Effect of Elevated Glucose Concentration on Membrane Voltage Regulation in Retinal Capillary Pericytes

The influence of elevated glucose concentration on resting membrane voltage, electrogenic Na⁺-K⁺-ATPase, and ATP-sensitive potassium channels (K_ATP channels) was studied in cultured bovine retinal capillary pericytes using conventional microelectrodes. The resting membrane voltage in cells grown in medium containing 5 mM glucose (control) averaged –27 ± 1.2 mV (mean ± SE, n = 26) and was not different from cells grown in medium containing 22.5 mM glucose (–26 1.2 mV, n = 26). Addition of ouabain (10⁻⁴ M), a specific inhibitor of the Na⁺-K⁺-ATPase, depolarized the membrane potential by 3.6 ± 0.4 mV (n = 10) in cells grown under control conditions and 0.7 ± 0.2 mV (n = 6) in cells grown under elevated glucose conditions. Thus, electrogenic activity of the Na⁺-K⁺-ATPase was significantly (P < 0.0001) reduced to 19% compared with control conditions. Electrogenic Na⁺-K⁺-ATPase activity could be partially restored (ouabain-induced depolarization ΔV = 2.0 ± 0.2 mV, n = 6) in cells grown with high glucose in the presence of the aldose reductase inhibitor tolrestat (10⁻⁵ M). The potassium channel opener Hoe 234 (10⁻⁶ M) induced membrane potential hyperpolarization in control cells (ΔV = 7.3 ± 1.2 mV, n = 13), which could be completely inhibited by the K_ATP channel blocker glibenclamide (10⁻⁷ M, n = 5). This indicates that pericytes possess K_ATP channels. The effect of K_ATP channels on membrane voltage was not significantly changed (P = 0.16) in cells cultured under high-glucose conditions (ΔV = 9.6 ± 2.0 mV, n = 6). Acute changes of glucose concentration did not affect the membrane voltage (n = 6). We conclude that high glucose concentrations alter the activity of the Na⁺-K⁺-ATPase in retinal pericytes via a mechanism involving the polyol metabolism. Therefore, hyperglycemia may alter regulation of membrane voltage and contractility of pericytes and, hence, the regulation of retinal microcirculation in diabetes. Altered microcirculation could be an important factor in the pathogenesis of diabetic retinopathy.