Raymond J. MacAllister

Acute Blood Pressure Lowering, Vasoprotective, and Antiplatelet Properties of Dietary Nitrate via Bioconversion to Nitrite

Diets rich in fruits and vegetables reduce blood pressure (BP) and the risk of adverse cardiovascular events. However, the mechanisms of this effect have not been elucidated. Certain vegetables possess a high nitrate content, and we hypothesized that this might represent a source of vasoprotective nitric oxide via bioactivation. In healthy volunteers, approximately 3 hours after ingestion of a dietary nitrate load (beetroot juice 500 mL), BP was substantially reduced (&Dgr;max −10.4/8 mm Hg); an effect that correlated with peak increases in plasma nitrite concentration. The dietary nitrate load also prevented endothelial dysfunction induced by an acute ischemic insult in the human forearm and significantly attenuated ex vivo platelet aggregation in response to collagen and ADP. Interruption of the enterosalivary conversion of nitrate to nitrite (facilitated by bacterial anaerobes situated on the surface of the tongue) prevented the rise in plasma nitrite, blocked the decrease in BP, and abolished the inhibitory effects on platelet aggregation, confirming that these vasoprotective effects were attributable to the activity of nitrite converted from the ingested nitrate. These findings suggest that dietary nitrate underlies the beneficial effects of a vegetable-rich diet and highlights the potential of a “natural” low cost approach for the treatment of cardiovascular disease.

Effect of inhibitors of the renin-angiotensin system and other antihypertensive drugs on renal outcomes : systematic review and meta-analysis

BACKGROUND A consensus has emerged that angiotensin-converting-enzyme (ACE) inhibitors and angiotensin-II receptor blockers (ARBs) have specific renoprotective effects. Guidelines specify that these are the drugs of choice for the treatment of hypertension in patients with renal disease. We sought to determine to what extent this consensus is supported by the available evidence. METHODS Electronic databases were searched up to January, 2005, for randomised trials assessing antihypertensive drugs and progression of renal disease. Effects on primary discrete endpoints (doubling of creatinine and end-stage renal disease) and secondary continuous markers of renal outcomes (creatinine, albuminuria, and glomerular filtration rate) were calculated with random-effect models. The effects of ACE inhibitors or ARBs in placebo-controlled trials were compared with the effects seen in trials that used an active comparator drug. FINDINGS Comparisons of ACE inhibitors or ARBs with other antihypertensive drugs yielded a relative risk of 0.71 (95% CI 0.49-1.04) for doubling of creatinine and a small benefit on end-stage renal disease (relative risk 0.87, 0.75-0.99). Analyses of the results by study size showed a smaller benefit in large studies. In patients with diabetic nephropathy, no benefit was seen in comparative trials of ACE inhibitors or ARBs on the doubling of creatinine (1.09, 0.55-2.15), end-stage renal disease (0.89, 0.74-1.07), glomerular filtration rate, or creatinine amounts. Placebo-controlled trials of ACE inhibitors or ARBs showed greater benefits than comparative trials on all renal outcomes, but were accompanied by substantial reductions in blood pressure in favour of ACE inhibitors or ARBs. INTERPRETATION The benefits of ACE inhibitors or ARBs on renal outcomes in placebo-controlled trials probably result from a blood-pressure-lowering effect. In patients with diabetes, additional renoprotective actions of these substances beyond lowering blood pressure remain unproven, and there is uncertainty about the greater renoprotection seen in non-diabetic renal disease.

Effect of remote ischaemic preconditioning on myocardial injury in patients undergoing coronary artery bypass graft surgery: a randomised controlled trial

BACKGROUND Whether remote ischaemic preconditioning, an intervention in which brief ischaemia of one tissue or organ protects remote organs from a sustained episode of ischaemia, is beneficial for patients undergoing coronary artery bypass graft surgery is unknown. We did a single-blinded randomised controlled study to establish whether remote ischaemic preconditioning reduces myocardial injury in these patients. METHODS 57 adult patients undergoing elective coronary artery bypass graft surgery were randomly assigned to either a remote ischaemic preconditioning group (n=27) or to a control group (n=30) after induction of anaesthesia. Remote ischaemic preconditioning consisted of three 5-min cycles of right upper limb ischaemia, induced by an automated cuff-inflator placed on the upper arm and inflated to 200 mm Hg, with an intervening 5 min of reperfusion during which the cuff was deflated. Serum troponin-T concentration was measured before surgery and at 6, 12, 24, 48, and 72 h after surgery. Analysis was by intention to treat. This trial is registered with ClinicalTrials.gov, number NCT00397163. FINDINGS Remote ischaemic preconditioning significantly reduced overall serum troponin-T release at 6, 12, 24, and 48 h after surgery. The total area under the curve was reduced by 43%, from 36.12 microg/L (SD 26.08) in the control group to 20.58 microg/L (9.58) in the remote ischaemic preconditioning group (mean difference 15.55 [SD 5.32]; 95% CI 4.88-26.21; p=0.005). INTERPRETATION We have shown that adult patients undergoing elective coronary artery bypass graft surgery at a single tertiary centre could benefit from remote ischaemic preconditioning, using transient upper limb ischaemia.

