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Effect of l -arginine infusion on systemic and renal hemodynamics in hypertensive patients
AJH
1999;12:8 –15
Effect of l-Arginine Infusion on Systemic and Renal Hemodynamics in Hypertensive Patients Yukihito Higashi, Tetsuya Oshima, Ryoji Ozono, Hideo Matsuura, Masayuki Kambe, and Goro Kajiyama
This study was designed to compare the renal endothelial function in patients with essential hypertension and normal renal function with that in hypertensive patients with renal insufficiency. We studied the effects of L-arginine (500 mg/kg intravenously over 30 min) on renal hemodynamics in 30 normotensive control subjects, 32 patients with mild to moderate essential hypertension who had normal renal function, and seven hypertensive patients with renal insufficiency who had a serum creatinine concentration >2.0 mg/mL and a glomerular filtration rate <50 mL/min/1.48 m2. LArginine infusion similarly reduced the mean blood pressure between the three groups (normotensive: 29.7% 6 0.7%, hypertensives with normal renal function: 210.2% 6 0.8%, and hypertensives with renal insufficiency: 28.2% 6 1.3%). The L-arginine–induced decrease in renal vascular resistance was smaller in essential hypertensive patients than in normotensive subjects (2 211.0% 6 2.2 v 219.8% 6 2.1%, P <.05). However, L-arginine had no effect on the renal vascular resistance in hypertensive patients with
renal insufficiency (1.6% 6 4.8%). Urine nitrite/nitrate levels in response to L-arginine significantly increased in the three groups in the following order: patients with renal insufficiency (47% 6 15%), essential hypertensive patients (87% 6 10%), and normotensive subjects (129% 6 12%). The glomerular filtration rate was unaffected by L-arginine in normotensive and essential hypertensive patients (3.1% 6 2.4% and 4.2% 6 2.5%), but significantly decreased in hypertensive patients with renal insufficiency (2 213.7% 6 6.1%). These findings suggest that the ability of the L-arginine–nitric oxide– cGMP pathway to relax the renal vascular tone may be impaired in essential hypertensive patients and more markedly blunted in hypertensive patients with renal insufficiency, in parallel with increasing serum creatinine concentrations. Am J Hypertens 1999;12:8 –15 © 1999 American Journal of Hypertension, Ltd.
Received February 13, 1998. Accepted August 11, 1998. From the First Department of Internal Medicine (YH, HM, GK), and Department of Clinical Laboratory Medicine (TO, RO, MK), Hiroshima University School of Medicine, Hiroshima, Japan. Presented in part at the Council for High Blood Pressure Research 50th Annual Fall Conference and Scientific Sessions, American Heart Association, Chicago, Illinois, September 17–20, 1996, and published in abstract form in Hypertension (1996;28:546). This study was supported in part by a Grant-in-Aid for Scientific
Research (08457639) from the Ministry of Education, Science and Culture of Japan and a Foundation for Total Health Promotion grant. Address correspondence and reprint requests to Yukihito Higashi, MD, PhD, First Department of Internal Medicine, Hiroshima University School of Medicine, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8551, Japan; e-mail: [email protected]. ac.jp
© 1999 by the American Journal of Hypertension, Ltd. Published by Elsevier Science, Inc.
KEY WORDS: L-Arginine,
nitric oxide, essential hypertension, hypertensive patients with renal insufficiency, renal hemodynamics.
0895-7061/99/$20.00 PII S0895-7061(98)00204-0
AJH–JANUARY 1999 –VOL. 12, NO. 1, PART 1
H
ypertension and renal failure often coexist, and it is widely accepted that hypertension can accelerate renal failure. The kidney plays an important role in blood pressure regulation, but its role in the development of hypertension has not been fully clarified. It is known that the kidney is a rich source of vasodilatory substances such as prostaglandins and endothelium-derived relaxing factors.1 It is possible that endotheliumderived relaxing factor/NO plays an important role in the regulation of hemodynamics in the kidney, specifically the renal vasculature. Recently, we also have reported that the administration of l-arginine has the beneficial effect of lowering blood pressure with increasing renal plasma flow (RPF) and decreasing renal vascular resistance (RVR) in both normotensive and hypertensive subjects with normal renal function.2–5 However, no attention has focused on the effects of endothelium-dependent renal vascular relaxation in hypertensive patients with renal insufficiency in vivo. We postulated that that l-arginine infusion might improve endothelial dysfunction even in hypertensive patients with renal failure. Therefore, the present study was designed to evaluate the effects of the intravenous administration of l-arginine on systemic and renal hemodynamics in normotensive control subjects, patients with essential hypertension who had normal renal function, and hypertensive patients with renal impairment. METHODS Subjects Thirty-two patients with essential hypertension (18 men and 14 women; mean age 47 6 2 years), seven hypertensive patients with renal insufficiency (five men and two women; mean age 50 6 5 years), and 30 normotensive subjects (19 men and 11 women; mean age 45 6 3 years) were evaluated. Hypertension was defined as a systolic pressure of .160 mm Hg or a diastolic pressure of .95 mm Hg while the subject was in the sitting position. Measurements were obtained on at least three different occasions in the outpatient clinic at the Hiroshima University School of Medicine. Patients with essential hypertension had serum creatinine concentrations of ,1.5 mg/dL (132.6 mmol/L) and glomerular filtration rates (GFR) of more than 50 mL/min/1.48 m2. Individuals with severe essential hypertension, who had a systolic blood pressure of .180 mm Hg or a diastolic pressure of .120 mm Hg, with objective signs of hypertensive end-organ disease were excluded. In addition, no patients with essential hypertension had a history of cardiovascular or cerebrovascular disease, diabetes mellitus, hypercholesterolemia, or liver disease. No antihypertensive drugs were taken by these patients for $4 weeks before the study. Hypertensive patients with renal insufficiency had serum creatinine concen-
ENDOTHELIAL FUNCTION AND RENAL INSUFFICIENCY
