Characterization of a Myo-Inositol (MI) efflux pathway

Purpose. To establish an association between hypertonicity, volume regulation, MI efflux and chloride channels. Methods. Cultured bovine lens epithelial cells (BLECs) respond to chronic sodium hypertonicity with the accumulation of intracellular MI. Utilizing BLECs acclimated to high osmolality and responding to a decrease in medium osmolality, a tonicity-activated mechanism for the release of MI was identified. Alternatively, using BLECs maintained in hypertonicity and hypergalactosemia, a polyol-activated mechanism for MI efflux was recognized. Using a partial cDNA encoding a swelling-induced, chloride channel from human nonpigmented ciliary epithelial cells (a gift from M. Coca-Prados), the mRNA abundance from cultured BLECs under hypertonic and isotonic conditions was determined by Northern blot analysis. Results. The chloride channel blocker, niflumic acid, as did Cl^- substitution, significantly inhibited both the tonicity-activated and polyol-activoted release of MI from cell to medium, suggesting that inositol efflux through either route is interactively associated with chloride movement. Northern blot analysis of BLEC mRNA hybridized to the cDNA probe showed that a specific BLEC chloride channel mRNA increased when secondary cultures were exposed to sodium hypertonic medium (473±6 mosm) over 6 hours. BLECs maintained in sodium hypertonic medium for 12 hours and rapidly transferred to isotonic medium (257±2 mosm) showed that the chloride channel mRNA significantly decreased during 12 hours. Conclusions. Both the tonicity-activated and polyol-activated loss of MI from cell to medium appear to be moderated via a common, niflumic acid-sensitive, chloride channel whose gene expression is responsive to extracellular osmotic fluctuations.