Competitive inhibition by glucose of myo-inositol incorporation into cultured porcine aortic endothelial cells

Abstract

To explore the significance of hyperglycaemia as a causal factor for the appearance of diabetic angiopathies we investigated aspects of myo-inositol metabolism in porcine aortic endothelial cells. myo-Inositol was shown to be a long-living metabolite. Its uptake into the cells was mediated by a high-affinity, Na +-dependent uptake system inhibitable by ouabain with an apparent K M of 18.6 μmol/l, which was responsible for more than 80% of total uptake at physiological myo-inositol concentrations. Inhibition of inositol uptake by d-glucose was exclusively competitive with an apparent K i of 24 mmol/l as shown by Lineweaver-Burk- and Dixon-plot analysis. The specificity of competitive inhibition was studied. l-Glucose which is stereochemically related to myo-inositol in the same way as the d-isomer proved to be an equally potent inhibitor. The hexoses d-galactose, d-mannose and d-fructose inhibited myo-inositol uptake to a minor extent. d-allose and 3-O- methyl- d-glucose had no inhibitory effect indicating that the OH-group of the carbon atom in 3 position is essential for the interaction with the carrier. The acyclic hexitol sorbitol also did not compete. As expected, the aldose reductase blocker sorbinil did not influence the carrier since there is no polyol pathway operating in porcine aortic endothelial cells. In accordance with the results of the uptake experiments, the incorporation of exogenous myo-inositol into membrane phosphatidylinositol was reduced at elevated extracellular glucose levels. The results raise the possibility that hyperglycaemia impairs endothelial inositol supply.