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trations of $2.0 mg/dL (range; 2.1 to 4.9 mg/dL (185.6 to 433.2 mmol/L) and GFR of ,50 mL/min/1.48 m2. These patients underwent no hemodialysis during the study and had no history of hemodialysis. Individuals with known renal diseases, such as glomerulonephritis, interstitial renal disease, and polycystic kidney, diabetic nephropathy, and renovascular stenosis, also were excluded. Five hypertensive patients with renal insufficiency had been treated with either diuretics, calcium antagonists, or a combination of both. No patients were receiving medication at the time of the study. Normotension was defined as a systolic pressure of ,140 mm Hg and a diastolic pressure of ,80 mm Hg. Normotensive control subjects had no history of serious diseases and took no medication for $4 weeks before the study. Informed consent was obtained from all subjects. The study protocol was approved by the ethics committee of the First Department of Internal Medicine. Protocol All subjects were hospitalized for $7 days and placed on a diet containing 170 mmol of sodium chloride/day for 1 week before the study. Throughout the study, the subjects ingested a constant amount of calories (40 cal/kg/day), potassium (100 mmol/day), and calcium (40 mmol/day). All subjects received meals prepared in the Hiroshima University Hospital kitchen. Compliance with the diet was assessed by measuring 24-h urinary sodium, chloride, and potassium excretion throughout the study. The l-arginine infusion study began at 8:30 am. Subjects fasted overnight for $12 h and were kept in the supine position in a quiet, dark, air-conditioned room maintained at a constant temperature (22°C to 25°C) throughout the study. A 19-gauge polyethylene catheter (Terumo Co, Tokyo, Japan) was inserted into the right antecubital vein for the infusion of para-aminohippurate (PAH), inulin, and l-arginine. Another catheter was inserted into the left antecubital vein to obtain blood samples. After a 30-min rest period, initial doses of PAH (8.0 mg/kg) and inulin (16 mg/kg) were each infused as a bolus. PAH and inulin were subsequently infused at constant rates of 12 and 20 mg/min, respectively, with a syringe pump (Terfusion; Terumo Co., Tokyo, Japan) throughout the study.6 Sixty minutes after the PAH and inulin infusions began, l-arginine (500 mg/ kg) was administered over 30 min with an infusion pump (PEI-1000; Pal Medical Co., Tokyo, Japan). A 30-min recovery period was allowed after the end of the l-arginine infusion. Blood pressure and heart rate measurements were performed with a TM2420 monitor (AND Co., Tokyo, Japan) every minute on the upper left arm. The mean blood pressure was calculated as the diastolic pressure plus one third of the pulse pressure. Blood samples were obtained for the determination of serum concentrations of PAH and
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inulin at baseline (0 min) and 15, 30, and 60 min after the start of l-arginine administration. Baseline fasting serum concentrations of total cholesterol, creatinine, glucose, and electrolytes also were obtained at 0 min. The urinary excretion of nitrite/nitrate (NOx), creatinine, and electrolytes were obtained during 1 h before and after the start of l-arginine infusion. Analytical Methods Samples of venous blood were placed in tubes containing EDTA-Na (1 mg/mL) and polystyrene tubes. EDTA tubes were promptly chilled in an ice bath. The plasma and serum were immediately separated by centrifugation at 3100 g at 4°C for 10 min and at 1000 g at room temperature for 10 min, and stored at 280°C until assayed. Serum concentrations of total cholesterol, creatinine, glucose, electrolytes, and urinary electrolytes were determined by routine chemical methods. Plasma renin activity was measured by radioimmunoassay (Gamma Coat PRA, Baxter Travenol Co., Tokyo, Japan). Urine concentration of NOx was assayed by colorimetric methods using NOx assay kits (Cayman Chemical Co, Ann Arbor, MI). Briefly, nitrate in the sample is converted to nitrite using nitrate reductase. The second step is the addition of Griess reagents7 (1% sulfonamide, 0.1% N-1-naphthylethylenediamide, and 5% HCl), which convert nitrite into a deep purple azo compound. The absorbance of this azo dye at 540 nm was measured by a microplate reader (M-Tmax; Wako Co, Osaka, Japan). The RPF was measured by the clearance of PAH, and serum PAH concentration was analyzed by spectrophotometry. The GFR was measured by the clearance of inulin,8 and the serum inulin concentration was analyzed by the anthrone method.9 The RVR was calculated as the mean blood pressure divided by renal blood flow, and FF as the GFR divided by RPF. The RPF, GFR, and RVR were normalized to the body surface area divided by 1.48 m2 (1.48 m2 being the average body surface area of the Japanese population). Drugs The l-arginine used for intravenous administration was l-arginine hydrochloride (MorishitaRuseru Pharmaceutical Co., Osaka, Japan). d-Arginine was d-arginine hydrochloride (Sigma Chemical Co., St. Louis, MO). The administered inulin was Inutest (Laevosan-Gesellschaft, Lintz, Austria), and the PAH was from Daiichi Pharmaceutical Co., Osaka, Japan. Statistical Analysis Comparisons of baseline parameters among the three groups were made using Student’s unpaired t test. Differences were compared using one way analysis of variance for repeated measures, followed by the Scheffe´ F test. Linear regression analysis was used to test the relationship among the peak RPF and RVR responses to l-arginine and the serum creatinine level. Multivariate analysis, using
AJH–JANUARY 1999 –VOL. 12, NO. 1, PART 1
