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(Hypertension. 2004;44:935.)
© 2004 American Heart Association, Inc.

Scientific Contributions

Upregulation of Vascular Arginase in Hypertension Decreases Nitric Oxide–Mediated Dilation of Coronary Arterioles

Cuihua Zhang; Travis W. Hein; Wei Wang; Matthew W. Miller; Theresa W. Fossum; Michelle M. McDonald; Jay D. Humphrey; Lih Kuo

From the Department of Medical Physiology (C.Z., T.W.H., W.W., L.K.), College of Medicine, Texas A&M University System Health Science Center, College Station, Tex; the Michael E. DeBakey Institute (M.W.M., T.W.F., M.M.M.), Department of Small Animal Medicine and Surgery, College of Veterinary Medicine, Texas A&M University, College Station, Tex; and the Department of Biomedical Engineering (J.D.H.), Dwight Look College of Engineering, Texas A&M University, College Station, Tex.

Correspondence to Lih Kuo, PhD, Department of Medical Physiology, College of Medicine, Texas A&M University System Health Science Center, Temple, TX 76504. E-mail LKUO{at}tamu.edu

	Abstract

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One characteristic of hypertension is a decreased endothelium-dependent nitric oxide (NO)-mediated vasodilation; however, the underlying mechanism is complex. In endothelial cells (ECs), L-arginine is the substrate for both NO synthase (NOS) and arginase. Because arginase has recently been shown to modulate NO-mediated dilation of coronary arterioles by reducing L-arginine availability, we hypothesized that upregulation of vascular arginase in hypertension contributes to decreased NO-mediated vasodilation. To test this hypothesis, hypertension (mean arterial blood pressure >150 mm Hg) was maintained for 8 weeks in pigs by aortic coarctation. Coronary arterioles from normotensive (NT) and hypertensive (HT) pigs were isolated and pressurized for in vitro study. NT vessels dilated dose-dependently to adenosine (partially mediated by endothelial release of NO) and sodium nitroprusside (endothelium-independent vasodilator). Conversely, HT vessels exhibited reduced dilation to adenosine but dilated normally to sodium nitroprusside. Adenosine-stimulated NO release was increased 3-fold in NT vessels but was reduced in HT vessels. Moreover, arginase activity was 2-fold higher in HT vessels. Inhibition of arginase activity by N-hydroxy-nor-L-arginine or incubation with L-arginine partially restored NO release and dilation to adenosine in HT vessels. Immunohistochemistry showed that arginase expression was increased but NOS expression was decreased in arteriolar ECs of HT vessels. These results suggest that NO-mediated dilation of coronary arterioles is inhibited in hypertension by an increase in arginase activity in EC, which limits L-arginine availability to NOS for NO production. The inability of arginase blockade or L-arginine supplementation to completely restore vasodilation may be related to downregulation of endothelial NOS expression.

Key Words: arginine • hypertension • microcirculation • nitric oxide synthase • vasodilation

	Introduction

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Hypertension is a major risk factor for coronary artery disease. One characteristic of hypertension that appears to be critical in the development of vascular disease is the impairment of endothelial function. For example, there is a markedly reduced endothelium-dependent vasodilation in both large and small coronary arteries from hypertensive humans and animal models of hypertension.¹ Mounting evidence suggests that this vascular impairment may be related to a diminished production and bioavailability of the potent vasodilator nitric oxide (NO), which may result from an increased vascular production of superoxide anion² or decreased endothelial levels of tetrahydrobiopterin^3,4 or L-arginine.⁵ Interestingly, administration of NO precursor L-arginine restores endothelium-mediated vasodilatory function in patients with essential hypertension,⁵ improves coronary hemodynamics in spontaneously hypertensive rats,⁶ and increases NO production and reduces blood pressure in hypertensive rats with⁷ or without⁸ renal failure. These results suggest the possible reduction of L-arginine availability for NO synthesis in hypertension. However, this phenomenon, generally known as the "L-arginine paradox," is somewhat surprising because endothelial production of NO is increased by exogenous L-arginine instead of endogenous L-arginine despite a saturating intracellular L-arginine concentration.⁹ Although the reason for the reduction of L-arginine bioavailability remains unknown, it is possible that the supply of L-arginine for endothelial NO synthase (eNOS) could be reduced by increased L-arginine utilization by other enzymes. This idea is raised because endothelial cells have been shown to metabolize L-arginine not only by NOS-forming NO but also by arginase-forming urea and ornithine.^9–11

