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Division of Hypertension and Vascular Research, Department of Internal Medicine, Henry Ford Hospital, Detroit, Michigan 48202
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
(pHi) 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 µM) decreased steady-state pHi 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
(JH) from 2.41 ± 0.66 to 0.97 ± 0.19 pmol · min
1 · mm
1,
55%. To assure that the decrease in
JH was due to NO,
another donor, nitroglycerin (NTG; 10 µM), was used. NTG decreased
JH from 1.65 ± 0.11 to 1.07 ± 0.24 pmol · min
1 · mm
1, 37%. To determine the
relative contributions of the apical and basolateral
Na+/H+
exchangers,
5-(N,N-dimethyl)amiloride
(DMA; 100 µM) was added to either bath or lumen. With DMA added to
the bath, SPM decreased JH 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 JH 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
JH. We conclude
that NO inhibits both apical and basolateral
Na+/H+
exchangers and consequently may play an important role in regulating pHi and may alter acid/base
balance by directly affecting bicarbonate absorption in the TAL.
acid-base balance; bicarbonate transport; natriuresis
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