stepwise regression, was performed to examine relationships among the peak RPF and RVR responses to l-arginine and age, gender, body mass index, cholesterol level, glucose level, plasma renin activity, and creatinine level. All results are presented as the mean 6 SEM. Multivariate analysis was performed with the SAS (Statistical Analysis System, SAS Inc., Cary, NC) REG procedure program package. Statistical significance was defined as a P , .05. RESULTS Clinical Characteristics The baseline clinical characteristics of the three groups of subjects are summarized in Table 1. The mean blood pressure and RVR were higher in essential hypertensive patients and hypertensive patients with renal insufficiency than in normotensive subjects. The RVR was higher in hypertensive patients with renal insufficiency than in patients with essential hypertension. The RPF, GFR, and FF values were less in hypertensive patients with renal insufficiency than in the other two groups, whereas the serum creatinine concentration was higher. Other parameters were similar among the three groups. Effects of L-Arginine on Mean Blood Pressure and Heart Rate The percent changes in mean blood pressure and heart rate above baseline levels during larginine infusion in the study groups are shown in Figure 1. Mean blood pressure promptly decreased after the intravenous administration of l-arginine, but quickly returned to the baseline level after the end of infusion. Conversely, heart rate gradually increased during l-arginine infusion, and gradually returned to the baseline level during the recovery period. There was no significant difference in the time course of changes in mean blood pressure and heart rate among the three groups. Effects of L-Arginine on Renal Hemodynamics Figure 2 shows the percent changes in parameters of renal hemodynamics (such as RPF, GFR, RVR, and FF) above baseline levels during l-arginine infusion in the study groups. The l-arginine–induced increase in RPF was less in patients with essential hypertension than in normotensive control subjects (10.1% 6 0.8% v 15.8% 6 0.9%, P , .05); conversely, RPF decreased in hypertensive patients with renal insufficiency in response to l-arginine (211.5% 6 5.0%). The decrease in RVR response to l-arginine was less in patients with essential hypertension than in normotensive subjects (214.3% 6 0.7% v 220.1% 6 0.8%; P , .05); RVR remained unchanged in hypertensive patients with renal insufficiency (4.8% 6 7.0%). l-Arginine infusion did not significantly alter the GFR in either normoten-
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TABLE 1. BASELINE CLINICAL CHARACTERISTICS IN NORMOTENSIVE SUBJECTS, ESSENTIAL HYPERTENSIVE PATIENTS, AND HYPERTENSIVE PATIENTS WITH RENAL INSUFFICIENCY
Parameter Body weight (kg) Body mass index (kg/m2) Mean blood pressure (mm Hg) Heart rate (beats/min) Serum creatinine (mmol/L) Total cholesterol (mmol/L) Serum glucose (mmol/L) Plasma renin activity (ng/L/sec) Urine NOx (mmol/mol creatinine) Renal hemodynamics RPF (mL/min/1.48 m2) GFR (mL/min/1.48 m2) RVR (mm Hg/mL/min/1.48 m2) FF
Normotensive Subjects (n 5 30) 65.6 6 2.9 25.1 6 1.1 80.1 6 1.4 60.1 6 2.1 88.9 6 6.9 4.4 6 0.3 5.0 6 0.2 0.27 6 0.04 127 6 17 603.6 6 25.4 90.1 6 2.3 0.084 6 0.006 0.148 6 0.006
Essential Hypertensive Patients (n 5 32)
Hypertensive Patients With Renal Insufficiency (n 5 7)
67.3 6 3.1 25.6 6 1.2 111.8 6 2.0* 59.7 6 1.9 89.3 6 7.5 4.3 6 0.3 4.9 6 0.1 0.29 6 0.04 119 6 16
63.7 6 5.6 24.7 6 2.1 114.2 6 6.1* 62.0 6 3.4 238.8 6 33.6*† 4.4 6 0.4 5.1 6 0.2 0.34 6 0.13 110 6 21
576.3 6 29.1 87.4 6 2.2 0.124 6 0.007* 0.147 6 0.005
328.2 6 35.1*† 31.4 6 4.4*† 0.220 6 0.020*† 0.104 6 0.013*†
NOx indicates nitrite/nitrate; RPF, renal plasma flow; GFR, glomerular filtration rate; RVR, renal vascular resistance; FF, filtration fraction. All results are presented as mean 6 SEM. * P , .001 v normotensive subjects. † P , .001 v essential hypertensive patients.
sive or hypertensive subjects (3.1% 6 2.4% or 4.2% 6 2.5%), but decreased the GFR in hypertensive patients with renal insufficiency (213.7% 6 5.3%). The FF was unchanged by l-arginine in both hypertensive groups (essential hypertensive subjects, 22.7% 6 2.1%; subjects with renal insufficiency, 22.7% 6 2.0%), but decreased in normotensive subjects (28.7% 6 2.0%). When multiple regression analysis was performed using the peak RPF response to l-arginine as a dependent variable, the serum creatinine level (P , .0001), age (P , .005), and mean blood pressure (P , .0006) were selected as independent variables. When multiple regression analysis was performed using the peak RVR response to l-arginine as a dependent variable, the serum creatinine level (P , .0001) and mean blood pressure (P , .03) were selected as independent variables. The multiple regression analysis equations are as follows: %RPF 5 42.4 20.10 z serum creatinine level 20.21 z age 2 0.11 z mean blood pressure, %RVR 5 239.4 1 0.09 z serum creatinine level 1 0.10 z mean blood pressure. Effects of L-Arginine on Urinary Excretion of NOx The baseline urine NOx levels were similar among the three groups of subjects (Table 1). The intravenous administration of l-arginine significantly increased urine NOx levels in the three groups, but this increase in NOx was less in hypertensive groups than in normotensive subjects (essential hypertensive subjects, 87% 6 10%; subjects with renal insufficiency, 47% 6
15% vs 129% 6 12%; P , .05, P , .01, respectively) and was less in hypertensive patients with renal insufficiency than in patients with essential hypertension (P , .05) (Figure 3). When multiple regression analysis was performed using the NOx response to l-arginine as a dependent variable, serum creatinine level (P , .01), age (P , .003), and mean blood pressure (P , .0001) were selected as independent variables. The multiple regression analysis equation is as follows: %NOx 5 280.4 2 0.12 z serum creatinine level 2 0.71 z age 2 0.79 z mean blood pressure. The maximal NOx response to l-arginine, which is a marker of nitric oxide production, was significantly correlated with the peak increases in RPF (r 5 0.69; P , .0001) and the peak decreases in RVR (r 5 20.76; P , .0001) (Figure 4). Side Effects The side effects associated with l-arginine infusion were minimal. Two subjects (one normotensive subject and one patient with essential hypertension) reported slight pain on the site where the catheter was inserted into the antecubital vein after the start of l-arginine infusion. Renal vascular responses to l-arginine were usually reversible in all subjects. Although clinical parameters such as the serum creatinine concentration were reexamined within a few days after the study in all hypertensive patients with renal insufficiency, none of the subjects demonstrated deterioration based on these parameters.
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FIGURE 1. Line graphs showing the effects of l-arginine infusion on mean blood pressure and heart rate in normotensive subjects (open circles), patients with essential hypertension (solid circles), and hypertensive patients with renal insufficiency (solid boxes). The responses of mean blood pressure and heart rate to l-arginine were similar among the three groups. All results are presented as the mean 6 SEM. The probability value refers to the comparison of time course curves using ANOVA for repeated measures.
