Effects of l-Arginine Infusion on Renal Hemodynamics in Patients With Mild Essential Hypertension
Abstract Previous studies have shown that endothelium-derived relaxing factor/nitric oxide plays an important role in the regulation of systemic and renal hemodynamics. The purpose of the present study was to determine whether endothelium-dependent renovascular relaxation was impaired in patients with mild essential hypertension who had normal renal plasma flow and glomerular filtration rate. We evaluated the effects of intravenous administration of l-arginine on blood pressure and renal hemodynamics in 13 patients with mild essential hypertension and 15 normotensive control subjects. l-Arginine infusion (500 mg/kg over 30 minutes) reduced mean blood pressure (from 82.5±2.5 to 76.3±2.6 mm Hg in hypertensive patients and from 106.1±3.0 to 97.5±2.9 mm Hg in control subjects; P<.001) and renovascular resistance (from 0.084±0.009 to 0.067±0.009 mm Hg · mL−1 · min−1 · [1.48 m2]−1 and from 0.105±0.010 to 0.093±0.011 mm Hg · mL−1 · min−1 · [1.48 m2]−1, respectively; P<.001). l-Arginine infusion increased renal plasma flow (from 602±36 to 698±40 mL · min−1 · [1.48 m2]−1, P<.05) in normotensive subjects but not in hypertensive subjects, and glomerular filtration rate was unaffected in both groups. Although the l-arginine–induced reduction in mean blood pressure was similar in both groups, the decline in renovascular resistance was smaller in hypertensive subjects. The response of renal plasma flow was also smaller in hypertensive subjects. These findings suggest that dysfunction of the l-arginine–nitric oxide pathway exists in the renal circulation even in mild essential hypertension with normal renal plasma flow and glomerular filtration rate. Thus, changes in renal endothelial function could be a cause rather than a consequence of hypertension.
Recent studies have demonstrated that the endothelium plays an important role in the regulation of vascular tone through release of various vasorelaxing and vasoconstricting factors.1 2 3 Endothelium-derived relaxing factor/nitric oxide, one of the most important vasorelaxing factors, is produced from l-arginine in the presence of nitric oxide synthase in the endothelium. Nitric oxide stimulates cytosolic guanylate cyclase and increases guanosine 3′,5′-cyclic monophosphate (cGMP) content in vascular smooth muscle cells, resulting in relaxation of vascular tone.4 5 Several investigators have reported that patients with essential hypertension have impaired endothelium-dependent vascular relaxation in the coronary or brachial arteries.6 7 8 9 Previous animal studies have shown that the renovascular bed is more sensitive to vasoactive agents than are the coronary or femoral vascular beds.10 Because the kidney is both a target for and a contributor to hypertension, the importance of renal circulation should be recognized. However, to our knowledge, there are no reports on renal artery endothelial function in humans in vivo. Furthermore, it is unclear whether endothelial dysfunction is a cause or a consequence of hypertension. Dysfunction of the l-arginine–nitric oxide pathway might be a secondary event involved in the maintenance and development, rather than the initiation, of hypertension, especially because most experimental data suggest that endothelial dysfunction develops as blood pressure increases.
The purpose of the present study was to determine whether there was impairment of endothelium-dependent renovascular relaxation in patients with mild essential hypertension who had normal renal function. We investigated how intravenously administered l-arginine modifies renal hemodynamics in patients with mild essential hypertension in comparison with normotensive control subjects.
