MYOINOSITOL TRANSPORT AND METABOLISM IN FETAL-BOVINE AORTIC ENDOTHELIAL-CELLS

The myo-inositol transport system in confluent fetal-bovine aortic end othelial cells was characterized after 7-10 days in subculture, at whi ch time the myo-inositol levels and rates of myo-[2-H-3]-inositol upta ke and incorporation into phospholipid had reached steady state. Kinet ic analysis indicated that the uptake occurred by both a high-affinity transport system with an apparent K(t) of 31 muM and V(max) of 45 pmo l/min per mg of protein, and a non-saturable low-affinity system. Upta ke was competitively inhibited by phlorrhizin, with a K(i) of 50 muM; phloretin was a non-competitive inhibitor, with half-maximal inhibitio n between 0.2 and 0.5 mM. Glucose was a weak competitive inhibitor, wi th a K(i) of 37 mM; galactose failed to inhibit uptake. A weak depende nce on Na+ for the initial rate of uptake was observed at 11 muM myo-i nositol. When fetal-bovine-serum (FBS)-supplemented medium, which cont ained 225 muM myoinositol, was used, the cells contained about 200 nmo l of myoinositol/mg of DNA. With adult-bovine-serum (ABS)supplemented medium, which contained 13 muM myo-inositol, the cells contained about 110 nmol/mg of DNA. Transport of 11 muM myo-[2-H-3]inositol was 18 an d 125 pmol/min per mg of DNA for cells grown in FBS and ABS respective ly. Kinetic analysis showed that for the cells grown in FBS the V(max) of the high-affinity system was decreased by 64%, whereas the K(t) re mained essentially unchanged. Increased cell myo-inositol levels were not associated with an increased rate of phosphatidylinositol synthesi s. After prolonged exposure of fetal endothelial cells to a myo-inosit ol concentration which approximated to a high fetal as opposed to a lo w adult blood level, cell myo-inositol levels doubled and high-affinit y transport underwent down-regulation.