Osmoregulatory alterations in myo-inositol uptake by bovine lens epithelial cells. Part 1: A hypertonicity-induced protein enhances myo-inositol transport.

PURPOSE The nature of the association between attenuated myo-inositol-concentrating capability, intracellular polyol accumulation, and hypertonicity-enhanced myo-inositol uptake was investigated in cultured bovine lens epithelial cells (BLECs) exposed to high ambient galactose. METHODS The kinetic characteristics of myo-inositol accumulation based on the measurement of in vitro myo-[3H]inositol (3H-MI) uptake was determined with cultured BLECs incubated in either high ambient galactose or galactose-free, physiological medium under experimental conditions that included both aldose reductase inhibition and elevation of extracellular osmotonicity. RESULTS The uptake of 3H-MI was lowered after chronic (20 hour) preincubation of cultured BLECs in 40 mmol/l galactose (i.e., conditions that would favor galactitol synthesis) compared with control cells in galactose-free, physiological medium. Acute exposure (3 hours) of cultured BLECs to a range of 10 to 40 mmol/l galactitol or 5.5 to 44 mmol/l galactose plus the aldose reductase inhibitor (ARI), sorbinil, established by Dixon plot that galactitol, but not galactose, inhibited both the high- and the low-affinity MI transport sites. MI uptake was markedly stimulated in cultured BLECs exposed to galactose-free, hyperosmotic medium by the addition of extracellular raffinose, mannitol, or sorbitol for 20 hours. The enhanced uptake involved increase in the maximal velocity without significant change in Km of both the high- and low-affinity MI transport sites, as indicated by Lineweaver-Burk analysis. However, a similar coadministration of 150 mmol/l sorbitol to the 40 mmol/l galactose (Gal) medium significantly increased, but failed to normalize, the MI uptake relative to that observed with galactose-free, physiological medium. The combined administration of 150 mmol/l sorbitol and the ARI, zopolrestat, to Gal significantly exceeded the MI uptake observed with physiological medium. Exposure of BLECs to cycloheximide for 20 hours did not affect MI uptake in cells maintained in 40 mmol/l galactose but inhibited the otherwise enhanced MI uptake in cells maintained in Gal plus 150 mmol/l sorbitol and zopolrestat in the omission versus the inclusion of cycloheximide. CONCLUSIONS These results suggest that bovine lens epithelial cells respond to hypertonic stress by elevating myo-inositol transport activity. The increase in MI uptake is due to an increase in the number (or, possibly, a change in the transporter turnover rate) of high- and low-affinity, sodium-dependent MI transporters expressed as a result of the osmotic shock stemming from exposure to hypertonic medium.