We have recently documented the constitutive expression of arginase-I in porcine coronary arterioles and demonstrated its functional role in modulating coronary microvascular tone under physiological¹² and pathophysiological conditions.¹³ Specifically, arginase appears to serve as an endogenous competitor of eNOS for L-arginine and to play a counteracting role in NO-mediated dilation in normal vessels.¹² Furthermore, the activity and expression of arginase-I can be increased after vascular insult (eg, ischemia–reperfusion) and subsequently can lead to the impairment of NO-mediated vasodilation.¹³ Interestingly, recent studies found that arginase activity is significantly increased in the heart and aorta of animals with various forms of hypertension.^14,15 It remains to be determined, however, whether arginase is increased in the coronary microvasculature during hypertension and whether this change modulates/impairs NO-mediated vasodilation. To address this issue, we tested the hypothesis that endothelial arginase is increased in hypertension, which causes a reduction of L-arginine availability to eNOS and thereby impairs NO-mediated vasodilation. Using an isolated vessel preparation, we examined the role of arginase in endothelium-dependent NO-mediated dilation of porcine coronary arterioles after 8 weeks of hypertension induced by aortic coarctation. The effect of hypertension on NO production, arginase activity, and eNOS and arginase protein expressions in isolated coronary arterioles was also investigated.

	Materials and Methods

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Animal Model
Animal care conformed to the guidelines of the University Laboratory Animal Care Committee, Texas A&M University (College Station, Tex). Renovascular hypertension was produced in pigs via suprarenal aortic coarctation as described in our previous study.¹⁶ Briefly, hypertension was induced in 6 mature (8- to 11-month-old) male, micro-minipigs (Sinclair Research Center, Inc, Columbia, Mo) weighing between 21 and 37 kg. Systemic blood pressure of the animals was monitored by telemetry. An inflatable, silicone occluder (In vivo Metrics, Inc; reinforced by Soloman Scientific) was prefilled with 50% dextrose solution and placed around the suprarenal aorta proximal to the diaphragm. Coarctation of the aorta was produced by slowly adding dextrose to the occluder, through a vascular access port, over a 2-week period until the 24-hour average mean arterial pressure was >150 mm Hg (target blood pressure). Occluder inflation was stopped once the target pressure was reached and maintained for 3 or more consecutive days. Five pigs were used as controls; these pigs had telemetry units and vascular access ports implanted, but they did not have occluder placement. The heart rate, activity level, and arterial blood pressure (systolic, diastolic, and mean) were recorded every 2 hours (30-second intervals) throughout the study period, and a daily average was obtained (24-hour average). After 8 weeks of hypertension, pigs were anesthetized with sodium pentobarbital (30 mg/kg), and the heart was fibrillated and excised for coronary microvessel isolation.

