Inhibition of Endothelium-Derived Relaxing Factor in A6 Cells

Abstract

The effects of endothelium-derived relaxing factor (EDRF) on Na⁺ transport in distal renal tubular A6 cells have been studied by inhibition of its synthesis with L-NAME (10^-2 mol/l). Na⁺ transport was monitored by measuring short-circuit current, cell voltage, transepithelial, apical and basolateral membrane conductances. EDRF production in A6 cells was tested by application of its substrate L-arginine.

The blockade of EDRF decreased significantly the Na⁺ current (11 %), membrane potential (5 mV) and basolateral conductance (33 %), but did not affect the apical membrane conductance. Activation of apical Na⁺ conductance by dexamethasone incubation (10^-7 mol/l) did not further influence the drop in Na⁺ current. The involvement of basolateral K⁺ channels in cell depolarization and in the reduction of basolateral conductance was tested in tissues with elevated basolateral K⁺/Cl^- conductance ratios (by increasing bath osmolarity) and by application of barium (0.5–10^-3 mol/l) a K⁺ channel blocker. The results showed that the effect of L-NAME on the short-circuit current was more pronounced in A6 cells with increased K⁺/Cl^- conductance ratios, but was almost nullified by barium. Finally, L-arginine fully restored the Na⁺ current, thus reversing the inhibition induced by L-NAME.

We conclude that EDRF is basally released in A6 cells. Inhibition of EDRF by L-NAME directly interferes with Na⁺ reabsorption. Since apical membrane conductance remains unchanged, the decrease in short-circuit current results from cell depolarization. The latter, together with the drop in basolateral conductance, might reflect inactivation of K⁺ channels.

Key Words:

• A6 cells
• endothelium-derived relaxing factor
• L-NAME
• sodium reabsorption
• potassium conductance