Thirteen inpatients with mild essential hypertension and 15 normotensive control subjects were studied. All subjects were Japanese. Hypertension was defined as systolic pressure of more than 140 mm Hg or diastolic pressure of more than 90 mm Hg, while the subject was in the sitting position, on at least three different occasions in the outpatient clinic at the Hiroshima University School of Medicine. Patients with secondary forms of hypertension were excluded by appropriate clinical and biochemical examination. Mild essential hypertension was defined as systolic pressure between 140 and 170 mm Hg and diastolic pressure between 90 and 105 mm Hg, with no objective signs of hypertensive end-organ disease. No patient had a history of cardiovascular or cerebrovascular disease, hypercholesterolemia, diabetes mellitus, liver disease, or renal disease. No antihypertensive drugs were taken for at least 4 weeks before the study. Normotension was defined as systolic pressure of less than 140 mm Hg and diastolic pressure of less than 80 mm Hg. Normotensive control subjects had no history of serious diseases and took no medication for at least 4 weeks before the study. The study protocol was approved by the ethics committee of the First Department of Internal Medicine, Hiroshima University School of Medicine; informed consent was obtained from all subjects.
The study began at 8:30 am. Both hypertensive and normotensive control subjects were kept on a diet containing 170 mmol sodium chloride per day for 1 week before the study. Subjects fasted overnight for at least 12 hours and were kept in a 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) was inserted into the right antecubital vein for the infusion of para-aminohippurate (PAH), inulin, and l-arginine, and another catheter was inserted into the left antecubital vein to obtain blood samples. After a 30-minute rest period, initial doses of PAH (8.0 mg/kg) and inulin (16 mg/kg) were infused as a bolus. PAH and inulin were subsequently infused at a constant rate of 12 and 20 mg/min, respectively, with a syringe pump (Terfusion; TERUMO Co) throughout the study.11 Sixty minutes after the PAH and inulin infusions began, l-arginine (500 mg/kg) was administered over 30 minutes with an infusion pump (PEI-1000; Pal Medical Co). A 30-minute 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) every minute on the upper left arm. Mean blood pressure was calculated as diastolic pressure plus one third of the pulse pressure. Blood samples were obtained for the determination of serum PAH, inulin, and plasma cGMP at 0 minutes and at 15, 30, and 60 minutes after the start of l-arginine administration. Baseline serum concentrations of total cholesterol, creatinine, and electrolytes and plasma renin activity were obtained at 0 minutes.
The l-arginine used for intravenous administration was l-arginine hydrochloride (Morishita Pharmaceutical Co), and the inulin was Inutest (Laevosan-Gesellschaft). The PAH was from Daiichi Pharmaceutical Co.
Serum concentrations of total cholesterol, creatinine, and electrolytes were determined by routine chemical methods. Plasma renin activity was measured by radioimmunoassay (Gamma Coat PRA, Baxter Travenol Co). Plasma cGMP levels were measured by radioimmunoassay using cGMP kits (Yamasa Shoyu Co). Renal plasma flow (RPF) was measured by the clearance of PAH. Serum PAH concentration was analyzed by spectrophotometry.12 Glomerular filtration rate (GFR) was measured by the clearance of inulin. Serum inulin concentration was analyzed by the anthrone method.13 Renovascular resistance (RVR) was calculated as the mean blood pressure divided by renal blood flow, and filtration fraction was calculated as GFR divided by RPF. RPF, GFR, and RVR were normalized to body surface area divided by 1.48 m2 (1.48 m2 being the average body surface area of the Japanese population).
Comparisons of baseline parameters between normotensive and hypertensive groups were made using Student’s unpaired t test. Differences were compared using ANOVA for repeated measures. All results are presented as mean±SEM. Statistical significance was defined as a value of P<.05.
The baseline clinical characteristics in 15 normotensive subjects and 13 patients with mild essential hypertension are shown in Table 1⇓. Mean blood pressure and RVR were significantly greater in patients with mild essential hypertension than in normotensive control subjects (mean blood pressure, 106.1±3.0 in patients with hypertension versus 82.5±2.5 mm Hg in normotensive subjects; RVR, 0.105±0.010 versus 0.084±0.009 mm Hg · mL−1 · min−1 · [1.48 m2]−1, respectively; P<.01). Other parameters did not differ significantly between the two groups.
