 Na and 3 H-3-O-methyl-D-glucose (3-O-MG) fluxes. RESULTS. In the presence of 5 mM glucose, the conjunctival tissue showed net Na + absorption in the mucosal-to-serosal direction at an approximate rate of 0.15 µEq/cm 2 /h. This net Na + absorption was abolished by serosally added 0.5 mM ouabain, but not affected by mucosally added 0.1 mM or 1 mM amiloride. There was a 40–60% reduction in net Na + absorption under the glucose-free condition or in the mucosal presence of 0.5 mM phlorizin. Moreover, serosally added ouabain and mucosally added phlorizin (both at 0.5 mM) significantly decreased the 3-O-MG permeability coefficient in the mucosal-to-serosal direction by about 70%, whereas mucosally instilled 0.1–1.0 mM amiloride was without any effect. Three-O-MG absorption in the mucosal-to-serosal direction appeared to be coupled with Na + transport with a 1:1 stoichiometry. In addition, this process exhibited temperature dependency, saturability, and directionality. CONCLUSION. Our findings are consistent with Na + -glucose co-transport as being one of the mechanisms for mucosal Na + entry into the epithelial cells of the pigmented rabbit conjunctiva.