Functional Assessment of Isolated Coronary Arterioles
Individual left ventricular subepicardial arterioles (1 mm in length; 60 to 120 µm inner diameter) were carefully dissected out for in vitro study as described previously.^17,18 Vessels were then cannulated with glass micropipettes, pressurized to 60 cm H₂O intraluminal pressure, and bathed in physiological salt solution. The inner diameters of coronary arterioles were measured using video microscopic techniques.¹⁸ After vessels developed stable basal tone in physiological salt solution at 36°C to 37°C, the dose-dependent dilations to endothelium-dependent agonists adenosine (0.1 nmol/L to 10 µmol/L), serotonin (0.1 nmol/L to 1 µmol/L), and bradykinin (0.1 nmol/L to 1 µmol/L) and to endothelium-independent agonists pinacidil (0.1 nmol/L to 1 µmol/L) and sodium nitroprusside (1 nmol/L to 10 µmol/L) were established. Three protocols were then performed to assess the involvement of NOS and arginase in the vasodilations to these agonists. First, the contribution of NO was examined by treating vessels with the NOS inhibitor, N^G-monomethyl-L-arginine (L-NMMA, 10 µmol/L, 30-minute extraluminal incubation). Second, to address whether the hypertension-induced vascular dysfunction was caused by an L-arginine deficiency, the NO-mediated response of coronary arterioles (from normal and hypertensive pigs) was assessed in the absence and presence of L-arginine (3 mmol/L, 60-minute intraluminal incubation). The specificity of L-arginine was examined by treating vessels with D-arginine (3 mmol/L). Third, the contribution of arginase in these vasodilations was examined by treating vessels with the arginase inhibitor -difluoromethylornithine (DFMO; 0.4 mmol/L, 60-minute intraluminal incubation; a gift from Ilex Oncology)¹⁹ or N-hydroxy-nor-L-arginine (NOHA; 0.1 mmol/L, 60-minute intraluminal incubation; Alexis).^20,21 Drugs were obtained from Sigma, except when specifically stated otherwise.

NO Assay
NO production from both normotensive and hypertensive coronary arterioles (5 to 7 vessels/sample, 100 µm inner diameter, 1 to 2 mm in length) was evaluated by measuring nitrite levels, a major stable breakdown product of NO, using a chemiluminescence NO analyzer (Siever Instruments) as described previously.^12,22 Briefly, after a 30-minute initial incubation at 37°C, 10 µL of adenosine (final concentration 1 µmol/L) was added to the vessel bath. The bathing solution was then collected for NO production assay after a 30-minute incubation. In another series of experiments, the vessels were pretreated with NOHA (0.1 mmol/L) for 30 minutes, and then the NO production in response to adenosine (final concentration of 1 µmol/L) was assayed after a 30-minute incubation. For the control groups, the vehicle solution or NOHA (0.1 mmol/L) was given to the vessel without the subsequent addition of adenosine. Protein levels in each tube were quantified by bicinchoninic acid protein assay (Pierce) and used as a basis to normalize the NO production.

Arginase Activity Assay
Coronary arterioles (5 to 7 vessels/sample, 100 µm inner diameter, 1 to 2 mm in length) from normotensive and hypertensive pigs were also isolated and prepared in lysis buffer for an arginase activity assay as described previously.¹² Briefly, vessel lysate (50 µL) was added into 75 µL of Tris·HCl (50 mmol/L, pH 7.5) containing 10 mmol/L MnCl₂. Heating the lysate at 55°C to 60°C for 10 minutes activated arginase. The hydrolysis reaction of L-arginine by arginase was performed by incubating the mixture containing activated arginase with 50 µL of L-arginine (0.5 mol/L, pH 9.7) at 37°C for 1 hour and was stopped by adding 400 µL of the acid solution mixture (H₂SO₄:H₃PO₄:H₂O=1:3:7). For calorimetric determination of urea, -isonitrosopropiophenone (25 µL, 9% in absolute ethanol) was then added and the mixture was heated at 100°C for 45 minutes. After placing the sample in the dark for 10 minutes at room temperature, the urea concentration was determined spectrophotometrically by the absorbance at 550 nm measured with a microplate reader (Bio-Tek Instruments). The amount of urea produced, after normalization with protein, was used as an index for arginase activity.

Immunohistochemical Analysis
Additional coronary arterioles (60 to 120 µm inner diameter) were prepared for immunohistochemical analysis as described previously.^12,13 Sections (12-µm-thick) were immunolabeled with mouse anti–arginase-I (1:40 dilution; Biosciences-Transduction Laboratories) or anti-eNOS monoclonal antibodies (1:40 dilution; Biosciences-Transduction Laboratories). Staining control tissues were exposed for the same duration to nonimmune mouse serum (1:40 dilution; Jackson ImmunoResearch Laboratories) in place of the primary antibody. The slides were observed and analyzed using the Ultima-Z 312 Confocal Microscope (Meridian Instruments). Normal control and experimental tissues were placed on the same slide and processed under the same conditions. Laser settings for image acquisition were identical for both control and experimental tissues.

