Inhibition of endothelium-derived relaxing factor in A6 cells

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.