| Background and objective:Hypertension is currently one of the most common diseases in the world. Its prevalence is more than 20% in the European and American adult population. National Nutrition and Health Survey data suggest that the prevalence of hypertension in the Chinese adult population was 18.8% in 2002. More importantly, hypertension is a major risk factor for cardiovascular disease （CVD） such as stroke, myocardial infarction, and chronic kidney diseases, and the World Health Organization ranks CVD as the world’s leading cause of death. It has been become a critical public health problem worldwide. Therefore, the prevention and control of hypertension has very important practical significance.The pathogenesis of primary hypertension is due to both the genetic susceptibility and environmental factors, and salt intake is one of major important environmental factors. The causal relation between habitual dietary salt intake and blood pressure has been established through many studies, and there is dose-response relationship between the dietary salt consumption and blood pressure. Reducing salt intake could significantly prevent elevation of blood pressure and development of cardiovascular diseases. Individuals vary with respect to the increase of blood pressure with increasing salt intake （salt sensitivity）. There is some evidence that salt sensitivity is associated with enhanced oxidative stress and increased superoxide anion production. High salt intake could increase superoxide anion generation and result in oxidative stress via increasing activity of NADPH oxidase. Excessive superoxide anion, a well-known quencher of nitric oxide （NO）, can substantially impair endothelial function and cause hypertension by decreasing NO bioavailability. Antioxidants could significantly improved endothelium-dependent vasodilation in essential hypertensive patients,even more decreased blood pressure in high-salt intake-induced hypertensive animals.WHO （World Health Organization） recommends population-wide decrease salt consumption to less than 5g/day. But most adult populations in China have average daily salt intakes higher than 12g, Governments take efforts to advocate current recommenda- tions of restriction salt intake in the population for preventing and controlling hypertension and cardiovascular diseases worldwide, but the efficacy is poor because of lack of health knowledge, habits of excessive salt consumption, adding excessive salt in food processing and preserving, and so on. Therefore, it is very important to explore and develop a simpler, and more convenient, practicable and effective approach to preventing high-salt intake-induce cardiovascular damage, Apart from restriction salt intake in whole population.Chili pepper is a popular and favorite vegetable. Capsaicin is a major pungent ingredient in Chili pepper and is used as a food additive, which is a highly selective agonist for the transient receptor potential vanilloid receptor 1 （TRPV1）. TRPV1 as a nonselective cation channel, activated TRPV1 channels increase intracellular Ca2+ concentration, in turn regulate multi-physiological functions. TRPV1 is mainly expressed in sensory neurons and brain, but is found in the vasculature including vascular smooth muscle cell and endothelium cell in recent years. The role of TRPV1 channel in cardiovascular diseases is growing a new research hotspot. Several studies support the hypotensive property of capsaicin via activation of TRPV1 receptor. The underlying mechanism may be related with capsaicin-sensitive sensory neurons releasing stored neuropeptides through a calcium-dependent mechanism via the binding of capsaicin to TRPV1 receptor. Those sensory neuropeptides including calcitonin gene-related peptide （CGRP） and substance P （SP）, which are potent vasodilators factors, exert a hypotensive effect. Recent study from our lab have revealed that long-term stimulation of TRPV1 by capsaicin activates PKA, increases eNOS phosphorylation and NO production, improves vasorelaxation , contributing to lowering blood pressure levels in genetically hypertensive rats.Previous studies indicate capsaicin could attenuate oxidative stress, increase urinary sodium excretion, and maintain water and salt homeostasis. But most previous studies are limited to short-term or in vitro studies. There is no report available so far for the natriuretic and antioxidant effect of long-term dietary capsaicin on high-salt intake-induced hypertension. We hypothesized that long-term activation of TRPV1 by capsaicin could attenuate oxidative stress, improve resistant arteries relaxation, increase urinary sodium excretion, improve salt sensitivity, and prevent high-salt intake-induced hypertension and target organ damage.To test the above hypothesis, in the present study, we first monitored the blood pressure by