Osmoregulatory alterations in myo-inositol uptake by bovine lens epithelial cells. Part 4: Induction pattern of Na⁺-myo-inositol cotransporter mRNA under hypertonic conditions denoting an early-onset, interactive, protective mechanism against water stress

Purpose. To examine the effect of hypertonicity on the induction of the Na⁺-myo-inositol (Na⁺-MI) cotransporter(s) in cultured bovine lens epithelial cells (BLECs). Methods. Na⁺-MI cotransporter 626-bp reverse transcription-polymerase chain reaction product amplified from lens cell RNA and aldose reductase (AR) cDNA probes were used to measure respective mRNA content by Northern blot analysis. Results. Northern blot analysis of BLEC mRNA hybridized to Na⁺-MI cotransporter cDNA showed that Na⁺-MI cotransporter mRNA increased when secondary cultures of BLECs were exposed to physiological medium supplemented with 116 mmol/l NaCl. A time course further revealed a maximal increase in Na⁺-MI cotransporter mRNA by 8 hours. Thereafter, the level of Na⁺-MI cotransporter mRNA steadily declined for the duration of the 72-hour incubation period despite continuous exposure of BLECs to hypertonicity. AR mRNA levels maximally increased by 24 h of cell exposure to hypertonic condition. Unlike Na⁺-MI cotransporter mRNA, AR mRNA remained elevated throughout the duration of the experiment. Hypertonic exposure resulted in a steady state accumulation of myo-inositol and sorbitol for 6 days. Inhibition of sorbitol formation prompted the intracellular myo- inositol content to a higher level. Conclusions. These data suggest that enhanced MI transport and accumulation, as an adaptive osmoregulatory response to hypertonicity in cultured BLECs, is a primary, early-onset, protective mechanism against water stress, succeeded by, enhanced sorbitol formation and accumulation, a secondary, late-onset protective mechanism. The lens appears to respond to the preliminary stages of hyperosmotic stress by induction of Na⁺-MI cotransporter mRNA, indicating that the myo-inositol carrier protein(s) play an initial responsive role in the management of osmotic stress. Lens water stress management is interactive because myo- inositol and sorbitol levels are regulated in concert.