Upregulation of myo-Inositol Transport Compensates for Competitive Inhibition by Glucose: An Explanation for the Inositol Paradox?

High glucose concentrations inhibit the uptake of myo-inositol into cells. However, whether this leads to a depletion of intracellular myo-inositol levels has been debated, because unchanged, decreased, and increased cellular myo-inositol levels all have been reported for diabetic tissues. To evaluate whether cells are capable of counterregulating impaired uptake, we have investigated myo-inositol uptake in porcine aortic endothelial cells under short- and long-term hyperglycemic conditions. Although increasing glucose concentrations inhibited acute myo-inositol uptake competitively, the uptake was increased markedly, when cells were already preincubated in a high glucose medium for >6 h. The stimulation was maximal at 20 mM of glucose with no further increase at 40 mM glucose. The same stimulation of uptake could be induced by 5 mM of glucose plus 35 mM of raffinose, whereas 35 mM of sorbitol or mannitol, which do not compete for myo-inositol uptake, were ineffective. Lineweaver-Burk analysis revealed an increased V_max for the induced myo-inositol transport activity, whereas the K_m for myo-inositol remained constant (18 μM). The upregulated inositol transporter was still Na⁺ and ATP dependent, indicating that the same carrier system was operating. Uptake returned to control values when cells were again exposed to normoglycemic medium conditions for an additional 24 h. When endothelial cells were incubated with D-[U-¹⁴C]glucose and 10 μM myo-[2-³H]inositol of equal specific radioactivity for 24 h, no ¹⁴C radioactivity was found in intracellular myo-inositol, indicating that conversion of glucose to myo-inositol was rather low.