radio-telemetry in mice fed a high-salt diet after 12-week dietary capsaicin intervention, and measured the weight of heart and detected the cardiac hypertrophy related molecules in heart. Then we evaluated vasorelaxant responses in mesenteric resistance arteries from mice after dietary intervention. Finally, we detected the levels of superoxide anion and NO in mesenteric resistance arteries, the levels of hydrogen peroxide and malondialdehyde in plasma. We also compared the 24-h urinary sodium excretion in mice after dietary intervention.Materials and Methods:The present study includes in vivo and in vitro experiments. In vivo models were C57BL/6J wild-type （WT） mice and TRPV1-null mutant （TRPV1–/–） mice fed with interventional diet. In vitro models include aortae and mesenteric arteries from mice. WT mice and TRPV1–/–were randomly grouped and fed with a normal-salt diet （NS）, high-salt diet （HS） or high-salt plus capsaicin diet （HS + Cap） for 12 weeks.1. The 24-hour ambulatory mean arterial pressure （MAP）, heart rate and locomotor activity were monitored by implantable radio-telemetry in mice after dietary intervention.2. Vasorelaxant responses in mesenteric resistance arteries from mice after dietary intervention evaluated by isotonic myograph.3. Superoxide anion and NO levels in mesenteric resistance arteries were detected by dihydroethidium （DHE） and 4, 5-diaminofluorescein diacetate （DAF-2DA） fluorescent stain. The levels of hydrogen peroxide and malondialdehyde in plasma were measured by spectrophotometry.4. Protein expressions of eNOS, p-eNOS in aorta and Akt, p-Akt, mTOR in heart from mice after dietary intervention were detected by immunoblotting.5. The 24-hour urine samples were collected during a fasting period from mice after dietary intervention. The levels of urinary sodium and urinary creatinine were measured by ion selective electrode and spectrophotometry, respectively.Results:1. Chronic capsaicin feeding prevented high-salt diet-induced elevation of MAP in WT mice during the night, but no difference was seen in TRPV1–/–mice. A similar but non-significant increase was observed during the day. Dietary capsaicin administration did not change heart rate and locomotor activity in mice that were fed a high-salt diet.2. Although the final heart weight was similar among the dietary intervention groups. High-salt diet significantly increased Heart-to-body weight ratio in mice of both genotypes. Chronic dietary capsaicin significantly prevented the increase of heart-to-body weight ratio in WT mice on a high-salt diet. However, this effect of capsaicin was absent in TRPV1–/– mice on a high-salt diet. Dietary capsaicin markedly down-regulated protein expression of p-Akt、mTOR in heart from WT mice on a high-salt diet, but no difference was seen in TRPV1–/– mice.3. Chronic dietary capsaicin significantly improved the endothelium-dependent relaxation in mesenteric resistance arteries from WT mice on a high-salt diet. However, this effect of capsaicin was absent in TRPV1–/– mice on a high-salt diet. There were not significant differences in Acetylcholine-induced relaxation after inhibited by L-NAME and endothelium-independent relaxation in response to nitroglycerin among different dietary groups.4. Dietary capsaicin significantly lowered superoxide anion production and increased NO levels in mesenteric arteries from WT mice on a high-salt diet, but not in TRPV1–/– mice fed the same diet. Dietary capsaicin also markedly decreased the plasma levels of H2O2 and MDA in mice on high-salt diet.5. Dietary capsaicin significantly decreased 24-hour urinary sodium excretion and urinary sodium/creatinine values during a fasting period in WT mice fed a high-salt diet, this effect of capsaicin was absent in TRPV1–/– mice fed a high-salt diet. Conclusions:1. Activation of TRPV1 by capsaicin attenuates high-salt intake-induced oxidative stress, reduces production superoxide anion, increases NO bioavailability, and improves high-salt intake-induced impairment of the endothelium-dependent relaxation in resistance arteries,prevents high-salt intake-induced hypertension.2. Activation of TRPV1 by capsaicin promotes urinary sodium excretion after salt loading, decreases salt sensitivity, contributing to alleviation of high-salt diet-induced oxidative stress.3. Activation of TRPV1 by capsaicin inhibits activity of Akt/mTOR pathway in heart in mice fed a high-salt diet, which may prevent high-salt intake-induced cardiac hypertrophy.4. Activation of TRPV1 by long-term dietary capsaicin might play a potential role in the prevention of the development of high-salt intake-induced hypertension and its related cardiac hypertrophy. Chronic dietary capsaicin may represent a simple and effective lifestyle intervention in populations with high-salt diet-induced hypertension and cardiovascular diseases. |
