Orally administered
nitrite exerts
antihypertensive effects associated with increased gastric
nitric oxide (NO) formation. While
reducing agents facilitate NO formation from
nitrite, no previous study has examined whether
antioxidants with reducing properties improve the
antihypertensive responses to orally administered
nitrite. We hypothesized that
TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) could enhance the hypotensive effects of
nitrite in hypertensive rats by exerting
antioxidant effects (and enhancing NO bioavailability) and by promoting gastric
nitrite-derived NO generation. The hypotensive effects of intravenous and oral
sodium nitrite were assessed in unanesthetized freely moving rats with
L-NAME (N(ω)-nitro-
L-arginine methyl ester; 100mg/kg; po)-induced
hypertension treated with
TEMPOL (18mg/kg; po) or vehicle. While
TEMPOL exerted
antioxidant effects in hypertensive rats, as revealed by lower plasma
8-isoprostane and vascular
reactive oxygen species levels, this
antioxidant did not affect the hypotensive responses to intravenous
nitrite. Conversely,
TEMPOL enhanced the dose-dependent hypotensive responses to orally administered
nitrite, and this effect was associated with higher increases in plasma
nitrite and lower increases in plasma
nitrate concentrations. In vitro experiments using electrochemical and chemiluminescence NO detection under variable pH conditions showed that
TEMPOL enhanced
nitrite-derived NO formation, especially at low pH (2.0 to 4.0).
TEMPOL signal evaluated by electron paramagnetic resonance decreased when
nitrite was reduced to NO under acidic conditions. Consistent with these findings, increasing gastric pH with
omeprazole (30mg/kg; po) attenuated the hypotensive responses to
nitrite and blunted the enhancement in plasma
nitrite concentrations and hypotensive effects induced by
TEMPOL.
Nitrite-derived NO formation in vivo was confirmed by using the NO scavenger
2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (C-
PTIO), which blunted the responses to oral
nitrite. Our results showed that
TEMPOL promotes
nitrite reduction to NO in the stomach and enhanced plasma
nitrite concentrations and the hypotensive effects of oral
sodium nitrite through mechanisms critically dependent on gastric pH. Interestingly, the effects of
TEMPOL on
nitrite-mediated
hypotension cannot be explained by increased NO formation in the stomach alone, but rather appear more directly related to increased plasma
nitrite levels and reduced
nitrate levels during
TEMPOL treatment. This may relate to enhanced
nitrite uptake or reduced
nitrate formation from NO or
nitrite.