Osmoregulatory Alterations in Myo-inositol Uptake by Bovine Lens Epithelial Cells. III. Effects of Cycloheximide and Colchicine on Na +-Myo-inositol Cotransporter Activity Under Hypertonic Conditions, Inhibition of a Plasma Membrane Osmotic Stress Protein

Abstract

Cultured bovine lens epithelial cells adapt to hypertonic sodium stress via an increase in Na +-myo-inositol cotransporter activity and accumulate myo-inositol. At least 12 hr of hypertonic exposure was necessary to enhance myo-inositol accumulation; and thereafter, uptake activity continued to increase throughout the duration of a 72-hr exposure period. Switching from hypertonic to isotonic medium for 24 hr reversed the otherwise elevated accumulation activity. The protein synthesis inhibitor, cycloheximide, did not affect myo-inositol uptake in isotonic medium but markedly decreased myo-inositol uptake in hypertonic medium. Cells exposed to hypertonic conditions and the microtubule disrupter, colchicine, similarly showed marked impairment of the otherwise enhanced myo-inositol uptake. These studies indicated that hypertonicity-induced elevation of Na +-myo-inositol cotransporter activity in cultured bovine lens epithelial cells is not solely attributed to the increased sodium gradient alone, but rather involves increased de novo synthesis of the Na +-myo-inositol cotransporter protein(s).