DISCUSSION l-Arginine significantly increased RPF in normotensive subjects and essential hypertensive patients with normal renal function, whereas it decreased RPF in hypertensive patients with renal insufficiency. l-Arginine did not induce a detectable change in RVR in hypertensive patients with renal insufficiency, whereas l-arginine significantly decreased RVR in the other groups. Changes in NOx levels evoked by larginine increased in the following order: hypertensive patients with renal insufficiency, patients with essential hypertension, and normotensive subjects. We predicted that hypertensive patients with renal insufficiency would have impaired endothelium-dependent renal vascular relaxation compared with es-
sential hypertensive patients with normal renal function and normal control subjects. In the present study, patients with essential hypertension who have normal renal function had a blunted renal vascular relaxation response to l-arginine infusion compared with normotensive subjects. This finding is consistent with findings documented in our previous reports2–5 and in other reports using forearm and coronary arteries.10,11 However, it was unexpected that l-arginine infusion not only did not cause renal vascular relaxation in the hypertensive patients with renal insufficiency, but even precipitated renovascular constriction. In hypertensive patients with renal sufficiency, the decrease in RPF and GFR is explained entirely by the reduction in perfusion pressure. Our findings suggest that those with renal insufficiency can adequately metabolize l-arginine to NO. This might be due to reduced delivery of the substrate or impaired access to NO synthase, or to reduced enzyme itself. The presence of other mechanisms for the autoregulation of renal hemodynamics during the hypotensive response to l-arginine in hypertensive patients with renal insufficiency cannot be eliminated by this study. The other possible explanation for the lack of an endothelium-dependent renal vascular response in hypertensive patients with renal insufficiency may involve a direct cytotoxic action of l-arginine–NO. Several investigators have reported the relationship between NO and cellular damage.12,13 Far advanced renal impairment, with severely damaged blood vessel endothelium, may represent direct injury by excessive NO. Conversely, the increasing production of NO may lead to vasodilation in normal to mildly damaged vessels. It also has been shown that superoxide is produced in excess in renal failure. In addition, Radi et al14 have demonstrated that peroxynitrite, which is prepared by allowing NO to react with superoxide, has a hydroxyl radical with a strong cytotoxic action. Therefore, counterregulatory effects of the renal response to l-arginine may be caused by this molecule. Another important finding in this study is that the renal vasodilating response to l-arginine was attenuated with increasing serum creatinine levels. When multiple regression analysis was performed using the peak RPF response to l-arginine in our subjects (who did not have abnormal metabolism of lipids or glucose) as the dependent variable, the serum creatinine level, age, and baseline mean blood pressure were selected as independent variables. Similarly, when the peak RVR response to l-arginine was selected as the dependent variable, the serum creatinine level and baseline mean blood pressure were selected as independent variables. The serum creatinine level was the strongest anticipated variable of a blunted renal vascular dilation response to l-arginine among the three variables. This finding may have important clinical
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FIGURE 2. Line graphs showing the effects of l-arginine infusion on parameters of renal hemodynamics, such as renal plasma flow, glomerular filtration rate, renal vascular resistance, and filtration fraction, in normotensive subjects (open circles), patients with essential hypertension (solid circles), and hypertensive patients with renal insufficiency (solid boxes). The responses of renal plasma flow to l-arginine were less in patients with essential hypertension than in normotensive subjects; the reverse relationship was found hypertensive patients with renal insufficiency. The response of renal vascular resistance was less in patients with essential hypertension than in normotensive subjects, but was unchanged in hypertensive patients with renal insufficiency. The response of the glomerular filtration rate was decreased in hypertensive patients with renal insufficiency, whereas these responses were unchanged in patients with essential hypertension and normotensive subjects. The response of the filtration fraction was decreased in normotensive subjects but was unchanged in other groups. All results are presented as the mean 6 SEM. The probability value refers to the comparison of time course curves using analysis of variance (ANOVA) for repeated measures. *P , .0001 versus normotensive subjects; †P , .0001 versus patients with essential hypertension.
relevance. The abnormality of l-arginine–NO– cGMP pathway involving the elevated creatinine level may play an important role in the development of renal impairment in hypertension. Our previous study showed that renal vasorelaxation and urine NOx responses were much greater during l-arginine infusion as compared with d-arginine.3 It has been reported that other amino acids that are not substrates for NO also produce renal vasodilation.15 Although the precise mechanism is unclear, only one third of the effects of l-arginine on RPF can be explained by these amino acid effects. These find-
ings may suggest that exogenous l-arginine infusion may cause a renal vasorelaxation mostly through the release of NO rather than a nonspecific effect of amino acid. However, d-arginine caused a small but significant increase in RPF. These data, coupled with the previous studies demonstrating the effect of other amino acids on RPF, suggest that all of the response to l-arginine is not due to NO. In addition, it is well known that l-arginine induces the release of a number of substances, including histamine and insulin, that may have vasoactive effects. Thus, we cannot deny the possibility that vasoactive agents other than NO may
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FIGURE 3. Bar graph showing the effects of l-arginine infusion on urine NOx concentrations in normotensive subjects (open bar), patients with essential hypertension (solid bar), and hypertensive patients with renal insufficiency (striped bar). The response of NOx to l-arginine was less in patients with essential hypertension than in normotensive subjects and was lower still in hypertensive patients with renal insufficiency. All results are presented as the mean 6 SEM. *P , .05, **P , .01 v normotensive subjects; †P , .05 v patients with essential hypertension.
be involved in the renal vasodilation induced by larginine. The use of specific NO synthase inhibitors such as N G -monomethyl-l-arginine and N G -nitro-l-argininemethylester, and agonists to stimulate NO release such as acetylcholine or bradykinin, would allow us to draw more specific conclusions concerning the role of the basal and stimulated release of NO in the renal circulation. In addition, to conclude the specificity of abnormalities in the l-arginine–NO pathway, it is useful to test the renal vasodilatory responses to other vasodilators such as prostaglandins. However, because the intravenous infusion of NO synthase inhibitors can increase the blood pressure and vascular resistance, these agents may lead to adverse effects in hypertensive patients. We therefore did not investigate these agents in certain aspects because of ethical considerations. Although the present results suggested that the endothelium-dependent renal vasodilation observed in response to l-arginine was impaired in patients with renal insufficiency as compared with patients without renal insufficiency, the number of patients with renal insufficiency is small. A large sample size of patients may allow more specific conclusions. Our results showed the marked differences in response to l-arginine between patients with similar GFR. Several lines of evidence suggest that endothelial dysfunction is related to cardiovascular risk factors
FIGURE 4. Scatter plot showing the relationship between the maximum percent change in NOx responses to l-arginine and maximum percent change in renal plasma flow response to larginine (top) and the maximum percent change in renal vascular resistance response (bottom), to l-arginine in normotensive subjects (open circles), patients with essential hypertension (solid circles), and hypertensive patients with renal insufficiency (solid triangles). The peak increase in NOx significantly correlated with the maximum percent change in the renal plasma flow response and the maximum percent change in renal vascular resistance response to l-arginine.
such as aging, high blood pressure, and atherosclerosis.3,10,16,17 Therefore, these factors other than GFR may contribute to differences in response to l-arginine in patients with similar degrees of renal insufficiency. In conclusion, the present study demonstrates that l-arginine–induced renal vascular dilatation is markedly impaired in hypertensive patients with renal insufficiency compared with normal control subjects and patients with essential hypertension who have normal renal function. Modifications in the l-argi-
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nine–NO-cGMP pathway associated with elevated creatinine levels may cause this blunted endotheliumdependent renovascular relaxation and may participate in the increasing prevalence of cardiovascular events in patients with renal insufficiency.
8.
9.
ACKNOWLEDGMENTS We thank Dr. Hiroaki Ikeda for the preparation of the Darginine hydrochloride, and Yuko Omura for her secretarial assistance.