Acute Systemic Inflammation Impairs Endothelium-Dependent Dilatation in Humans

BackgroundWe tested the hypothesis that endothelial dysfunction underlies the association between an acute inflammatory episode and the transiently increased risk of a cardiovascular event by examining the effects of an experimental inflammatory stimulus on endothelium-dependent vasodilation. Methods and ResultsSalmonella typhi vaccine was used to generate a systemic inflammatory response in healthy volunteers. In 12 subjects, dilatation of the brachial artery to flow and to sublingual nitroglycerin (NTG) was recorded (conduit vessel response), and in 6 subjects, venous occlusion plethysmography was used to measure forearm blood flow during intrabrachial infusion of the endothelium-dependent dilators acetylcholine (ACh) and bradykinin (BK) and the endothelium-independent dilators NTG and verapamil (resistance vessel response). Responses were assessed 16 hours before and 8 and 32 hours after vaccination. Vaccination resulted in elevations in white cell count and serum levels of interleukin-6 and interleukin-1 receptor antagonist. Eight hours after vaccination, resistance vessel responses to BK (P =0.0099) and ACh (P =0.0414) were markedly attenuated, and brachial artery flow-mediated dilatation was depressed. Resistance vessel responses to verapamil and NTG were unchanged, as was the conduit vessel response to NTG. Thirty-two hours after vaccination, resistance vessel responses to BK and ACh had returned to normal. ConclusionsS typhi vaccine generates a mild inflammatory reaction associated with temporary but profound dysfunction of the arterial endothelium in both resistance and conduit vessels to both physical and pharmacological dilator stimuli. This finding might explain the association between infection and inflammation and the enhanced risk of an acute cardiovascular event.

Regulation of nitric oxide synthesis by dimethylarginine dimethylaminohydrolase

1 Dimethylarginine dimethylaminohydrolase (DDAH), an enzyme that metabolizes the endogenous nitric oxide synthase inhibitors NG‐monomethyl‐L‐arginine and NG, NG‐dimethy‐L‐arginine to citrulline, was identified by Western blotting in rat and human tissue homogenates. 2 S‐2‐amino‐4(3‐methylguanidino)butanoic acid (4124W) inhibited the metabolism of [14C]‐NG‐monomethyl‐L‐arginine to [14C]‐citrulline by rat liver homogenates (IC50 416 ± 66μm; n = 9), human cultured endothelial cells (IC50 250 ± 34 μm; n = 9)and isolated purified dimethylarginine dimethylaminohydrolase. 3 Addition of 4124W to culture medium increased the accumulation of endogenously‐generated NG, NG‐dimethy‐L‐arginine in the supernatant of human cultured endothelial cells from 3.1 ± 0.3 to 5 ± 0.7 μm (n=15; P < 0.005). 4 4124W (1 μm‐1 mM) had no direct effect on endothelial nitric oxide synthase activity but caused endothelium‐dependent contraction of rat aortic rings (1 mM 4124W increased tone by 81.5 ± 9.6% of that caused by phenylephrine 100 nM). This effect was reversed by L‐arginine (100 μm). 4124W reversed endothelium‐dependent relaxation of human saphenous vein (19.2 ± 6.7% reversal of bradykinin‐induced relaxation at 1 mM 4124W). 5 These data suggest that inhibition of dimethylarginine dimethylaminohydrolase increases the intracellular concentration of NG, NG‐dimethyl‐L‐arginine sufficiently to inhibit nitric oxide synthesis. Inhibiting the activity of DDAH may provide an alternative mechanism for inhibition of nitric oxide synthases and changes in the activity of DDAH could contribute to pathophysiological alterations in NO generation.

