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 intracel lular metabolism by two putative mechanisms: reciprocal osmoregulatory depletion of other organic osmolytes e.g., myo-inositol, and alterati ons 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 di fferent mechanisms: the ''osmolyte'' hypothesis,ia depletion of intrac ellular myo-inositol (the cosubstrate for phosphatidylinositolsynthase ) and the ''redos'' hypothesis through enhanced de novo synthesis from triose phosphates. The osmolyte hypothesis predicts diminished phosph oinositide-derived arachidonyldiacylglycerol, while the redox hypothes is predicts increased total diacylglycerol and phosphatidic acid. In h igh aldose reductase expressing retinal pigment epithelial cells, gluc ose-induced, aldose reductase inhibitor-sensitive CDP-diglyceride accu mulation and inhibition of P-32-incorporation into phosphatidylinosito l paralleled myo-inositol depletion (but not cytoplasmic redox, that w as unaffected bg glucose) and depletion of arachidonyl-diacylglycerol. 3 mM pyruvate added to the culture medium left cellular redox unalter ed, but stimulated Na+-dependent myo-inositol uptake, accumulation, an d incorporation into phosphatidylinositol. These results favor myo-ino sitol depletion rather than altered redox as the primary cause of gluc ose-induced aldose reductase-related defects in phospholipid metabolis m in cultured retinal pigment epithelial cells.