Osmoregulatory alterations in myo-inositol uptake by bovine lens epithelial cells: Part 5: Mechanism of the myo-inositol efflux pathway

Purpose. Cultured bovine lens epithelial cells (BLECs) exposed to sodium hypertonicity respond with an accumulation of intracellular myo-inositol. Using BLECs initially maintained at hypertonicity and reacting to a decrease in medium osmolality, a mechanism for the tonicity-activated release of myo- inositol was recognized. Alternatively, BLECs acclimated to sodium hypertonicity and subsequently transferred to high sodium osmolality plus hypergalactosemia rapidly accumulate intracellular galactitol, an experimental manipulation that permitted characterization of the role of sugar alcohols in polyol-activated myo-inositol efflux. The authors identify a communal transport route for tonicity-activated and polyol-activated myo- inositol release from cell to medium and demonstrate an association for myo- inositol efflux with chloride movement. Methods. Two distinct experimental approaches were designed to delineate the physiological circumstances that initiate myo-inositol efflux. For tonicity-induced inositol efflux, BLECs were maintained at confluence in sodium hypertonic medium (473 ± 6 mOsm) for 48 hours; afterward, the medium was replaced with isotonic medium (285 ± 4 mOsm) containing 40 mM galactose ± Sorbinil. For polyol-induced inositol release, hypertonically adapted BLECs were transferred to fresh sodium hypertonic medium containing 40 mM galactose (513 ± 10 mOsm). Results. On reduction in medium osmolality, intracellular myo-inositol was lost because of a rapid, transient efflux during the first 30 minutes, which was followed by a slow, sustained decrease in efflux during the next 12 hours. Inhibition of aldose reductase activity substantially diminished myo-inositol efflux from cell to galactose-containing, isotonic medium. Administration of phloretin significantly inhibited both tonicity-activated and polyol- activated myo-inositol release, as did the chloride channel blocker, niflumic acid. Conclusions. In cultured bovine lens epithelial cells, tonicity- activated movement of myo-inositol from cell to medium and myo-inositol efflux as induced by intracellular polyol accumulation appear to be interactively associated with chloride movement and moderated by a common anionic (chloride) channel, carrier-mediated transport protein, or both.