10.
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1999;12:8 –15
Effect of l-Arginine Infusion on Systemic and Renal Hemodynamics in Hypertensive Patients Yukihito Higashi, Tetsuya Oshima, Ryoji Ozono, Hideo Matsuura, Masayuki Kambe, and Goro Kajiyama
This study was designed to compare the renal endothelial function in patients with essential hypertension and normal renal function with that in hypertensive patients with renal insufficiency. We studied the effects of L-arginine (500 mg/kg intravenously over 30 min) on renal hemodynamics in 30 normotensive control subjects, 32 patients with mild to moderate essential hypertension who had normal renal function, and seven hypertensive patients with renal insufficiency who had a serum creatinine concentration >2.0 mg/mL and a glomerular filtration rate <50 mL/min/1.48 m2. LArginine infusion similarly reduced the mean blood pressure between the three groups (normotensive: 29.7% 6 0.7%, hypertensives with normal renal function: 210.2% 6 0.8%, and hypertensives with renal insufficiency: 28.2% 6 1.3%). The L-arginine–induced decrease in renal vascular resistance was smaller in essential hypertensive patients than in normotensive subjects (2 211.0% 6 2.2 v 219.8% 6 2.1%, P <.05). However, L-arginine had no effect on the renal vascular resistance in hypertensive patients with
renal insufficiency (1.6% 6 4.8%). Urine nitrite/nitrate levels in response to L-arginine significantly increased in the three groups in the following order: patients with renal insufficiency (47% 6 15%), essential hypertensive patients (87% 6 10%), and normotensive subjects (129% 6 12%). The glomerular filtration rate was unaffected by L-arginine in normotensive and essential hypertensive patients (3.1% 6 2.4% and 4.2% 6 2.5%), but significantly decreased in hypertensive patients with renal insufficiency (2 213.7% 6 6.1%). These findings suggest that the ability of the L-arginine–nitric oxide– cGMP pathway to relax the renal vascular tone may be impaired in essential hypertensive patients and more markedly blunted in hypertensive patients with renal insufficiency, in parallel with increasing serum creatinine concentrations. Am J Hypertens 1999;12:8 –15 © 1999 American Journal of Hypertension, Ltd.
Received February 13, 1998. Accepted August 11, 1998. From the First Department of Internal Medicine (YH, HM, GK), and Department of Clinical Laboratory Medicine (TO, RO, MK), Hiroshima University School of Medicine, Hiroshima, Japan. Presented in part at the Council for High Blood Pressure Research 50th Annual Fall Conference and Scientific Sessions, American Heart Association, Chicago, Illinois, September 17–20, 1996, and published in abstract form in Hypertension (1996;28:546). This study was supported in part by a Grant-in-Aid for Scientific
Research (08457639) from the Ministry of Education, Science and Culture of Japan and a Foundation for Total Health Promotion grant. Address correspondence and reprint requests to Yukihito Higashi, MD, PhD, First Department of Internal Medicine, Hiroshima University School of Medicine, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8551, Japan; e-mail: [email protected]. ac.jp
© 1999 by the American Journal of Hypertension, Ltd. Published by Elsevier Science, Inc.
KEY WORDS: L-Arginine,
nitric oxide, essential hypertension, hypertensive patients with renal insufficiency, renal hemodynamics.
0895-7061/99/$20.00 PII S0895-7061(98)00204-0
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H
ypertension and renal failure often coexist, and it is widely accepted that hypertension can accelerate renal failure. The kidney plays an important role in blood pressure regulation, but its role in the development of hypertension has not been fully clarified. It is known that the kidney is a rich source of vasodilatory substances such as prostaglandins and endothelium-derived relaxing factors.1 It is possible that endotheliumderived relaxing factor/NO plays an important role in the regulation of hemodynamics in the kidney, specifically the renal vasculature. Recently, we also have reported that the administration of l-arginine has the beneficial effect of lowering blood pressure with increasing renal plasma flow (RPF) and decreasing renal vascular resistance (RVR) in both normotensive and hypertensive subjects with normal renal function.2–5 However, no attention has focused on the effects of endothelium-dependent renal vascular relaxation in hypertensive patients with renal insufficiency in vivo. We postulated that that l-arginine infusion might improve endothelial dysfunction even in hypertensive patients with renal failure. Therefore, the present study was designed to evaluate the effects of the intravenous administration of l-arginine on systemic and renal hemodynamics in normotensive control subjects, patients with essential hypertension who had normal renal function, and hypertensive patients with renal impairment. METHODS Subjects Thirty-two patients with essential hypertension (18 men and 14 women; mean age 47 6 2 years), seven hypertensive patients with renal insufficiency (five men and two women; mean age 50 6 5 years), and 30 normotensive subjects (19 men and 11 women; mean age 45 6 3 years) were evaluated. Hypertension was defined as a systolic pressure of .160 mm Hg or a diastolic pressure of .95 mm Hg while the subject was in the sitting position. Measurements were obtained on at least three different occasions in the outpatient clinic at the Hiroshima University School of Medicine. Patients with essential hypertension had serum creatinine concentrations of ,1.5 mg/dL (132.6 mmol/L) and glomerular filtration rates (GFR) of more than 50 mL/min/1.48 m2. Individuals with severe essential hypertension, who had a systolic blood pressure of .180 mm Hg or a diastolic pressure of .120 mm Hg, with objective signs of hypertensive end-organ disease were excluded. In addition, no patients with essential hypertension had a history of cardiovascular or cerebrovascular disease, diabetes mellitus, hypercholesterolemia, or liver disease. No antihypertensive drugs were taken by these patients for $4 weeks before the study. Hypertensive patients with renal insufficiency had serum creatinine concen-
ENDOTHELIAL FUNCTION AND RENAL INSUFFICIENCY