Data Analysis
At the end of each functional experiment, the vessel was relaxed with sodium nitroprusside (100 µmol/L) to obtain its maximal diameter at 60 cm H₂O intraluminal pressure.²³ All diameter changes in response to agonists were normalized to the vasodilation in response to 100 µmol/L sodium nitroprusside and expressed as a percentage of maximal dilation. All data are presented as mean±SE. Comparisons of dose responses to agonists and NO production between groups (normotensive versus hypertensive) and within groups (before and after pharmacological treatment) were performed using 2-way analysis of variance (ANOVA) with or without a repeated-measures design, followed by Fisher protected least significant difference multiple range test. Arginase activity and basal tone were compared by a paired Student t test. A value of P<0.05 was considered significant.

	Results

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Systemic Hemodynamics
The mean arterial blood pressure of pigs subjected to aortic coarctation (170±10 mm Hg; n=6) was significantly higher than that of control pigs (90±5 mm Hg; n=5). In the control pigs, the mean heart rate during the study period was 97±3 bpm. The mean heart rate for the hypertensive pigs was 103±9 bpm before occlusion and 83±5 bpm after reaching the target pressure (P>0.05 versus control group). The body weights were not significantly different (P>0.05) between normotensive (43±4 kg) and hypertensive (41±5 kg) pigs.

NO-Mediated Coronary Arteriolar Dilations
In coronary arterioles isolated from normotensive pigs, both adenosine (Figure 1A) and serotonin (Figure 1B) produced a dose-dependent dilation. In the presence of NOS inhibitor L-NMMA (10 µmol/L), the basal vascular tone was slightly increased but did not reach statistical significance (before L-NMMA: 64±1% of maximal diameter; after L-NMMA: 62±2% of maximal diameter; P>0.05); however, the dilation of these vessels to adenosine and serotonin was significantly inhibited (P<0.05; Figure 1). Although vessels isolated from hypertensive pigs had a tendency for higher basal tone to develop (normotensive: 68±2% of maximal diameter; hypertensive: 65±1% of maximal diameter), this did not reach statistical significance (P>0.05). Similar to the decreased dilations to adenosine and serotonin after L-NMMA in normotensive vessels, the vasodilatory responses to these agonists were significantly attenuated in hypertensive vessels. In contrast to the normotensive control vessels, addition of L-NMMA had no inhibitory effect on the dilation of hypertensive vessels to either adenosine (Figure 1A) or serotonin (Figure 1B). The dilations of the hypertensive vessels were restored by a subsequent incubation with L-arginine (3 mmol/L; Figure 2) but not D-arginine (3 mmol/L, n=3; data not shown), suggesting that a reduction in L-arginine availability contributed to the impairment of NO-mediated vasodilation.

View larger version (19K): [in this window] [in a new window]   Figure 1. Coronary arterioles isolated from normotensive (NT) pigs dilated in a dose-dependent manner to adenosine (n=5; A) and serotonin (n=5; B). L-NMMA attenuated these vasodilatory responses (n=5). Vasodilations to adenosine (n=9) and serotonin (n=6) were attenuated in the coronary arterioles isolated from the hypertensive (HT) pigs. L-NMMA did not have any effect on these vasodilations (adenosine, n=5; serotonin, n=5). *P<0.05 between groups by 2-way ANOVA or within groups by repeated-measures 2-way ANOVA.

View larger version (18K): [in this window] [in a new window]   Figure 2. Coronary arterioles isolated from normotensive (NT) pigs dilated in a dose-dependent manner to adenosine (n=5; A) and serotonin (n=5; B). The dilation of coronary arterioles from hypertensive (HT) pigs to adenosine (n=5) and serotonin (n=5) was significantly attenuated. Intraluminal administration of L-arginine (3 mmol/L) restored the impaired vasodilatory responses (n=4). *P<0.05 between groups by 2-way ANOVA or within groups by repeated-measures 2-way ANOVA.

