EFFECTS OF GLUCOSE ON SORBITOL PATHWAY ACTIVATION, CELLULAR REDOX, AND METABOLISM OF MYOINOSITOL, PHOSPHOINOSITIDE, AND DIACYLGLYCEROL IN CULTURED HUMAN RETINAL-PIGMENT EPITHELIAL-CELLS

Sorbitol (aldose reductase) pathway flux in diabetes perturbs intracellular metabolism by two putative mechanisms: reciprocal osmoregulatory depletion of other organic osmolytes e.g., myo-inositol, and alterations in NADPH/NADP(+) and/or NADH/NAD(+). The ''osmolyte'' and ''redos'' hypotheses predict secondary elevations in CDP-diglyceride, the rate-limiting precursor for phosphatidylinositol synthesis, but through different mechanisms: the ''osmolyte'' hypothesis,ia depletion of intracellular myo-inositol (the cosubstrate for phosphatidylinositolsynthase) and the ''redos'' hypothesis through enhanced de novo synthesis from triose phosphates. The osmolyte hypothesis predicts diminished phosphoinositide-derived arachidonyldiacylglycerol, while the redox hypothesis predicts increased total diacylglycerol and phosphatidic acid. In high aldose reductase expressing retinal pigment epithelial cells, glucose-induced, aldose reductase inhibitor-sensitive CDP-diglyceride accumulation and inhibition of P-32-incorporation into phosphatidylinositol paralleled myo-inositol depletion (but not cytoplasmic redox, that was unaffected bg glucose) and depletion of arachidonyl-diacylglycerol. 3 mM pyruvate added to the culture medium left cellular redox unaltered, but stimulated Na+-dependent myo-inositol uptake, accumulation, and incorporation into phosphatidylinositol. These results favor myo-inositol depletion rather than altered redox as the primary cause of glucose-induced aldose reductase-related defects in phospholipid metabolism in cultured retinal pigment epithelial cells.