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trations of $2.0 mg/dL (range; 2.1 to 4.9 mg/dL (185.6 to 433.2 mmol/L) and GFR of ,50 mL/min/1.48 m2. These patients underwent no hemodialysis during the study and had no history of hemodialysis. Individuals with known renal diseases, such as glomerulonephritis, interstitial renal disease, and polycystic kidney, diabetic nephropathy, and renovascular stenosis, also were excluded. Five hypertensive patients with renal insufficiency had been treated with either diuretics, calcium antagonists, or a combination of both. No patients were receiving medication at the time of the study. Normotension was defined as a systolic pressure of ,140 mm Hg and a diastolic pressure of ,80 mm Hg. Normotensive control subjects had no history of serious diseases and took no medication for $4 weeks before the study. Informed consent was obtained from all subjects. The study protocol was approved by the ethics committee of the First Department of Internal Medicine. Protocol All subjects were hospitalized for $7 days and placed on a diet containing 170 mmol of sodium chloride/day for 1 week before the study. Throughout the study, the subjects ingested a constant amount of calories (40 cal/kg/day), potassium (100 mmol/day), and calcium (40 mmol/day). All subjects received meals prepared in the Hiroshima University Hospital kitchen. Compliance with the diet was assessed by measuring 24-h urinary sodium, chloride, and potassium excretion throughout the study. The l-arginine infusion study began at 8:30 am. Subjects fasted overnight for $12 h and were kept in the supine position in a quiet, dark, air-conditioned room maintained at a constant temperature (22°C to 25°C) throughout the study. A 19-gauge polyethylene catheter (Terumo Co, Tokyo, Japan) was inserted into the right antecubital vein for the infusion of para-aminohippurate (PAH), inulin, and l-arginine. Another catheter was inserted into the left antecubital vein to obtain blood samples. After a 30-min rest period, initial doses of PAH (8.0 mg/kg) and inulin (16 mg/kg) were each infused as a bolus. PAH and inulin were subsequently infused at constant rates of 12 and 20 mg/min, respectively, with a syringe pump (Terfusion; Terumo Co., Tokyo, Japan) throughout the study.6 Sixty minutes after the PAH and inulin infusions began, l-arginine (500 mg/ kg) was administered over 30 min with an infusion pump (PEI-1000; Pal Medical Co., Tokyo, Japan). A 30-min recovery period was allowed after the end of the l-arginine infusion. Blood pressure and heart rate measurements were performed with a TM2420 monitor (AND Co., Tokyo, Japan) every minute on the upper left arm. The mean blood pressure was calculated as the diastolic pressure plus one third of the pulse pressure. Blood samples were obtained for the determination of serum concentrations of PAH and
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inulin at baseline (0 min) and 15, 30, and 60 min after the start of l-arginine administration. Baseline fasting serum concentrations of total cholesterol, creatinine, glucose, and electrolytes also were obtained at 0 min. The urinary excretion of nitrite/nitrate (NOx), creatinine, and electrolytes were obtained during 1 h before and after the start of l-arginine infusion. Analytical Methods Samples of venous blood were placed in tubes containing EDTA-Na (1 mg/mL) and polystyrene tubes. EDTA tubes were promptly chilled in an ice bath. The plasma and serum were immediately separated by centrifugation at 3100 g at 4°C for 10 min and at 1000 g at room temperature for 10 min, and stored at 280°C until assayed. Serum concentrations of total cholesterol, creatinine, glucose, electrolytes, and urinary electrolytes were determined by routine chemical methods. Plasma renin activity was measured by radioimmunoassay (Gamma Coat PRA, Baxter Travenol Co., Tokyo, Japan). Urine concentration of NOx was assayed by colorimetric methods using NOx assay kits (Cayman Chemical Co, Ann Arbor, MI). Briefly, nitrate in the sample is converted to nitrite using nitrate reductase. The second step is the addition of Griess reagents7 (1% sulfonamide, 0.1% N-1-naphthylethylenediamide, and 5% HCl), which convert nitrite into a deep purple azo compound. The absorbance of this azo dye at 540 nm was measured by a microplate reader (M-Tmax; Wako Co, Osaka, Japan). The RPF was measured by the clearance of PAH, and serum PAH concentration was analyzed by spectrophotometry. The GFR was measured by the clearance of inulin,8 and the serum inulin concentration was analyzed by the anthrone method.9 The RVR was calculated as the mean blood pressure divided by renal blood flow, and FF as the GFR divided by RPF. The RPF, GFR, and RVR were normalized to the body surface area divided by 1.48 m2 (1.48 m2 being the average body surface area of the Japanese population). Drugs The l-arginine used for intravenous administration was l-arginine hydrochloride (MorishitaRuseru Pharmaceutical Co., Osaka, Japan). d-Arginine was d-arginine hydrochloride (Sigma Chemical Co., St. Louis, MO). The administered inulin was Inutest (Laevosan-Gesellschaft, Lintz, Austria), and the PAH was from Daiichi Pharmaceutical Co., Osaka, Japan. Statistical Analysis Comparisons of baseline parameters among the three groups were made using Student’s unpaired t test. Differences were compared using one way analysis of variance for repeated measures, followed by the Scheffe´ F test. Linear regression analysis was used to test the relationship among the peak RPF and RVR responses to l-arginine and the serum creatinine level. Multivariate analysis, using
AJH–JANUARY 1999 –VOL. 12, NO. 1, PART 1