Inorganic Nitrate Supplementation Lowers Blood Pressure in Humans. Role for Nitrite-Derived NO

Ingestion of dietary (inorganic) nitrate elevates circulating and tissue levels of nitrite via bioconversion in the entero-salivary circulation. In addition, nitrite is a potent vasodilator in humans, an effect thought to underlie the blood pressure–lowering effects of dietary nitrate (in the form of beetroot juice) ingestion. Whether inorganic nitrate underlies these effects and whether the effects of either naturally occurring dietary nitrate or inorganic nitrate supplementation are dose dependent remain uncertain. Using a randomized crossover study design, we show that nitrate supplementation (KNO3 capsules: 4 versus 12 mmol [n=6] or 24 mmol of KNO3 (1488 mg of nitrate) versus 24 mmol of KCl [n=20]) or vegetable intake (250 mL of beetroot juice [5.5 mmol nitrate] versus 250 mL of water [n=9]) causes dose-dependent elevation in plasma nitrite concentration and elevation of cGMP concentration with a consequent decrease in blood pressure in healthy volunteers. In addition, post hoc analysis demonstrates a sex difference in sensitivity to nitrate supplementation dependent on resting baseline blood pressure and plasma nitrite concentration, whereby blood pressure is decreased in male volunteers, with higher baseline blood pressure and lower plasma nitrite concentration but not in female volunteers. Our findings demonstrate dose-dependent decreases in blood pressure and vasoprotection after inorganic nitrate ingestion in the form of either supplementation or by dietary elevation. In addition, our post hoc analyses intimate sex differences in nitrate processing involving the entero-salivary circulation that are likely to be major contributing factors to the lower blood pressures and the vasoprotective phenotype of premenopausal women.

Heterogenous Nature of Flow-Mediated Dilatation in Human Conduit Arteries In Vivo: Relevance to Endothelial Dysfunction in Hypercholesterolemia

Abstract — Flow-mediated dilatation (FMD) of conduit arteries is dependent on an intact endothelium, although the mechanisms are not fully understood. Using high-resolution ultrasound, we examined the role of endothelial mediators in radial artery dilatation in response to transient (short period of reactive hyperemia) and sustained (prolonged period of reactive hyperemia, hand warming, or an incremental infusion of acetylcholine into the distal radial artery) hyperemia. After short episodes of reactive hyperemia, FMD was abolished by local infusion of the nitric oxide synthesis inhibitor NGmonomethyl-l-arginine (5.3±1.2% versus 0.7±0.7%, P <0.001). In contrast, basal vessel diameter and dilatation after prolonged episodes of reactive hyperemia, hand warming, and distal infusion of acetylcholine were not attenuated by nitric oxide synthesis inhibition. Inhibition of cyclooxygenase or local autonomic nervous system blockade also had no effect on FMD. Patients with hypercholesterolemia exhibited reduced FMD in response to transient hyperemia, but the response to sustained hyperemia was normal. These data suggest heterogeneity of endothelial responses to blood flow that are dependent on the characteristics of the flow stimulus. Dilatation after brief episodes of hyperemia is mediated by release of nitric oxide, whereas dilatation during sustained hyperemia is unaffected by NO synthesis inhibition. Hypercholesterolemia seems to differentially affect these pathways with impairment of the nitric oxide–dependent pathway and preservation of non nitric oxide–mediated dilatation to sustained flow stimuli.

Defective acute inflammation in Crohn's disease : a clinical investigation

BACKGROUND The cause of Crohns disease has not been mechanistically proven. We tested the hypothesis that the disease is a form of immunodeficiency caused by impaired innate immunity. METHODS We investigated inflammatory responses in patients and controls by quantifying neutrophil recruitment and cytokine production after acute trauma, interleukin 8 secretion by cultured monocyte-derived macrophages after exposure to inflammatory mediators, and local inflammatory and vascular changes in response to subcutaneous injection of heat-killed Escherichia coli. FINDINGS In patients with Crohns disease, trauma to rectum, ileum, or skin led to abnormally low neutrophil accumulation (differences from healthy individuals of 79%, n=8, p=0.0003; 57%, n=3, p=0.05; 50%, n=13, p<0.0001, respectively) and lower production of proinflammatory interleukin 8 (63%, n=7, p=0.003; 63%, n=3, p=0.05; 45%, n=8, p<0.0001) and interleukin 1beta (50%, n=8, p=0.0005). Interleukin 8 secretion by cultured macrophages was reduced after exposure to acute wound fluid (38%, n=50, p<0.0001), C5a (48%, n=41, p=0.0005), or tumour necrosis factor alpha (52%, n=27, p<0.0001). Local inflammatory reaction to inoculation with E coli was attenuated, as quantified by changes in bloodflow (ileal disease 50%, n=6, p=0.01; colonic disease 77%, n=6, p=0.0003). This response was mediated by nitric oxide in controls, was increased by sildenafil in patients, and was not related to CARD15 genotype. INTERPRETATION In Crohns disease, a constitutionally weak immune response predisposes to accumulation of intestinal contents that breach the mucosal barrier of the bowel wall, resulting in granuloma formation and chronic inflammation. Polymorphisms in CARD15 do not underlie this phenotype, but incapacitate the NOD2 pathway that can compensate for impairment of innate inflammation. Current treatment of secondary chronic inflammation might exaggerate the underlying lesion and promote chronic disease.

