Prolonged incubation with elevated glucose inhibits the regulatory response to shrinkage of cultured human retinal pigment epithelial cells.

Abstract

Transport defects by retinal pigment epithelial (RPE) and other cells are observed in experimental models of diabetes mellitus. Recent studies have established that glucose concentration, per se, is the critical risk factor in the pathogenesis of diabetic complications. This study was designed to test whether transport alterations could be produced in the simplest model of diabetes, sustained exposure of cultured cells to a high-glucose environment. The regulatory transport responses to acute changes in cell volume were measured in order to assess the effects of glucose on a range of transport processes. Continuous lines of nontransformed human retinal pigment epithelial (hRPE) cells were grown for two weeks with either 5.6 low glucose (LG) or 26.0 high glucose (HG) mM in paired experiments. The cell volumes of suspended cells were studied in hypo-, iso- and hypertonic solutions containing the same ionic composition. Hypotonic swelling triggered a regulatory volume decrease (RVD), inhibited by reducing the chemical driving force for K+ efflux, or blocking K+ channels (with Ba2+) or Cl- channels (with NPPB). Thus, the RVD of the hRPE cells likely reflects efflux of K+ and Cl- through parallel channels. Shrinkage caused a regulatory volume increase (RVI), which was inhibited by blocking Na+/H+ (with dimethylamiloride) or Cl-/HCO3- exchange (with DIDS). Bumetanide inhibited the RVI significantly only when the K+ concentration was increased above the baseline level. Therefore, the RVI under our baseline conditions likely reflects primarily Na+/H+ and Cl-/HCO3- antiport exchange. Growth in high-glucose medium had no substantial effect on the RVD, but reduced the rate constant of the RVI by approximately 50%. The RVI was unaffected by growth in high-mannitol medium. Stimulation of protein kinase C (PKC) with DiC8 increased the RVI of HG-cells, but not of LG-cells. The DiC8-induced stimulation was bumetanide insensitive and abolished by 1 mM amiloride. Other transport effects of PKC (on the RVD) were unaltered in the HG-cells. We conclude that sustained elevation of extracellular glucose, per se, can downregulate the Na+/H+ antiport of target cells, an effect noted in streptozotocin-treated rats, and that this downregulation does not reflect interruption of the PKC-signaling pathway.