Arginase II inhibition prevents nitrate tolerance

BACKGROUND AND PURPOSE

Nitrate tolerance, the loss of vascular responsiveness with continued use of nitrates, remains incompletely understood and is a limitation of these therapeutic agents. Vascular superoxide, generated by uncoupled endothelial NOS (eNOS), may play a role. As arginase competes with eNOS for L-arginine and may exacerbate the production of reactive oxygen species (ROS), we hypothesized that arginase inhibition might reduce nitrate tolerance.

EXPERIMENTAL APPROACH

Vasodilator responses were measured in aorta from C57Bl/6 and arginase II knockout (argII -/-) mice using myography. Uncoupling of eNOS, determined as eNOS monomer : dimer ratio, was assessed using low-temperature SDS-PAGE and ROS levels were measured using L-012 and lucigenin-enhanced chemiluminescence.

KEY RESULTS

Repeated application of glyceryl trinitrate (GTN) on aorta isolated from C57Bl/6 mice produced a 32-fold rightward shift of the concentration-response curve. However this rightward shift (or resultant tolerance) was not observed in the presence of the arginase inhibitor (s)-(2-boronethyl)-L-cysteine HCl (BEC; 100 µM) nor in aorta isolated from argII -/- mice. Similar findings were obtained after inducing nitrate tolerance in vivo. Repeated administration of GTN in human umbilical vein endothelial cells induced uncoupling of eNOS from its dimeric state and increased ROS levels, which were reduced with arginase inhibition and exogenous L-arginine. Aortae from GTN tolerant C57Bl/6 mice exhibited increased arginase activity and ROS production, whereas vessels from argII -/- mice did not.

CONCLUSION AND IMPLICATIONS

Arginase II removal prevents nitrate tolerance. This may be due to decreased uncoupling of eNOS and consequent ROS production.

l-Arginine transporters in cardiovascular disease: A novel therapeutic target

The amino acid l-arginine participates in a variety of key biochemical and physiological activities, including its well-recognized role as the key substrate for nitric oxide (NO) biosynthesis. The current review describes the cellular influences on arginine metabolism with particular focus on the transport of l-arginine in the endothelium. It details the processes by which intracellular and extracellular levels of l-arginine may influence nitric oxide production and further documents the imbalance that is evident in various cardiovascular disease states. In man, impairment of l-arginine transport has been observed in hypertension, heart failure, and renal disease, and it may thus be a relevant therapeutic target for rectification of nitric oxide pathogenesis in these conditions.

Advanced glycation of apolipoprotein A-I impairs its anti-atherogenic properties

AIMS/HYPOTHESIS

AGE contribute to the pathogenesis of diabetic complications, including dyslipidaemia and atherosclerosis. However, the precise mechanisms remain to be established. In the present study, we examined whether AGE modification of apolipoprotein A-I (apoA-I) affects its functionality, thus altering its cardioprotective profile.

MATERIALS AND METHODS

The ability of AGE-modified apoA-I to facilitate cholesterol and phospholipid efflux, stabilise ATP-binding cassette transporter A1 (ABCA1) and inhibit expression of adhesion molecules in human macrophages and monocytes was studied.

RESULTS

The ability of AGE-modified apoA-I to promote cholesterol efflux from THP-1 macrophages, isolated human monocytes and from ABCA1-transfected HeLa cells was significantly reduced (>70%) compared with unmodified apoA-I. This effect was reversed by preventing AGE formation with aminoguanidine or reversing AGE modification using the cross-link breaker alagebrium chloride. AGE-modification of HDL also reduced its capacity to promote cholesterol efflux. AGE-apoA-I was also less effective than apoA-I in stabilising ABCA1 in THP-1 cells as well as in inhibiting expression of CD11b in human monocytes.

CONCLUSIONS/INTERPRETATION

AGE modification of apoA-I considerably impairs its cardioprotective, antiatherogenic properties, including the ability to promote cholesterol efflux, stabilise ABCA1 and inhibit the expression of adhesion molecules. These findings provide a rationale for targeting AGE in the management of diabetic dyslipidaemia.

