Mechanisms of impairment of endothelium-dependent relaxation to acetylcholine in Watanabe heritable hyperlipidaemic rabbit aortas

Redox control of endothelial function and dysfunction: molecular mechanisms and therapeutic opportunities

The endothelium is essential for the maintenance of vascular homeostasis. Central to this role is the production of endothelium-derived nitric oxide (EDNO), synthesized by the endothelial isoform of nitric oxide synthase (eNOS). Endothelial dysfunction, manifested as impaired EDNO bioactivity, is an important early event in the development of various vascular diseases, including hypertension, diabetes, and atherosclerosis. The degree of impairment of EDNO bioactivity is a determinant of future vascular complications. Accordingly, growing interest exists in defining the pathologic mechanisms involved. Considerable evidence supports a causal role for the enhanced production of reactive oxygen species (ROS) by vascular cells. ROS directly inactivate EDNO, act as cell-signaling molecules, and promote protein dysfunction, events that contribute to the initiation and progression of endothelial dysfunction. Increasing data indicate that strategies designed to limit vascular ROS production can restore endothelial function in humans with vascular complications. The purpose of this review is to outline the various ways in which ROS can influence endothelial function and dysfunction, describe the redox mechanisms involved, and discuss approaches for preventing endothelial dysfunction that may highlight future therapeutic opportunities in the treatment of cardiovascular disease.

N,N-Diacetyl-l-cystine Improves Endothelial Function in Atherosclerotic Watanabe Heritable Hyperlipidaemic Rabbits

N,N-diacetyl-L-cystine (DiNAC), a novel immunomodulator, stimulates contact sensitivity/delayed type hypersensitivity reactions in mice induced by oxazolone and reduces atherosclerosis in Watanabe heritable hyperlipidaemic (WHHL) rabbits. Forty-week-old WHHL rabbits were given DiNAC (3 micromol/kg per day) for 8 weeks, and endothelium-mediated dilatation was investigated in vivo using pulse wave analysis. A significant improvement in endothelial function was found after 3 weeks of treatment, which was further improved after 8 weeks. For experiments on isolated blood vessels, 40-week-old rabbits were treated for 3 weeks. Treatment did not affect plasma lipid levels. At termination, aortic rings from the thoracic and abdominal aorta were contracted with phenylephrine in vitro. Concentration-effect curves to acetylcholine and the calcium ionophore A 23187 were used to measure endothelium-mediated vasodilatation, and nitroprusside to elicit endothelium-independent relaxations. Abdominal aorta relaxations were generally larger than in thoracic aorta. DiNAC improved endothelium-dependent relaxations in the abdominal but not in the thoracic aorta. This effect was independent of the degree of atherosclerosis. It is concluded that DiNAC improved endothelial function in atherosclerotic rabbit arteries in vivo and in vitro, and may represent a new treatment modality for atherosclerosis-related diseases.

Oxidative stress and endothelial nitric oxide bioactivity

The endothelium plays an important role in the maintenance of vascular homeostasis. Central to this role is the endothelial production of nitric oxide (NO), synthesized by the constitutively expressed endothelial isoform of nitric oxide synthase. Vascular diseases, including hypertension, diabetes, and atherosclerosis, are characterized by impaired endothelium-derived NO bioactivity that may contribute to clinical cardiovascular events. Growing evidence indicates that impaired endothelium-derived NO bioactivity is due, in part, to excess vascular oxidative stress. This review outlines how different forms of oxidative stress can impact on NO bioactivity and discusses strategies to prevent oxidative stress-induced endothelial dysfunction.

