Restoration of Na+-K+ Pump Activity and Resting Membrane Potential by myo-Inositol Supplementation in Neuroblastoma Cells Chronically Exposed to Glucose or Galactose

Abstract

myo-Inositol uptake by culture neuroblastoma cells at a concentration of myo-inositol <50 μM was largely Na+ dependent. Exposing neuroblastoma cells to media supplemented with increasing concentrations of myo-inositol resulted in an increase in myo-inositol accumulation and intracellular content, but myo-inositol incorporation into phospholipids was not increased. The data indicate that myo-inositol exists as separate pools in neuroblastoma cells, and one or more of these pools may contribute to phospholipid synthesis. Exposing neuroblastoma cells to an increased concentration of glucose caused a decrease in myo-inositol uptake by two separate mechanisms. Acute exposure of the cells to 30 mM glucose caused a myo-inositol concentration–dependent decrease in Na+-dependent myo-inositol uptake. We propose that the acute inhibition of myo-inositol uptake by glucose is likely due to a competitive type of inhibition. Chronic exposure of cells to media containing 30 mM glucose or 30 mM galactose also caused decreases in myo-inositol uptake and incorporation into inositol phospholipids and intracellular myo-inositol content. This decrease in myo-inositol metabolism persisted at a higher concentration of external myo-inositol than the acute inhibition. Supplementing media containing 30 mM glucose or 30 mM galactose with 250 μM myo-inositol restored myo-inositol metabolism and content. The inhibition of myo-inositol uptake by cells chronically exposed to increased concentrations of glucose or galactose was due to a noncompetitive type of inhibition that was blocked by the addition of sorbinil. Chronic exposure of neuroblastoma cells to media containing 30 mM glucose or 30 mM galactose caused a decrease in Na+-K+-ATPase transport activity and resting membrane potential. Both of these abnormalities were corrected by supplementing the media with 250 μM myo-inositol. myo-lnositol depletion in neuroblastoma cells exposed to increased concentrations of glucose may occur by two different mechanisms and may be responsible for alterations in Na+-K+-ATPase transport activity and resting membrane potential.