The isoflavone metabolite dehydroequol produces vasodilatation in human resistance arteries via a nitric oxide-dependent mechanism

BACKGROUND

Isoflavones (phytoestrogens) offer potential cardioprotective benefits. We recently reported on the vasodilatory activity of the isoflavone metabolite, dehydroequol, in rat isolated aortic ring preparations. In the current study, we examine the effect of this metabolite on the vascular haemodynamic profile in human forearm resistance arteries.

METHODS AND RESULTS

Responses to brachial artery infusion of dehydroequol (0.1, 0.3, 1 and 3 micromol/min) in forearm resistance arteries were obtained in six healthy males. These were done, on two separate occasions, in the absence and presence of endogenous nitric oxide synthase inhibition using NG-monomethyl-L-arginine, with sufficient sodium nitroprusside to maintain vascular tone. Dehydroequol produced a dose-dependent increase in forearm blood flow from 2.44 +/- 0.37 (basal) to 5.25 +/- 1.07 mL/100 mL/min (P < 0.05) at dehydroequol 3 micromol/min. Responses to dehydroequol were significantly dampened with inhibition of endogenous nitric oxide synthase (at 3 micromol/min: % increase in forearm blood flow fell from 114.3 +/- 22.81 to 19.45 +/- 9.19; P < 0.01).

CONCLUSION

This is the first report of dehydroequol, a metabolite derived from the isoflavone diadzein, demonstrating potent vasodilatory properties in human resistance arteries via a nitric oxide-dependent mechanism.

Hypercholesterolemia and Endothelium Dysfunction: Role of Dietary Supplementation as Vascular Protective Agents

There is increasing evidence that dietary supplementation, such as L-arginine, anti-oxidant vitamins, soy phytoestrogens, flavonoids and omega-3 fatty acids exert vascular protective benefits particularly in terms of restoring endothelial function in cardiovascular disease states. The endothelium has been a major focus over the past 20 years as being a primary site at which dysfunction occurs in association with, and contributing to, vascular pathologies. Such states include mild compromise of the cardiovascular system as observed in smokers, hypercholesterolemics and hypertensives, through to end-point heart failure. This review will focus on the experimental and clinical evidence examining the effect of nutriceuticals on vascular function, in particular endothelium-derived factors, and argues that there is a role for nutriceuticals in the clinical management of the cardiovascular compromised individual.

Evidence for altered vascular responses to exogenous endothelin-1 in patients with advanced cirrhosis with restoration of the normal vasoconstrictor response following successful liver transplantation

BACKGROUND AND AIMS

There is evidence that dampened responses to endogenous vasoconstrictors contribute to the hyperdynamic circulation that is characteristic of advanced cirrhosis. The aim of this study was to determine whether there is an altered vascular responsiveness to the endothelium derived constricting factor endothelin-1 (ET-1) in patients with decompensated chronic liver disease which might contribute to this abnormal circulatory state, and whether normal endothelin responses are restored following liver transplantation.

METHODS

Using forearm plethysmography, we studied the vascular response to an intra-arterial ET-1 infusion in six patients with end stage cirrhosis, before and after liver transplantation, compared with six normal control subjects. Responses to the selective endothelin A (ET(A)) receptor subtype antagonist, BQ123, were also examined.

RESULTS

The forearm vessels of patients with cirrhosis vasodilated in response to ET-1 infusion while in healthy controls a marked vasoconstriction response was observed (p<0.0001, area under the curve time-blood flow was normal compared with the cirrhosis groups, ANOVA). Prior to commencement of liver transplant surgery, cirrhotic patients were confirmed to have a hyperdynamic circulation with a high cardiac index (4.07 (0.23) l/min/m(2) (normal range 2.8-3.6 l/min/m(2))) and low systemic vascular resistance index (1284 (115) dynxs/cm(5)/m(2) (normal range 1760-2600 dynxs/cm(5)/m(2))). Following transplantation, normal vasoconstrictor responses to ET-1 were restored. Responses to BQ123 were not different in patients with advanced cirrhosis compared with controls.

CONCLUSION

In patients with end stage cirrhosis, ET-1 produces vasodilatation at a dose that causes marked vasoconstriction in normal control subjects. This effect is not attributable to impairment of ET(A) receptor responses. Our findings suggest that altered endothelin responses may contribute to the generalised dilatation of the circulation that occurs in patients with advanced liver disease.