Decreased arterial responses in WHHL rabbits, an animal model of spontaneous hypercholesterolemia and atherosclerosis

We examined changes in blood pressure and blood flow of the arteries of WHHL and Japanese white rabbits after intravenous bolus injections of acetylcholine (3.0 micrograms/kg), bradykinin (0.5 microgram/kg), and sodium nitroprusside (3.0 micrograms/kg) under a condition of anesthesia. These vasodilators lowered the blood pressure and increased the blood flow in WHHL and Japanese white rabbits. The changes in the hemodynamic parameters of WHHL rabbits after injection of sodium nitroprusside were similar to those of Japanese white rabbits. This suggests that the relaxation response of the tunica media was not diminished in WHHL rabbits. In contrast, the changes in the hemodynamic parameters of WHHL rabbits after injection of acetylcholine or bradykinin were significantly lower than those in Japanese white rabbits. In the histopathological and immunohistological examination, atherosclerotic lesions were observed in the ascending aortas of WHHL rabbits. In the surface of the atheromatous plaques, CD31-positive endothelial cells disappeared partly and the accumulation of RAM-11-positive macrophages was observed in these regions. In addition, plasma NO2- and NO3- levels of WHHL rabbits were significantly lower than those of Japanese white rabbits. These findings suggest that relaxation responses derived from arterial endothelial cells were probably depressed in WHHL rabbits due to dysfunction or denudation of the arterial endothelial cells.

Downregulation of vascular soluble guanylate cyclase induced by high salt intake in spontaneously hypertensive rats

1. Cyclic guanosine monophosphate (cyclic GMP)-mediated mechanism plays an important role in vasodilatation and blood pressure regulation. We investigated the effects of high salt intake on the nitric oxide (NO) - cyclic GMP signal transduction pathway regulating relaxation in aortas of spontaneously hypertensive rats (SHR). 2. Four-week-old SHR and normotensive Wistar-Kyoto rats (WKY) received a normal salt diet (0.3% NaCl) or a high salt diet (8% NaCl) for 4 weeks. 3. In aortic rings from SHR, endothelium-dependent relaxations in response to acetylcholine (ACh), adenosine diphosphate (ADP) and calcium ionophore A23187 were significantly impaired by the high salt intake. The endothelium-independent relaxations in response to sodium nitroprusside (SNP) and nitroglycerin were also impaired, but that to 8-bromo-cyclic GMP remained unchanged. On the other hand, high salt diet had no significant effects on the relaxations of aortic rings from WKY. 4. In aortas from SHR, the release of NO stimulated by ACh was significantly enhanced, whereas the production of cyclic GMP induced by either ACh or SNP was decreased by the high salt intake. 5. Western blot analysis showed that the protein level of endothelial NO synthase (eNOS) was slightly increased, whereas that of soluble guanylate cyclase (sGC) was dramatically reduced by the high salt intake. 6. These results indicate that in SHR, excessive dietary salt can result in downregulation of sGC followed by decreased cyclic GMP production, which leads to impairment of vascular relaxation in responses to NO. It is notable that chronic high salt intake impairs the sGC/cyclic GMP pathway but not the eNOS/NO pathway.

Reduced lung endothelial angiotensin-converting enzyme activity in Watanabe hyperlipidemic rabbits in vivo

We investigated pulmonary endothelial function in vivo in 12- to 18-mo-old male Watanabe heritable hyperlipidemic (WHHL; n = 7) and age- and sex-matched New Zealand White (n = 8) rabbits. The animals were anesthetized and artificially ventilated, and the chest was opened and put in total heart bypass. The single-pass transpulmonary utilizations of the angiotensin-converting enzyme (ACE) substrate [(3)H]benzoyl-Phe-Ala-Pro (BPAP) and the 5'-nucleotidase (NCT) substrate [(14)C]AMP were estimated, and the first-order reaction parameter A(max)/K(m), where A(max) is the product of enzyme mass and the catalytic rate constant and K(m) is the Michaelis-Menten constant, was calculated. BPAP transpulmonary utilization and A(max)/K(m) were reduced in WHHL (1.69 +/- 0.16 vs. 2.9 +/- 0.44 and 599 +/- 69 vs. 987 +/- 153 ml/min in WHHL and control rabbits, respectively; P < 0.05 for both). No differences were observed in the AMP parameters. BPAP K(m) and A(max) values were estimated separately under mixed-order reaction conditions. No differences in K(m) values were found (9.79 +/- 1 vs. 9.9 +/- 1.31microM), whereas WHHL rabbit A(max) was significantly decreased (5.29 +/- 0.88 vs. 7. 93 +/- 0.8 micromol/min in WHHL and control rabbits, respectively; P < 0.05). We conclude that the observed pulmonary endothelial ACE activity reduction in WHHL rabbits appears related to a decrease in enzyme mass rather than to alterations in enzyme affinity.