Effects of l-Arginine on Blood Pressure and Heart Rate
Fig 1⇓ shows the effects of l-arginine administration on mean blood pressure and heart rate. In a preliminary study, l-arginine was administered for 60 minutes to examine changes in blood pressure. Blood pressure reached a plateau at 20 minutes. Therefore, we performed the l-arginine infusion for 30 minutes. After l-arginine infusion began, mean blood pressure promptly decreased and after 30 minutes declined from 82.5±2.5 to 76.3±2.6 mm Hg (P<.001) in normotensive subjects and from 106.1±3.0 to 97.5±2.9 mm Hg (P<.001) in hypertensive subjects. A prompt return to baseline levels occurred after the end of the infusion in both groups. The percentage changes in mean blood pressure above the baseline were similar in the two groups (normotensive subjects, −8.2±2.1%; hypertensive subjects, −10.1±1.5%). Changes in systolic and diastolic pressures were exactly paralleled by changes in mean blood pressure (data not shown). Conversely, heart rate gradually increased, and after 30 minutes went from 62.4±2.7 to 68.6±2.4 beats per minute (P<.001) in normotensive subjects and from 58.6±2.0 to 65.5±2.9 beats per minute (P<.001) in hypertensive subjects. Heart rate gradually returned to the baseline level during the rest period. The percent changes in heart rate were also similar in the two groups.
Effects of l-Arginine on Renal Hemodynamics
Table 2⇓ shows the effects of l-arginine administration on RPF, GFR, RVR, and filtration fraction. During the l-arginine infusion, RPF was significantly increased in normotensive control subjects, but not in patients with hypertension (Table 2⇓, Fig 2a⇓). RVR was significantly reduced in both normotensive control subjects and hypertensive patients. The percentage reduction in RVR was smaller in patients with hypertension than in normotensive control subjects (−11.3±2.2% versus −19.1±2.1%, P<.001) (Fig 2b⇓). Filtration fraction was significantly decreased in normotensive control subjects, but not in patients with hypertension. The administration of l-arginine did not significantly change GFR in either normotensive control subjects or patients with hypertension.
Effects of l-Arginine on Plasma cGMP Level
The administration of l-arginine increased plasma cGMP levels in normotensive control subjects from 2.2±0.3 to 4.1±0.4 pmol/mL (P<.001) and in patients with essential hypertension from 2.5±0.4 to 3.8±0.6 pmol/mL (P<.001) at 30 minutes. The percentage increase in plasma cGMP levels was smaller in patients with essential hypertension than in normotensive control subjects (54±7% versus 83±8%, P<.05) (Fig 3⇓).
The vascular endothelium modulates blood vessel function through the production of vasoactive factors such as endothelium-derived relaxing factor, endothelium-derived hyperpolarizing factor, endothelin, prostacyclin, and superoxide anions.2 3 Endothelium-derived relaxing factor/nitric oxide is released under basal conditions and in response to many physiological stimuli, including circulating hormones, shear stress, and autacoids. Nitric oxide not only causes direct vasodilation but also inhibits adhesion and aggregation of platelets and participates in suppression of vascular smooth muscle cell proliferation.1 14 Therefore, it is postulated that abnormalities in the nitric oxide pathway may play a vital role in cardiovascular disease, including hypertension. In fact, it has been clearly shown in animal models that nitric oxide inhibitor induces hypertension.15 In addition, endothelial cell morphology and function are altered in acute and chronic experimental hypertension,15 16 17 and nitric oxide–related endothelium-dependent vascular relaxation is impaired.17
Because the test used to assess the function of the l-arginine–nitric oxide pathway is invasive, there is less information about the pathway in humans. Several investigators have demonstrated that endothelium-dependent forearm vasodilation in response to endothelium-dependent vasodilators such as acetylcholine is reduced in patients with essential hypertension.6 7 8 9 This impaired response to acetylcholine in the human forearm circulation may be related to diminished nitric oxide release.6 Recently, however, Cockcroft et al18 reported that endothelium-dependent forearm vasodilation induced by muscarinic agonists was preserved in patients with essential hypertension. This discrepancy may result from the heterogeneity in pathophysiology and the severity of essential hypertension. Because most experimental findings have suggested that endothelial function is impaired as blood pressure increases and that the degree of dysfunction is related to the severity of blood pressure elevation,19 it is unclear whether dysfunctional endothelium is a cause or a consequence of hypertension. Therefore, subjects with mild essential hypertension who had no objective signs of end-organ damage were carefully selected for the present study.
