TEMPOL enhances the antihypertensive effects of sodium nitrite by mechanisms facilitating nitrite-derived gastric nitric oxide formation

Abstract

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.

Letter by Pinheiro et al Regarding Article, “Unexpected Effect of Proton Pump Inhibitors: Elevation of the Cardiovascular Risk Factor Asymmetric Dimethylarginine”

“…4,5 Importantly, the PPI omeprazole consistently attenuated nitriteinduced reduction in blood pressure by ≈40%. 4,5 This major inhibitory effect of omeprazole on the responses to oral nitrite suggests that PPIs impair the cardiovascular responses to dietary nitrite (and nitrate) and decrease NO activity in the cardiovascular system. Moreover, omeprazole completely blunted tempol-induced increases in plasma nitrite concentrations and improved reductions in blood pressure associated with oral administration of nitrite.…”

“…Moreover, omeprazole completely blunted tempol-induced increases in plasma nitrite concentrations and improved reductions in blood pressure associated with oral administration of nitrite. 5 This is important because it is highly probable that antioxidant substances in the diet improve the protective effects of nitrite-rich diets, and omeprazole prevents all of these effects. 5 These recent findings showing that omeprazole reduces the effects of oral nitrite indicate that PPIs may contribute to decrease NO activity in the cardiovascular system by a variety of mechanisms, and, therefore, the consequences may be more severe than anticipated.…”

“…5 This is important because it is highly probable that antioxidant substances in the diet improve the protective effects of nitrite-rich diets, and omeprazole prevents all of these effects. 5 These recent findings showing that omeprazole reduces the effects of oral nitrite indicate that PPIs may contribute to decrease NO activity in the cardiovascular system by a variety of mechanisms, and, therefore, the consequences may be more severe than anticipated. PPIs increase asymmetrical diemethylarginine levels and impair endogenous NO synthase-dependent NO formation, 1 and this may be aggravated by impaired NO generation from nitrite, thus further disrupting vascular homeostasis.…”

Efeitos de drogas inibidoras da secreção ácida do estômago sobre as respostas hipotensoras do nitrito de sódio

“…Estudos recentes do nosso grupo mostraram que o omeprazol reduz significativamente os efeitos hipotensores de nitrito de sódio em ratos hipertensos por interferir na formação gástrica de espécies relacionadas ao NO através do aumento do pH gástrico (Pinheiro et al 2012;Amaral et al 2013;Pinheiro et al 2015). Como mostrado no tópico anterior, o tratamento com omeprazol atenua o aumento nas concentrações plasmática de nitrosotiol relacionado a diminuição do efeito hipotensor do nitrito, sugerindo que a formação de nitrosotiol gástrico conduz, pelo menos em parte, o efeito anti-hipertensivos do nitrito de sódio (Pinheiro et al 2015).…”

“…Assim, nosso grupo demonstrou recentemente que o pré-tratamento com omeprazol interfere no efeito anti-hipertensivo do nitrito de sódio (Pinheiro et al 2012). No presente estudo, reproduzimos estes achados prévios, sendo que o pré-tratamento com omeprazol diminuiu cerca da metade do efeito hipotensor do nitrito de sódio e esses dados corroboram os valores já apresentados na literatura (Pinheiro et al 2012;Amaral et al 2013). Comprovada a interferência do omeprazol no efeito hipotensor do nitrito, fez-se necessária a análise de outra droga que interfira no pH ácido do estômago e sua possível relação com esses efeitos do nitrito, já que esses achados podem ter implicâncias clínicas importantes para pacientes que fazem uso de inibidores da secreção gástrica.…”