To determine whether arginase contributed to hypertension-induced vascular dysfunction, the role of arginase in modulating NO-mediated vasodilation was first examined in coronary arterioles isolated from normotensive pigs. Administration of arginase inhibitor, NOHA (0.1 mmol/L), did not alter resting vascular tone (before NOHA: 63±1% of maximal diameter; after NOHA: 65±2% of maximal diameter; P>0.05); however, in contrast to L-NMMA, NOHA potentiated vasodilations in response to adenosine (Figure 3A) and serotonin (Figure 3B). Another arginase inhibitor, DFMO (0.4 mmol/L), evoked similar effects as NOHA on agonist-induced vasodilations (n=4; data not shown). In a similar manner as L-arginine, NOHA (0.1 mmol/L) restored the dilation of the hypertensive vessels in response to adenosine and serotonin (Figure 3). There was no difference in the vasodilations to K_ATP channel opener pinacidil, endothelium-dependent cytochrome P-450 monooxygenase activator bradykinin,²⁴ and endothelium-independent vasodilator sodium nitroprusside between coronary arterioles isolated from normotensive and hypertensive pigs (Figure 4).

View larger version (20K): [in this window] [in a new window]   Figure 3. Coronary arterioles isolated from normotensive (NT) pigs dilated in a dose-dependent manner to adenosine (n=5; A) and to serotonin (n=5; B). Arginase inhibitor NOHA (0.1 mmol/L) enhanced vasodilations to adenosine (n=5) and serotonin (n=5). The dilation of vessels from hypertensive (HT) pigs to adenosine (n=6) and serotonin (n=6) was significantly attenuated. After intraluminal administration of NOHA (0.1 mmol/L, n=4), the impaired vasodilatory responses were restored. *P<0.05 between groups by 2-way ANOVA or within groups by repeated-measures 2-way ANOVA.

View larger version (12K): [in this window] [in a new window]   Figure 4. The dilations of coronary arterioles from hypertensive (HT) and normotensive (NT) pigs, in response to pinacidil (n=5; A), bradykinin (n=4; B), and sodium nitroprusside (n=5; C) were not different.

Effect of Hypertension on Arginase Activity and NO Production
Further support for the role of arginase in hypertension-induced vascular dysfunction was provided from the assay of arginase activity in the coronary arteriolar lysate. Arginase activity in vessels from normotensive pigs was 280 nmol urea per milligram of protein, whereas in the vessels from hypertensive animals arginase activity was significantly increased to 550 nmol urea per milligram of protein (P<0.05, Figure 5A). To support the functional study, NO production from coronary arterioles in response to adenosine was assayed. Whereas the amount of basal NO production was 70 nmol per gram of protein for the normotensive control vessels, there was an almost 3-fold increase in NO production after adding adenosine (Figure 5B). Subsequent incubation of the vessels with NOHA significantly enhanced the adenosine-stimulated production of NO (Figure 5B). The NOHA did not alter NO production in the absence of adenosine (data not shown). In contrast, basal (30 nmol nitrite per gram of protein) and adenosine-stimulated NO productions were significantly attenuated in vessels isolated from hypertensive pigs. After incubation of these vessels with NOHA, however, the adenosine-stimulated NO production increased to a level similar to that observed in the normal control vessels treated with adenosine alone.

View larger version (15K): [in this window] [in a new window]   Figure 5. Arginase activity and NO production in coronary arterioles. A, Arginase activity (ie, urea production) was significantly greater in coronary arterioles isolated from hypertensive (HT) pigs (n=4) than from normotensive (NT) pigs (n=4). n indicates number of experiments. *P<0.05 versus control. B, NO production by the coronary arterioles was measured by chemiluminescence detection of nitrite. In NT vessels, adenosine (ADO) stimulated an increase in NO production (n=4). In HT vessels, both the baseline (vehicle) and adenosine-stimulated NO production were significantly diminished (n=4). Concomitant incubation with NOHA potentiated the adenosine-stimulated NO production of both the NT and HT vessels (n=4). n indicates number of experiments. *P<0.05 vs vehicle; **P<0.05 vs ADO; P<0.05 vs NT.