In addition, it has been reported that there is a discrepancy between the responses to vasoactive factors in different vascular beds. Renal vasculature is one of the main targets of hypertension, and it is very sensitive to changes in endothelial function.10 Although there is an important role for the kidney in the development, maintenance, and pathophysiology of hypertension, little investigation has been done on the effects of nitric oxide on renal hemodynamics in humans. The present study is the first to report the in vivo effects of nitric oxide on renal circulation in hypertensive patients. The intravenous administration of l-arginine, a precursor of nitric oxide, increased RPF in normotensive control subjects but not in patients with essential hypertension. The l-arginine–induced reduction in RVR was less in patients with mild essential hypertension than in normotensive control subjects. These findings suggest that endothelial dysfunction may exist even in the early stages of hypertension and implicate endothelial dysfunction as a cause of hypertension.
It is impossible to deny the possibility that the disturbed response to l-arginine in essential hypertension might result from mechanisms independent of endothelium or nitric oxide. The ability of amino acids such as glycine to induce vasodilation and renal hyperfiltration has been reported in animals and humans.20 21 However, in the present study, the l-arginine–induced increase in plasma cGMP was smaller in patients with essential hypertension than in normotensive control subjects. Therefore, an impaired renovascular relaxation in response to l-arginine infusion in essential hypertension may be due to disturbances in the synthesis or release of nitric oxide. However, further studies are necessary to confirm the specificity of the l-arginine effect. Furthermore, the sensitivity of the vascular smooth muscle cells to nitric oxide may be attenuated. However, because many investigators have reported that the response to endothelium-independent vasodilators, such as sodium nitroprusside, before and after administration of nitric oxide synthesis inhibitors is preserved in patients with essential hypertension,6 7 8 9 it is unlikely that altered function of vascular smooth muscle cells could participate in this mechanism.
In experimental animal models, GFR was reported to be decreased after treatment with l-arginine nitric oxide synthesis inhibitors such as NG-nitro-l-arginine-methyl ester and NG-monomethyl-l-arginine and increased or unchanged with l-arginine treatment.10 22 Although we found that RPF was increased in normotensive control subjects during l-arginine infusion, GFR was not significantly altered. Baylis et al22 have reported an insignificant increase in the GFR response to l-arginine alone despite an increase in RPF in the normal rat kidney, which explains why GFR was relatively protected and unchanged in proportion to the change in RPF.
We have shown that intravenous administration of l-arginine induced a reduction in systemic blood pressure in humans. This result is consistent with the report by Nakaki et al.23 However, several investigators have demonstrated in animals that l-arginine infusion failed to change systemic hemodynamics, although it could reverse the effects of nitric oxide synthesis inhibitors during simultaneous administration.16 17 There may be species-related differences in the role of the l-arginine–nitric oxide pathway in the regulation of blood pressure.
In conclusion, the present study indicates that endothelium-dependent renovascular relaxation in response to l-arginine is impaired even in patients with mild essential hypertension who have normal RPF and GFR and no proteinuria. Because an increase in the circulating cGMP level is blunted in hypertensive patients, the reduced synthesis or release of nitric oxide may result in this disturbed response. The masked endothelial dysfunction may play an important role in the development and maintenance of essential hypertension.
This study was supported, in part, by a Foundation for Total Health Promotion grant (1992). The authors thank Yuko Omura for her secretarial assistance.
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