OSMOREGULATORY ALTERATIONS IN MYOINOSITOL UPTAKE BY BOVINE LENS EPITHELIAL-CELLS .5. MECHANISM OF THE MYOINOSITOL EFFLUX PATHWAY

Purpose. Cultured bovine lens epithelial cells (BLECs) exposed to sodi um hypertonicity respond with an accumulation of intracellular myo-ino sitol. Using BLECs initially maintained at hypertonicity and reacting to a decrease in medium osmolality, a mechanism for the tonicity-activ ated release of myo-inositol was recognized. Alternatively, BLECs accl imated to sodium hypertonicity and subsequently transferred to high so dium osmolality plus hypergalactosemia rapidly accumulate intracellula r galactitol, an experimental manipulation that permitted characteriza tion of the role of sugar alcohols in polyol-activated myo-inositol ef flux. The authors identify a communal transport route for tonicity-act ivated and polyol-activated myo-inositol release from cell to medium a nd demonstrate an association for myo-inositol efflux with chloride mo vement. 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 mO sm) containing 40 mM galactose +/- Sorbinil. For polyol-induced inosit ol release, hypertonically adapted BLECs were transferred to fresh sod ium hypertonic medium containing 40 mM galactose (513 +/- 10 mOsm). Re sults. On reduction in medium osmolality, intracellular myo-inositol w as lost because of a rapid, transient efflux during the first 30 minut es, which was followed by a slow, sustained decrease in efflux during the next 12 hours. Inhibition of aldose reductase activity substantial ly diminished myo-inositol efflux from cell to galactose-containing, i sotonic medium. Administration of phloretin significantly inhibited bo th tonicity-activated and polyol-activated myo-inositol release, as di d the chloride channel blocker, niflumic acid. Conclusions. In culture d bovine lens epithelial cells, tonicity-activated movement of myo-ino sitol from cell to medium and myo-inositol efflux as induced by intrac ellular polyol accumulation appear to be interactively associated with chloride movement and moderated by a common anionic (chloride) channe l, carrier-mediated transport protein, or both.