stepwise regression, was performed to examine relationships among the peak RPF and RVR responses to l-arginine and age, gender, body mass index, cholesterol level, glucose level, plasma renin activity, and creatinine level. All results are presented as the mean 6 SEM. Multivariate analysis was performed with the SAS (Statistical Analysis System, SAS Inc., Cary, NC) REG procedure program package. Statistical significance was defined as a P , .05. RESULTS Clinical Characteristics The baseline clinical characteristics of the three groups of subjects are summarized in Table 1. The mean blood pressure and RVR were higher in essential hypertensive patients and hypertensive patients with renal insufficiency than in normotensive subjects. The RVR was higher in hypertensive patients with renal insufficiency than in patients with essential hypertension. The RPF, GFR, and FF values were less in hypertensive patients with renal insufficiency than in the other two groups, whereas the serum creatinine concentration was higher. Other parameters were similar among the three groups. Effects of L-Arginine on Mean Blood Pressure and Heart Rate The percent changes in mean blood pressure and heart rate above baseline levels during larginine infusion in the study groups are shown in Figure 1. Mean blood pressure promptly decreased after the intravenous administration of l-arginine, but quickly returned to the baseline level after the end of infusion. Conversely, heart rate gradually increased during l-arginine infusion, and gradually returned to the baseline level during the recovery period. There was no significant difference in the time course of changes in mean blood pressure and heart rate among the three groups. Effects of L-Arginine on Renal Hemodynamics Figure 2 shows the percent changes in parameters of renal hemodynamics (such as RPF, GFR, RVR, and FF) above baseline levels during l-arginine infusion in the study groups. The l-arginine–induced increase in RPF was less in patients with essential hypertension than in normotensive control subjects (10.1% 6 0.8% v 15.8% 6 0.9%, P , .05); conversely, RPF decreased in hypertensive patients with renal insufficiency in response to l-arginine (211.5% 6 5.0%). The decrease in RVR response to l-arginine was less in patients with essential hypertension than in normotensive subjects (214.3% 6 0.7% v 220.1% 6 0.8%; P , .05); RVR remained unchanged in hypertensive patients with renal insufficiency (4.8% 6 7.0%). l-Arginine infusion did not significantly alter the GFR in either normoten-
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TABLE 1. BASELINE CLINICAL CHARACTERISTICS IN NORMOTENSIVE SUBJECTS, ESSENTIAL HYPERTENSIVE PATIENTS, AND HYPERTENSIVE PATIENTS WITH RENAL INSUFFICIENCY
Parameter Body weight (kg) Body mass index (kg/m2) Mean blood pressure (mm Hg) Heart rate (beats/min) Serum creatinine (mmol/L) Total cholesterol (mmol/L) Serum glucose (mmol/L) Plasma renin activity (ng/L/sec) Urine NOx (mmol/mol creatinine) Renal hemodynamics RPF (mL/min/1.48 m2) GFR (mL/min/1.48 m2) RVR (mm Hg/mL/min/1.48 m2) FF
Normotensive Subjects (n 5 30) 65.6 6 2.9 25.1 6 1.1 80.1 6 1.4 60.1 6 2.1 88.9 6 6.9 4.4 6 0.3 5.0 6 0.2 0.27 6 0.04 127 6 17 603.6 6 25.4 90.1 6 2.3 0.084 6 0.006 0.148 6 0.006
Essential Hypertensive Patients (n 5 32)
Hypertensive Patients With Renal Insufficiency (n 5 7)
67.3 6 3.1 25.6 6 1.2 111.8 6 2.0* 59.7 6 1.9 89.3 6 7.5 4.3 6 0.3 4.9 6 0.1 0.29 6 0.04 119 6 16
63.7 6 5.6 24.7 6 2.1 114.2 6 6.1* 62.0 6 3.4 238.8 6 33.6*† 4.4 6 0.4 5.1 6 0.2 0.34 6 0.13 110 6 21
576.3 6 29.1 87.4 6 2.2 0.124 6 0.007* 0.147 6 0.005
328.2 6 35.1*† 31.4 6 4.4*† 0.220 6 0.020*† 0.104 6 0.013*†
NOx indicates nitrite/nitrate; RPF, renal plasma flow; GFR, glomerular filtration rate; RVR, renal vascular resistance; FF, filtration fraction. All results are presented as mean 6 SEM. * P , .001 v normotensive subjects. † P , .001 v essential hypertensive patients.
sive or hypertensive subjects (3.1% 6 2.4% or 4.2% 6 2.5%), but decreased the GFR in hypertensive patients with renal insufficiency (213.7% 6 5.3%). The FF was unchanged by l-arginine in both hypertensive groups (essential hypertensive subjects, 22.7% 6 2.1%; subjects with renal insufficiency, 22.7% 6 2.0%), but decreased in normotensive subjects (28.7% 6 2.0%). When multiple regression analysis was performed using the peak RPF response to l-arginine as a dependent variable, the serum creatinine level (P , .0001), age (P , .005), and mean blood pressure (P , .0006) were selected as independent variables. When multiple regression analysis was performed using the peak RVR response to l-arginine as a dependent variable, the serum creatinine level (P , .0001) and mean blood pressure (P , .03) were selected as independent variables. The multiple regression analysis equations are as follows: %RPF 5 42.4 20.10 z serum creatinine level 20.21 z age 2 0.11 z mean blood pressure, %RVR 5 239.4 1 0.09 z serum creatinine level 1 0.10 z mean blood pressure. Effects of L-Arginine on Urinary Excretion of NOx The baseline urine NOx levels were similar among the three groups of subjects (Table 1). The intravenous administration of l-arginine significantly increased urine NOx levels in the three groups, but this increase in NOx was less in hypertensive groups than in normotensive subjects (essential hypertensive subjects, 87% 6 10%; subjects with renal insufficiency, 47% 6
15% vs 129% 6 12%; P , .05, P , .01, respectively) and was less in hypertensive patients with renal insufficiency than in patients with essential hypertension (P , .05) (Figure 3). When multiple regression analysis was performed using the NOx response to l-arginine as a dependent variable, serum creatinine level (P , .01), age (P , .003), and mean blood pressure (P , .0001) were selected as independent variables. The multiple regression analysis equation is as follows: %NOx 5 280.4 2 0.12 z serum creatinine level 2 0.71 z age 2 0.79 z mean blood pressure. The maximal NOx response to l-arginine, which is a marker of nitric oxide production, was significantly correlated with the peak increases in RPF (r 5 0.69; P , .0001) and the peak decreases in RVR (r 5 20.76; P , .0001) (Figure 4). Side Effects The side effects associated with l-arginine infusion were minimal. Two subjects (one normotensive subject and one patient with essential hypertension) reported slight pain on the site where the catheter was inserted into the antecubital vein after the start of l-arginine infusion. Renal vascular responses to l-arginine were usually reversible in all subjects. Although clinical parameters such as the serum creatinine concentration were reexamined within a few days after the study in all hypertensive patients with renal insufficiency, none of the subjects demonstrated deterioration based on these parameters.
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FIGURE 1. Line graphs showing the effects of l-arginine infusion on mean blood pressure and heart rate in normotensive subjects (open circles), patients with essential hypertension (solid circles), and hypertensive patients with renal insufficiency (solid boxes). The responses of mean blood pressure and heart rate to l-arginine were similar among the three groups. All results are presented as the mean 6 SEM. The probability value refers to the comparison of time course curves using ANOVA for repeated measures.
