The effects of elevated glucose on Na⁺/K⁺-ATPpase of cultured bovine retinal pigment epithelial cells measured by a new nonradioactive rubidium uptake assay

The effects of simulated hyperglycemia on the Na⁺/K⁺-ATPase activity of cultured bovine retinal pigment epithelial (RPE) cells were investigated. Total Rb⁺ uptake, measured by a chromatographic method, was decreased 20- 30% by 55.5 mM glucose relative to 5.55 mM glucose for culture periods of 2 to 28 days. An acute hyperglycemic stress (<1 week) had no effect on ouabain inhibition of Rb⁺ uptake or ouabain binding to RPE cells (IC₅₀ = 55 nM for both processes) and did not alter the IC₅₀ value (near 10 nM) for binding of strophanthidin, another selective Na⁺/K⁺-ATPase inhibitor. A small increase in the apparent K(m) of Rb⁺ for Na⁺/K⁺-ATPase accompanied the decrease in maximal Rb⁺ uptake at 55.5 mM glucose. The continuous presence of AL-1576, an aldose reductase inhibitor (ARI), normalized the effect of severe hyperglycemia on Rb⁺ uptake in the chronic (28 days) but not the acute exposure protocols. Thus, decreased efficiency of Na⁺/K⁺-ATPase caused by chronic accumulation of intracellular sorbitol can account for previously reported functional and structural alterations in the RPE cell layer of diabetic rodents. The results of the present study suggest that hyperglycemia-induced loss of Na⁺/K⁺-ATPase function in RPE cells, which responds to aldose reductase inhibitor treatment, contributes to the pathogenesis of diabetic retinopathy.