Redox control of vascular nitric oxide bioavailability

NO is an important component of vascular homeostasis and abnormal NO bioactivity has been implicated in number of disease states with important public health implications. One clear mechanism of impaired NO bioactivity and vascular disease is excess vascular oxidative stress. There is now a wealth of developing data that manipulation of vascular antioxidant stress is the considerable influence of the biologic activity of endothelium-derived NO. It remains to be seen if this influence can be exploited in a manner that truly alters the course of human disease.

Characterization of nitric oxide- and prostaglandin-independent relaxation in response to acetylcholine in rabbit renal artery

1. We investigated the characterization of acetylcholine (ACh)-induced NG-nitro-L-arginine methyl ester (L-NAME)- and indomethacin (IND)-resistant relaxations, which can be mediated by endothelium-derived hyperpolarizing factor (EDHF), in rabbit renal arterial rings. 2. The relaxations were inhibited by SKF 525A, a cytochrome P450 inhibitor, but were not affected by other inhibitors, namely clotrimazole, 17-octadecynoic acid and alpha-naphthoflavone. Furthermore, 11,12-epoxyeicosatrienoic acid, a cytochrome P450 metabolite, did not relax arterial rings. 3. Arterial relaxations were significantly attenuated by charybdotoxin and iberiotoxin, but not by apamin, all K+ channel blockers. 4. In a sandwich bioassay experiment, ACh-induced L-NAME- and IND-resistant relaxations were not transferred to the detector site. 5. Relaxations were also significantly attenuated by 1-heptanol and 18 alpha-glycyrrhetinic acid, gap junctional coupling inhibitors. 6. These results indicate that, in the rabbit renal artery, L-NAME- and IND-resistant relaxations are mediated by factors other than cytochrome P450-derived arachidonic acid metabolites, which may be able to diffuse into the lumen but be partly transferred via myoendothelial gap junctions to adjacent vascular smooth muscle cells and relax muscles by opening high-conductance Ca(2+)-activated K+ channels.

Diet-induced changes in endothelial-dependent relaxation of the rat aorta

PURPOSE

Hypertension (HTN), hyperlipidemia (HLP), and hyperinsulinemia are known risk factors for the development of cardiovascular disease. Each has independently been shown to be associated with impaired endothelial function, as demonstrated by decreased endothelial derived relaxation (EDR). Previous work in our laboratory has shown that rats fed a high-fat sucrose (HFS) diet will become insulin resistant, hypertriglyceridemic, and hypertensive. We hypothesize that the development of these diet-induced risk factors is associated with endothelial dysfunction and a significant decrease in EDR. Furthermore, the endothelial dysfunction will be improved by returning to a normal (low-fat complex carbohydrate (LFCC)) diet.

METHODS

Adult, male Fischer rats were fed either a LFCC or a HFS diet for 6 months (n = 8 in each group). A third group of rats (SWITCH) was fed a HFS diet for 6 months and then changed to a LFCC diet for 4 weeks. Blood pressure was measured via the tail-cuff method weekly. The rats were sacrificed and aortic ring segments were placed in physiologic tissue baths for measurement of vascular reactivity to various agents. Arterial ring segments were constricted with potassium chloride (K) and phenylephrine (PE). Endothelial-dependent vasorelaxation was measured with acetylcholine (Ach), bradykinin (BK), and calcium ionophore (CA). Endothelial-independent relaxation was measured using sodium nitroprusside (NTP).

RESULTS

The HFS diet group developed HTN compared to LFCC group. Vasoconstriction to K and PE were similar in all groups. Vasorelaxation to Ach, BK, and CA was significantly decreased in the HFS group, but returned to baseline in the diet-switched group, as did the systolic blood pressure. There were no differences in relaxation to NTP.

CONCLUSIONS

HFS diet-induced HTN is associated with significantly decreased EDR. Switching to a low-fat diet reverses this effect. The vascular smooth muscle contraction and endothelial-independent relaxation are not affected by the diet-induced risk factors. There is a direct and reversible effect of an HFS diet on endothelial function and blood pressure.