DISCUSSION l-Arginine significantly increased RPF in normotensive subjects and essential hypertensive patients with normal renal function, whereas it decreased RPF in hypertensive patients with renal insufficiency. l-Arginine did not induce a detectable change in RVR in hypertensive patients with renal insufficiency, whereas l-arginine significantly decreased RVR in the other groups. Changes in NOx levels evoked by larginine increased in the following order: hypertensive patients with renal insufficiency, patients with essential hypertension, and normotensive subjects. We predicted that hypertensive patients with renal insufficiency would have impaired endothelium-dependent renal vascular relaxation compared with es-
sential hypertensive patients with normal renal function and normal control subjects. In the present study, patients with essential hypertension who have normal renal function had a blunted renal vascular relaxation response to l-arginine infusion compared with normotensive subjects. This finding is consistent with findings documented in our previous reports2–5 and in other reports using forearm and coronary arteries.10,11 However, it was unexpected that l-arginine infusion not only did not cause renal vascular relaxation in the hypertensive patients with renal insufficiency, but even precipitated renovascular constriction. In hypertensive patients with renal sufficiency, the decrease in RPF and GFR is explained entirely by the reduction in perfusion pressure. Our findings suggest that those with renal insufficiency can adequately metabolize l-arginine to NO. This might be due to reduced delivery of the substrate or impaired access to NO synthase, or to reduced enzyme itself. The presence of other mechanisms for the autoregulation of renal hemodynamics during the hypotensive response to l-arginine in hypertensive patients with renal insufficiency cannot be eliminated by this study. The other possible explanation for the lack of an endothelium-dependent renal vascular response in hypertensive patients with renal insufficiency may involve a direct cytotoxic action of l-arginine–NO. Several investigators have reported the relationship between NO and cellular damage.12,13 Far advanced renal impairment, with severely damaged blood vessel endothelium, may represent direct injury by excessive NO. Conversely, the increasing production of NO may lead to vasodilation in normal to mildly damaged vessels. It also has been shown that superoxide is produced in excess in renal failure. In addition, Radi et al14 have demonstrated that peroxynitrite, which is prepared by allowing NO to react with superoxide, has a hydroxyl radical with a strong cytotoxic action. Therefore, counterregulatory effects of the renal response to l-arginine may be caused by this molecule. Another important finding in this study is that the renal vasodilating response to l-arginine was attenuated with increasing serum creatinine levels. When multiple regression analysis was performed using the peak RPF response to l-arginine in our subjects (who did not have abnormal metabolism of lipids or glucose) as the dependent variable, the serum creatinine level, age, and baseline mean blood pressure were selected as independent variables. Similarly, when the peak RVR response to l-arginine was selected as the dependent variable, the serum creatinine level and baseline mean blood pressure were selected as independent variables. The serum creatinine level was the strongest anticipated variable of a blunted renal vascular dilation response to l-arginine among the three variables. This finding may have important clinical
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FIGURE 2. Line graphs showing the effects of l-arginine infusion on parameters of renal hemodynamics, such as renal plasma flow, glomerular filtration rate, renal vascular resistance, and filtration fraction, in normotensive subjects (open circles), patients with essential hypertension (solid circles), and hypertensive patients with renal insufficiency (solid boxes). The responses of renal plasma flow to l-arginine were less in patients with essential hypertension than in normotensive subjects; the reverse relationship was found hypertensive patients with renal insufficiency. The response of renal vascular resistance was less in patients with essential hypertension than in normotensive subjects, but was unchanged in hypertensive patients with renal insufficiency. The response of the glomerular filtration rate was decreased in hypertensive patients with renal insufficiency, whereas these responses were unchanged in patients with essential hypertension and normotensive subjects. The response of the filtration fraction was decreased in normotensive subjects but was unchanged in other groups. All results are presented as the mean 6 SEM. The probability value refers to the comparison of time course curves using analysis of variance (ANOVA) for repeated measures. *P , .0001 versus normotensive subjects; †P , .0001 versus patients with essential hypertension.
relevance. The abnormality of l-arginine–NO– cGMP pathway involving the elevated creatinine level may play an important role in the development of renal impairment in hypertension. Our previous study showed that renal vasorelaxation and urine NOx responses were much greater during l-arginine infusion as compared with d-arginine.3 It has been reported that other amino acids that are not substrates for NO also produce renal vasodilation.15 Although the precise mechanism is unclear, only one third of the effects of l-arginine on RPF can be explained by these amino acid effects. These find-
ings may suggest that exogenous l-arginine infusion may cause a renal vasorelaxation mostly through the release of NO rather than a nonspecific effect of amino acid. However, d-arginine caused a small but significant increase in RPF. These data, coupled with the previous studies demonstrating the effect of other amino acids on RPF, suggest that all of the response to l-arginine is not due to NO. In addition, it is well known that l-arginine induces the release of a number of substances, including histamine and insulin, that may have vasoactive effects. Thus, we cannot deny the possibility that vasoactive agents other than NO may
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FIGURE 3. Bar graph showing the effects of l-arginine infusion on urine NOx concentrations in normotensive subjects (open bar), patients with essential hypertension (solid bar), and hypertensive patients with renal insufficiency (striped bar). The response of NOx to l-arginine was less in patients with essential hypertension than in normotensive subjects and was lower still in hypertensive patients with renal insufficiency. All results are presented as the mean 6 SEM. *P , .05, **P , .01 v normotensive subjects; †P , .05 v patients with essential hypertension.
be involved in the renal vasodilation induced by larginine. The use of specific NO synthase inhibitors such as N G -monomethyl-l-arginine and N G -nitro-l-argininemethylester, and agonists to stimulate NO release such as acetylcholine or bradykinin, would allow us to draw more specific conclusions concerning the role of the basal and stimulated release of NO in the renal circulation. In addition, to conclude the specificity of abnormalities in the l-arginine–NO pathway, it is useful to test the renal vasodilatory responses to other vasodilators such as prostaglandins. However, because the intravenous infusion of NO synthase inhibitors can increase the blood pressure and vascular resistance, these agents may lead to adverse effects in hypertensive patients. We therefore did not investigate these agents in certain aspects because of ethical considerations. Although the present results suggested that the endothelium-dependent renal vasodilation observed in response to l-arginine was impaired in patients with renal insufficiency as compared with patients without renal insufficiency, the number of patients with renal insufficiency is small. A large sample size of patients may allow more specific conclusions. Our results showed the marked differences in response to l-arginine between patients with similar GFR. Several lines of evidence suggest that endothelial dysfunction is related to cardiovascular risk factors
FIGURE 4. Scatter plot showing the relationship between the maximum percent change in NOx responses to l-arginine and maximum percent change in renal plasma flow response to larginine (top) and the maximum percent change in renal vascular resistance response (bottom), to l-arginine in normotensive subjects (open circles), patients with essential hypertension (solid circles), and hypertensive patients with renal insufficiency (solid triangles). The peak increase in NOx significantly correlated with the maximum percent change in the renal plasma flow response and the maximum percent change in renal vascular resistance response to l-arginine.
such as aging, high blood pressure, and atherosclerosis.3,10,16,17 Therefore, these factors other than GFR may contribute to differences in response to l-arginine in patients with similar degrees of renal insufficiency. In conclusion, the present study demonstrates that l-arginine–induced renal vascular dilatation is markedly impaired in hypertensive patients with renal insufficiency compared with normal control subjects and patients with essential hypertension who have normal renal function. Modifications in the l-argi-
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nine–NO-cGMP pathway associated with elevated creatinine levels may cause this blunted endotheliumdependent renovascular relaxation and may participate in the increasing prevalence of cardiovascular events in patients with renal insufficiency.
8.
9.
ACKNOWLEDGMENTS We thank Dr. Hiroaki Ikeda for the preparation of the Darginine hydrochloride, and Yuko Omura for her secretarial assistance.
10.
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