Nitric oxide inhibits sodium/hydrogen exchange activity in the thick ascending limb

Nitric oxide (NO) inhibits transport in various nephron segments, and the thick ascending limb (TAL) expresses nitric oxide synthase (NOS). However, the effects of NO on TAL transport have not been extensively studied. We tested the hypothesis that NO inhibits apical and basolateral Na+/H+ exchange by the TAL by measuring intracellular pH (pH(i)) of isolated, perfused rat TALs using the fluorescent dye 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF). The NO donor spermine NONOate (SPM, 10 mu M) decreased steady-state pH(i) in medullary TALs from 7.18 +/- 0.13 to 7.13 +/- 0.14 (P < 0.02), whereas controls did not decrease significantly. We next measured the buffering capacity of medullary TALs and the rate at which they recovered from acid loads to investigate the mechanism whereby NO reduces steady-state pHi. SPM decreased H+ flux (J(H)) from 2.41 +/- 0.66 to 0.97 +/- 0.19 pmol . min(-1) . mm(-1) 55%. To assure that the decrease in J(H) was due to NO, another donor, nitroglycerin (NTG; 10 mu M), was used. NTG decreased J(H) from 1.65 +/- 0.11 to 1.07 +/- 0.24 pmol . min(-1) . mm-l, 37%. To determine the relative contributions of the apical and basolateral Na+/H+ exchangers, 5-(N,N-dimethyl)-amiloride (DMA; 100 mu M) was added to either bath or lumen. With DMA added to the bath, SPM decreased J(H) from 4.78 +/- 1.08 to 2.74 +/- 0.54 pmol . min(-1) . mm(-1), an inhibition of 41%; and with DMA added to the lumen, SPM decreased J(H) from 2.31 +/- 0.29 to 1.74 +/- 0.27 pmol . min(-1) . mm(-1), a reduction of 26%. Addition of DMA alone to both bath and lumen resulted in an 87% inhibition of J(H). We conclude that NO inhibits both apical and basolateral Na+/H+ exchangers and consequently may play an important role in regulating pH(i) and may alter acid/base balance by directly affecting bicarbonate absorption in the TAL.