Effect of Hypertension on Arginase-I and eNOS Expressions in Coronary Arterioles
To determine whether hypertension altered the expression of arginase-I and eNOS in coronary arterioles, isolated subepicardial arterioles were subjected to immunohistochemical study. The background level of staining was determined in a vessel treated with the nonimmune mouse serum, secondary antibody, and fluorescein avidin D (data not shown). The coronary arterioles isolated from normotensive pigs expressed a low level of signal for arginase-I, as represented by a pseudo-color spectral display, compared with that in arterioles from the hypertensive pigs (Figure 6A). The arginase-I expression was markedly increased in both endothelial and smooth muscle cells in the vessels isolated from hypertensive pigs (Figure 6A). In contrast to the arginase-I, eNOS protein expression was significantly reduced in the coronary vessels isolated from hypertensive pigs (Figure 6B).

View larger version (37K): [in this window] [in a new window]   Figure 6. Immunohistochemical detection of arginase and eNOS in coronary arterioles. Cross-section views of fluorescein-labeled vessels after treatment with either anti–arginase-I or anti-eNOS primary antibodies are shown as a pseudo-color spectral display. A, Moderate levels of arginase-I signal, as represented by the color pallet, were detected in both the endothelium (arrow head) and smooth muscle (arrow) of the normotensive (NT) vessel. The arginase-I signal intensity in both the endothelial and smooth muscle cells of the hypertensive (HT) vessel was increased. B, High levels of eNOS signal were detected in the endothelial cells of the NT vessel. The eNOS signal intensity in the endothelium of the HT vessel was markedly decreased. Data shown are representative of 4 separate experiments.

	Discussion

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It is well-recognized that an important manifestation of systemic hypertension is the structural changes of both coronary vessels and the myocardium. In addition to the structural changes, the corresponding functional alteration is also well-documented. For example, coronary vasodilator reserve was found to be reduced in hypertensive patients as well as in experimental hypertension.^25–27 However, the underlying mechanisms for the functional abnormalities of coronary arterioles in arterial hypertension remain incompletely understood. Interestingly, in vivo studies indicate that coronary flow regulation in hypertensive animals under resting conditions is near normal, but the flow supply is significantly reduced during metabolic stress.^26,27 Because adenosine is involved in the metabolic regulation of coronary blood flow, it is reasonable to speculate that the coronary response to adenosine might be impaired in hypertension. In vivo studies support this view, showing that coronary vasodilatory reserve for adenosine is significantly reduced in humans and animals with chronic hypertension.^26–28 Using isolated vessel approaches, we also found that adenosine-induced dilation was impaired in coronary arterioles from hypertensive pigs. Because we have previously shown that coronary arteriolar dilation to adenosine is mediated by the release of NO from the endothelium and the activation of K_ATP channels in vascular smooth muscle,²² it is speculated that the reduced response to adenosine might result from the impairment of either or both of these pathways. However, coronary arterioles from hypertensive and normotensive pigs exhibited identical vasodilation to K_ATP channel activator pinacidil (Figure 4), suggesting that the reduced adenosine response is not related to K_ATP channel function. We also found that coronary arteriolar dilation in response to another endothelium-dependent NO-mediated vasodilator serotonin¹² was impaired, but the dilations in response to the activation of endothelium-dependent cytochrome P-450 monooxygenase pathway (by bradykinin)²⁴ and the endothelium-independent NO donor sodium nitroprusside remained normal. The ability of the arterioles to respond to exogenous NO, but not to endothelium-dependent NO-mediated agonists such as adenosine and serotonin, appears to suggest a selective inhibition of the endothelial NO pathway for coronary arteriolar dilation in pigs with aortic coarctation–induced hypertension.

