Glucose transport across isolated bovine pigment epithelium

Glucose transport across the isolated bovine pigment epithelium was studied in vitro by mounting the tissue in a chamber and exposing the apical and basal surfaces to separate solutions. Analysis of the total unidirectional glucose fluxes, for both directions, indicates that a carrier-mediated transport system exists with a significant net flux in the choroid to retina direction. The apparent kinetic constants of carrier-mediated glucose transport are: K_m = 9·5 mm, V_max = 0·71 μmol/cm²/hr for the retina to choroid direction, and K_m = 27 mm, V_max = 2·1 μmol/cm²/hr for the choroid to retina direction. Comparison of the kinetic parameters show that only the maximum velocities were significantly different. The total unidirectional glucose fluxes were insensitive to phloridzin (0·1 mm), sodium reduction (50%) and potassium reduction (90%). Phloretin (0·1 mm) and calcium reduction (90%) were effective in reducing the total unidirectional flux significantly in both directions. These findings suggest that glucose transport across bovine pigment epithelium involves a carrier-mediated transport system, and calcium is an effective modulator of the unidirectional glucose flux.