Role Of Uncoupling Protein 2 In High-Salt Intake-Induced Vascular Dysfunction

Backgrounds and objectives:The relationship between high-salt intake and hypertension has been highlighted worldwidely. Numerous epidemiological studies have demonstrated that salt intake is positively correlated to the arterial blood pressure (BP) and target organ damage, while cutting down on salt and eating no more than 6g per day could lower BP by 2-8 mmHg. Several epidemiological investigations suggest that only some individuals are susceptible to salt with regard to an increase in BP, while a body of individuals is not salt sensitive. Recently, the molecular events involved in this differential susceptibility toward dietary salt intake with respect to hypertension remain elusive. The efforts to address this issue would further our understanding of the molecular mechanisms involved in high-salt intake-induced hypertension and vascular dysfunction and benefit for developing the strategies for treating salt-sensitive hypertension.There is some evidence that salt sensitivity is associated with enhanced oxidative stress and increased superoxide production. Excessive superoxide anion, a well-known quencher of nitric oxide (NO), can substantially impair endothelial function and cause hypertension by decreasing NO bioavailability. These evidences suggest that the enzymes regulating the level of superoxide anion might mediate the salt-induced vascular damage. The mitochondrial uncoupling protein 2 (UCP2) is a kind of mitochondrial inner membrane transporter that dissipates the proton gradient and reduces the production of mitochondrial superoxide anion. Moreover, it has been reported that a common polymorphism in human UCP2 gene is associated with an increased incidence of hypertension. These evidences suggest that UCP2 might be involved in high-salt intake-induced hypertension and vascular dysfunction.The present study aimed to investigate whether UCP2 ablation exacerbates high-salt intake-induced elevation of BP and vascular dysfunction by overproducing superoxide and decreasing NO bioavailability using UCP2 knockout (KO, UCP2^-/-) mice and wild-type (WT, UCP^2+/+) littermates. Materials and methods:Homozygous UCP2^-/- mice on a C57BL/6J genetic background and WT littermates, 6–8 weeks of age, were housed under 12/12-h day/night conditions, provided with free access to tap water and randomly divided into two groups that received a normal-salt (NS, 0.5% sodium chloride, n=12) or a high-salt (HS, 8% sodium chloride, n=12) diet, respectively, for 24 weeks. Then, the following measurements were performed.1. The mRNA expression of UCP2 in aortae and mesenteric resistance arteries was detected by RT-PCR.2. Indirect systolic BP measurements were performed in conscious, restrained mice by tail-cuff plethysmography.3. Mean arterial BP of mice breathing spontaneously was detected with a pressure transducer.4. Concentration-response curves for phenylephrine (PE), acetylcholine (ACh) and nitroglycerin (NTG) were recorded by a four-chamber wire myograph.5. Superoxide anion levels in aortae and mesenteric resistance arteries were assessed by dihydroethidium (DHE) fluorescence.6. NO levels in aortae and mesenteric resistance arteries were assessed by DAF-2DA fluorescence.Results:1. UCP2 transcripts were detected in both aortae and mesenteric resistance arteries of WT mice but not UCP2^-/- mice.2. Systolic BP was significantly higher in UCP2^-/- mice on a HS diet compared with UCP2^-/- mice on a NS diet (P<0.01). In contrast, systolic BP in WT mice was similar between the NS and HS diet group. These results were confirmed by direct mean arterial BP measurements.3. PE-induced contraction was significantly augmented and ACh-elicited relaxation was obviously impaired in mesenteric resistance arteries from both WT and UCP2^-/- mice on a HS diet compared with mice on a NS diet (all P<0.01). However, under the HS intake challenge, PE-induced contraction and ACh-induced relaxation was remarkably enhanced by UCP2 ablation. Moreover, the ACh-induced relaxation in the presence of the eNOS inhibitor L-NG-nitrolarginine methyl ester (L-NAME) was almost completely abolished in WT and UCP2^-/- mice on either a HS diet or a NS diet. Additionally, NTG-induced relaxation of mesenteric resistance arteries was unaffected in WT and UCP2^-/- mice either on a HS diet or a NS diet.4. DHE fluorescence was significantly higher in both aortae and mesenteric resistance arteries from WT and UCP2^-/- mice on a HS diet compared to those on a NS diet (P<0.05 or P<0.01). Importantly, DHE fluorescence in aortae and mesenteric resistance arteries from HS diet-treated UCP2^-/- mice was markedly higher than those from HS diet-treated WT littermates (all P<0.01). DAF-2DA fluorescence was significantly lower in both aortae and mesenteric resistance arteries from WT and UCP2^-/- mice on a HS diet compared to those on a NS diet (P<0.05 or P<0.01). Importantly, DAF-2DA fluorescence in aortae and mesenteric resistance arteries from HS diet-treated UCP2^-/- mice was markedly lower than that from HS diet-treated WT littermates.Conclusion:1. UCP2 ablation exacerbates HS diet-induced elevation of BP and impairment of vascular reactivity.2. UCP2 ablation enhances HS diet–induced increase in superoxide production and decrease in NO bioavailability.3. UCP2 might play an important role in the prevention of the development of salt sensitivity-related hypertension and vascular dysfunction...