In the present study, the diminished endothelium-dependent NO-mediated dilation in hypertensive vessels was comparable to the reduced dilation observed in the normal vessels treated with L-NMMA, suggesting that the endothelium-dependent NO-mediated responses were nearly abolished in hypertension. This idea was supported further by data showing that the agonist-stimulated NO production in hypertensive vessels was reduced. The slight increase in agonist-stimulated NO production appears to reflect nonvasoactive levels of NO (ie, below the threshold for vasodilation) or, alternatively, may be attributed to the different experimental conditions of pressurized vessels (functional study) versus nonpressurized vessels (biochemical study). It should be noted that the baseline NO production in hypertensive vessels was significantly reduced. This may explain the tendency of increased basal tone in hypertensive vessels, although it did not reach statistical significance. Nevertheless, to the best of our knowledge, the present results provide the first direct evidence that relatively short-term hypertension (8 weeks) can lead to impaired agonist-induced NO production and subsequent NO-mediated dilation in the coronary microcirculation.

Our findings on the improvement of NO-mediated dilations of coronary arterioles from the hypertensive animals by L-arginine suggest that a reduction in L-arginine availability was involved in the vascular dysfunction. This observation is in accord with previous evidence in which oral administration of L-arginine improves endothelium-dependent NO-mediated flow-mediated dilation of the brachial artery in patients with essential hypertension.⁵ Nevertheless, the specific cellular mechanisms contributing to the dysregulation of L-arginine remain obscure. Interestingly, besides the NOS isoforms, arginase is another major L-arginine–consuming enzyme that converts L-arginine to L-ornithine and urea. Arginase is expressed most abundantly in the liver for ammonia detoxification via the urea cycle. Recent studies indicate that some extrahepatic tissues/cells, including endothelial and smooth muscle cells, which do not possess complete urea cycle enzymes, also express arginase.^10,29 Moreover, our recent findings support an endogenous role of vascular arginase in regulating NO-mediated dilation of coronary arterioles.¹² It appears that the arginase residing in the endothelium contributes to the regulation of NO-mediated vasodilation under normal conditions, because inhibition of arginase activity significantly enhances endothelium-dependent NO-mediated coronary arteriolar dilations (Figure 3) and increases the stimulated NO production in normal blood vessels.¹² However, exogenous administration of L-arginine, but not D-arginine, to arterioles specifically enhances agonist-stimulated NO-mediated vasodilations.¹² This L-arginine–dependent NO response was also observed in various microvascular beds,^30–32 including human coronary microvessels.³³ Collectively, these results suggest that L-arginine can be a limiting factor for agonist-stimulated NO synthesis in coronary microvessels and that arginase expressed in the endothelium can compete with NOS for their common substrate L-arginine and thus influence NO production.

Because the overall NO production for dilation in coronary arterioles can be modulated by arginase activity, it is plausible that upregulation of vascular arginase in hypertension causes a reduction of L-arginine availability to NOS and thereby compromises NO-mediated vasodilation. Our present findings showed that arginase protein expression and activity were significantly greater in the hypertensive vessels than in the normotensive control vessels. Furthermore, the arginase inhibitors NOHA and DFMO were able to improve the NO-mediated vasodilation and NO production of coronary arterioles isolated from hypertensive pigs. Of the 2 distinct arginase isoforms that have been identified,³⁴ our previous findings demonstrate that the arginase-I, but not arginase-II, is expressed in coronary arterioles, with the cellular localization appearing to be in both the endothelium and vascular smooth muscle.¹² Interestingly, a recent study showed that aortic arginase activity was elevated in established hypertension of deoxycorticosterone acetate–salt rats compared with the normotensive counterparts, and that increased arginase-I activity could accelerate the development of hypertension in these animals.¹⁴ In concert, our present data demonstrate a physiological role of endothelial arginase in modulating NO-mediated dilation of coronary arterioles and a putative pathophysiological role of its upregulation in microvascular dysfunction associated with hypertension.

