UP-REGULATION OF MYOINOSITOL TRANSPORT COMPENSATES FOR COMPETITIVE-INHIBITION BY GLUCOSE - AN EXPLANATION FOR THE INOSITOL PARADOX

High glucose concentrations inhibit the uptake of myo-inositol into ce lls. However, whether this leads to a depletion of intracellular myo-i nositol levels has been debated, because unchanged, decreased, and inc reased cellular myo-inositol levels all have been reported for diabeti c tissues. To evaluate whether cells are capable of counterregulating impaired uptake, we have investigated myo-inositol uptake in porcine a ortic endothelial cells under short- and long-term hyperglycemic condi tions. 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. Th e 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 ineffecti ve. Lineweaver-Burk analysis revealed an increased V(max) for the indu ced myo-inositol transport activity, whereas the K(m) for myo-inositol remained constant (18 muM). The upregulated inositol transporter was still Na+ and ATP dependent, indicating that the same carrier system w as operating. Uptake returned to control values when cells were again exposed to normoglycemic medium conditions for an additional 24 h. Whe n endothelial cells were incubated with D-[U-C-14]glucose and 10 muM m yo-[2-H-3]inositol of equal specific radioactivity for 24 h, no C-14 r adioactivity was found in intracellular myo-inositol, indicating that conversion of glucose to myo-inositol was rather low. A pulse-chase ex periment with myo-[2-H-3]inositol revealed that intracellular myo-inos itol was rather long lived, underlining the important role of uptake f or cellular myo-inositol content. These results suggested that endothe lial cells responded to competitive inhibition by upregulation of tran sport activity, thereby compensating the inhibitory effect of glucose. Accordingly, no decreases of endothelial phosphatidylinositol could b e found during incubation with the high glucose medium. The findings o ffer an explanation for the unchanged or even paradoxical increases of tissue myo-inositol observed in diabetic tissues.