Ischemic Preconditioning Prevents Endothelial Injury and Systemic Neutrophil Activation During Ischemia-Reperfusion in Humans In Vivo

BACKGROUND Endothelial dysfunction leading to neutrophil infiltration of tissues has been implicated in tissue injury caused by ischemia-reperfusion (IR). Tissue injury during IR can be reduced by prior ischemic preconditioning (IPC). In humans, it is unclear whether endothelial dysfunction occurs during IR or whether IPC offers protection against endothelial dysfunction and inflammatory cell activation. We studied the effects of experimental IR on endothelial and neutrophil function in the human forearm in vivo and examined the protection afforded by IPC. METHOD AND RESULTS The forearm was made ischemic for 20 minutes by inflating a blood pressure cuff to 200 mm Hg. We assessed endothelial function of conduit (radial artery flow-mediated dilation) and resistance vessels (blood flow responses to intra-arterial infusion of the endothelium-dependent dilator acetylcholine) in healthy volunteers before and after IR. IR reduced flow-mediated dilation of the radial artery at 15 minutes of reperfusion (7.7+/-1.5% to 3.5+/-0.9%) and the dilator response of resistance vessels to acetylcholine at 15, 30, and 60 minutes of reperfusion. IR did not reduce the dilator response of the radial artery to glyceryltrinitrate and only caused a small reduction of glyceryltrinitrate-induced dilation of resistance vessels at 60 minutes of reperfusion. IR caused an increase in neutrophil CD11b expression and platelet-neutrophil complexes in the circulating blood. IPC (three 5-minute episodes of ischemia) before IR prevented endothelial dysfunction and neutrophil activation. CONCLUSIONS A clinically relevant period of ischemia-reperfusion causes profound and sustained endothelial dysfunction and systemic neutrophil activation. IPC attenuates both of these effects in humans.

Transient limb ischemia induces remote preconditioning and remote postconditioning in humans by a K(ATP)-channel dependent mechanism.

Background— Transient limb ischemia administered before a prolonged ischemic insult has systemic protective effects against ischemia-reperfusion (IR) injury (remote ischemic preconditioning [RIPC]). It has been demonstrated that protection from IR can be achieved by brief periods of ischemia applied at a remote site during an injurious ischemic event (remote postconditioning [RPostC]). Using an in vivo model of endothelial IR injury, we sought to determine whether RPostC occurred in humans and whether it shared mechanistic similarities with RIPC. Methods and Results— Endothelial function was assessed by flow-mediated dilation before and after IR (20 minutes of arm ischemia followed by reperfusion). RIPC was induced by conditioning cycles of 5 minutes of ischemia and reperfusion on the contralateral arm or leg before IR. For RPostC induction, conditioning cycles were administered during the ischemic phase of IR. Oral glibenclamide was used to determine the dependence of RIPC and RPostC on KATP channels. IR caused a significant reduction in flow-mediated dilation in healthy volunteers (baseline, 9.3±1.2% versus post-IR, 3.3±0.7%; P<0.0001) and patients with atherosclerosis (baseline, 5.5±0.6% versus post-IR, 2.3±0.5%; P<0.01). This reduction was prevented by RIPC (post-IR+RIPC: healthy volunteers, 7.2±0.5% [P<0.0001 versus post-IR]; patients, 4.5±0.3% [P<0.01 versus post-IR]) and RPostC (post-IR+RPostC: 8.0±0.5%; P<0.0001 versus post-IR). The protective effects of RIPC and RPostC were blocked by glibenclamide. Conclusions— This study demonstrates for the first time in humans that RPostC can be induced by transient limb ischemia and is as effective as RIPC in preventing endothelial IR injury. RIPC and RPostC share mechanistic similarities, with protection being dependent on KATP channel activation. These results suggest that remote conditioning stimuli could be protective in patients with acute ischemia about to undergo therapeutic reperfusion.