It has been suggested that the mechanisms associated with the impairment of endothelium-dependent vasodilation during hypertension may be multifactorial. In the present study, arginase inhibition or L-arginine administration did not fully restore normal vasodilatory function (ie, vasodilation attained by normotensive control vessels in the presence of NOHA [Figure 3] or excess L-arginine¹² of hypertensive vessels). Furthermore, combined administration of NOHA and L-arginine did not further improve the vasodilatory response to adenosine (data not shown). Collectively, all of these results indicate that additional inhibitory mechanisms may be involved. Three potential mechanisms are postulated from previous studies and the present study. First, the increased levels of vascular superoxide anions in hypertension might directly inactivate NO.^35,36 Second, the improvement of endothelium-dependent NO-mediated response by the addition of synthetic tetrahydrobiopterin, or its intermediate donor sepiapterin, to vessel rings from animals with hypertension^3,4 suggests that diminished levels of this eNOS cofactor may contribute to the endothelial dysfunction related to NO deficiency. Third, the downregulation of eNOS expression in the coronary arterioles from hypertensive animals, as revealed by immunohistochemistry herein, is another potential mechanism. Although the aforementioned mechanisms could contribute to the impairment of NO-mediated dilation in hypertension, the predominant mechanism in the present model appears to be related to the upregulation of arginase.

Although the present study did not identify the culprit for regulating arginase expression in hypertension, it is reasonable to speculate that increased local production of pro-inflammatory factors such as cytokines and/or angiotensin II mediate the specific enzyme activation because production of these factors have been implicated in the cardiovascular complications of hypertension,^37,38 and they have been shown to inhibit endothelium-dependent dilation in the coronary circulation.^39,40 In particular, the release of the cytokine tumor necrosis factor- from monocytes has been shown to be increased in hypertensive patients.³⁷ Other studies have shown that tumor necrosis factor- contributes to the increase in arginase activity in bovine pulmonary arterial endothelial cells⁴¹ and inhibits relaxation of bovine coronary arteries to endothelium-dependent NO-mediated agonists.³⁹ Although there appears to be no evidence at this time for a direct influence of angiotensin II on vascular arginase, a recent study has shown that an angiotensin II AT₁ receptor inhibitor, irbezantane, inhibited the increase in arginase activity in erythrocytes from hypertensive patients.⁴² Furthermore, it should be noted that the hemodynamic changes, ie, high intraluminal pressure per se or compensatory changes in coronary blood flow/shear stress associated with cardiac hypertrophy, may also be involved in the upregulation of arginase. Future studies are warranted to ascertain the putative contribution of tumor necrosis factor- and other pro-inflammatory cytokines, angiotensin II, or mechanical stress to the regulation of arginase expression and the arginase-induced impairment of endothelium-dependent NO-mediated dilation in hypertension.

Perspectives
This study provides evidence for physiological and pathophysiological roles of arginase in the coronary microcirculation. The results further support the idea that arginase expressed in the endothelium can regulate NO-mediated vascular function in normal vessels and provide the first evidence that the upregulation of this enzyme in hypertension can impair NO-mediated vasodilation. An additional mechanism that may exacerbate the impaired NO-mediated response appears to be the downregulation of eNOS. Because interstitial levels of both adenosine (from myocardium and vascular cells) and serotonin (from platelets, mast cells, and sympathetic nerve endings) are elevated in myocardium during metabolic stress, it is conceivable that the impairment of NO-mediated responses to these endogenous vasodilators under this condition may lead to the deficiency of coronary perfusion and possibly exacerbate suppressed cardiac functions during hypertension. We anticipate that a better understanding of the vascular arginase/NOS signaling pathway and affinity of L-arginine, as well as reaction kinetics of each enzyme in the intact vascular tissue, could yield important information for therapeutic treatment of coronary vasomotor dysfunction associated with hypertension.

	Acknowledgments

 
This study was supported by National Heart, Lung, and Blood Institute grant HL-64372.

Received May 24, 2004; first decision June 8, 2004; accepted September 24, 2004.

	References

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