Modulation of myo-[³H]inositol uptake by glucose and sorbitol in cultured bovine lens epithelial cells: II. Characterization of high- and low-affinity myo-inositol transport sites

Myo-[³H]inositol accumulation in cultured bovine lens epithelial cells (BLECs) occurred by high- and low-affinity, Na⁺-dependent transport sites. The high- and low-affinity transport systems had a Km of 27 ± 4 and 157 ± 22 μmol/l, respectively, and V(max) of 652 ± 35 and 2952 ± 308 pmol/mg protein/hr, respectively. The uptake of myo-[³H]inositol was lowered after chronic (20 hr) incubation of cultured cells in 40 mmol/l glucose throughout the concentration range for 1.5-400 μmol/l myo-inositol. The coadministration of sorbinil (0.1 mmol/l) to 40 mmol/l glucose partially prevented the inhibitory effect of glucose on myo-[³H]inositol uptake. Although the aldose reductase inhibitor prevented the inhibitory effect of glucose on the low-affinity transport site, a glucose-sensitive process for myo-[³H]inositol uptake on the high-affinity transport site was uncovered by Lineweaver-Burk analysis. Acute exposure (3 hr) of cultured BLECs maintained in physiologic medium (Eagle's minimal essential medium, 5.5 mmol/l glucose) to a range of 5.5-44 mmol/l glucose plus sorbinil also caused a decrease in myo-[³H]inositol uptake. Dixon plot analysis confirmed that the acute effect of glucose was the result of competitive inhibition of the high-affinity myo- inositol transport site. Acute exposure of cultured cells to 10-40 mmol/l sorbitol also diminished the accumulation of myo-[³H]inositol. Dixon plot analysis established that the acute effect of exogenous sorbitol was the result of competitive inhibition of the low-affinity myo-inositol transport site. These data support the contention that myo-inositol transport in BLECs is maintained by at least two processes: a glucose-sensitive, relative sorbitol insensitive, sodium-dependent, high-affinity myo-inositol transport system; and a sorbitol (aldose reductase) sensitive, sodium-dependent, low- affinity myo-inositol transport system.