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CLINICAL SCIENCE |

Departments of *Nephrology and
Clinical Neurophysiology, University Medical Center Utrecht, The Netherlands
Correspondence to Dr. Peter J. Blankestijn, Department of Nephrology, Room F03.226, University Medical Center, P.O. Box 85500, 3508 GA Utrecht, The Netherlands. Phone: +31-30-2507336; Fax: +31-30-2543492; E-mail: p.j.blankestijn{at}azu.nl
| Abstract |
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| Introduction |
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There is some indication that sympathetic nervous activity is also involved. Experimental data have indicated that the presence of parenchymal injury results in neurogenic hypertension (3).Muscle sympathetic nerve activity (MSNA) is increased in both dialysis and predialysis patients, whereas bilaterally nephrectomized patients have MSNA identical to control subjects (46). Recently, we reported that MSNA is increased in hypertensive patients with polycystic kidney disease and still normal kidney function (7). Thus, the available data indicate that renal parenchymal changes can cause sympathetic hyperactivity. Sympathetic activity increases with age (79) and is feedback-regulated by baroreflex control and volume status (10). In patients with chronic renal disease, volume status may vary substantially. Therefore, it is critical that this be taken into account when assessing sympathetic activity in individual patients.
The overall hypothesis for the present studies was that sympathetic activity is "inappropriately" increased in patients with renal parenchymal disease. In view of the above mentioned, the specific aims were (1) to establish that sympathetic activity is increased in relation to the volume status of these patients and (2) to establish that sympathetic hyperactivity is a feature of chronic renal disease and not of reduced number of nephrons but intact parenchymal structure.
| Materials and Methods |
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Fifty-seven patients (polycystic kidney disease, n = 31; IgA nephropathy, n = 5; obstructive uropathy disease, n = 5; chronic pyelonephritis, n = 2; nephrosclerosis, n = 3; focal sclerosis, n = 1; mesangial proliferative glomerulonephritis, n = 1; interstitial nephritis, n = 2; analgesic nephropathy, n = 2; Alport syndrome, n = 2; or unknown, n = 3), 57 control subjects, and seven kidney donors were studied.
Protocol
The institutional committee for studies in humans approved the protocol. All subjects gave their written informed consent. Patients were studied when taken off antihypertensive medication for at least 2 wk. Diuretics were continued to maintain normovolemia in 26 of the 57 patients but were not taken on the study day. Vitamin D supplements, phosphate binders, and/or HMG-CoA-reductase inhibitors were continued as well.
A subgroup of seven patients was studied twice, when clinically normovolemic (i.e., no clinical signs of fluid overload) and when taken off diuretics for >7 d. During both study sessions, patients were without angiotensin-converting enzyme (ACE) inhibition or angiotensin II antagonist. An additional group of eight control subjects were studied twice as well, while on high-salt diet (regular diet + salt supplements: 200 mmol/d for 5 d) and while on low-salt diet (30 mmol/d for 5 d, and frusemide, 20 mg twice daily, during the first 2 d). The order of the investigations regarding changes in volume status was randomized. Fluid status was assessed by extracellular volume (ECV) measurements (see below). Seven living kidney donors were investigated shortly before (2 to 12 wk) and after (12 to 58 wk) unilateral nephrectomy.
All subjects underwent an identical set of measurements, in supine position in a quiet room with an ambient temperature of 22 to 24°C. The protocol, which is described in more detail previously (5,7), included measurement of supine BP, heart rate, MSNA, baroreflex sensitivity, ECV. and plasma renin activity (PRA). On the day before the MSNA measurement, subjects collected 24-h urine.
BP and heart rate were measured in a recumbent position with a standard mercury sphygmomanometer; means of three measurements are presented. During the baroreceptor sensitivity assessments, BP and heart rate were recorded continuously by finger plethysmography (11). MSNA was recorded with a unipolar tungsten microelectrode placed in a muscle nerve fascicle of the peroneal nerve (6,7,12). Success rate of obtaining an adequate neural signal is approximately 85%. The interbeat intervals were measured from the ECG. From the interbeat interval, the heart rate is computed. An intravenous cannula for infusion and blood sample collection was inserted into an antecubital vein. After instrumentation, the subjects rested for 20 min. Baseline measurements for BP, heart rate, and MSNA were obtained, and blood was sampled for PRA and bromide. Next, the arterial baroreflex sensitivity was assessed as the response of MSNA and of heart rate to changes in BP induced by subsequent continuous infusion of sodium nitroprusside and phenylephrine (5,7). Bromide distribution volume was used as an index of ECV (13). Plasma bromide levels range between 1 and 3 mmol/L, which is well below the therapeutic and toxic levels. Normalization for lean body mass allows comparison between men and women (14). The normal range in our laboratory is 273 to 334 ml/kg lean body mass. PRA was measured by RIA (15).
Data Analysis
Data are mean ± SD, unless indicated otherwise. MSNA was expressed as the number of bursts of sympathetic activity per minute or as the number of bursts per 100 heart beats to correct for differences in heart rate. Intraobserver and interobserver reproducibility are 4.5 ± 0.5% and 6.2 ± 0.7%. Baroreflex sensitivity is quantified as described elsewhere (5,7).
Statistical Analyses
PRA was analyzed after logarithmic transformation. Baseline characteristics of patients and control subjects were compared by unpaired t test. Only independent variables were included in the regression analysis. Analysis of covariance was used to calculate differences between slopes when appropriate. Pearson correlation coefficients were calculated followed by stepwise linear regression when appropriate. P < 0.05 was considered to be statistically significant.
| Results |
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Patients and control subjects were matched for age, gender, and body mass index. Creatinine clearance in patients was lower and ranged from 10 to 118 ml/min. Despite that ECV did not differ between the two groups, BP, MSNA, and PRA were markedly higher in the patients (all <0.001; Table 1). BP did not correlate with MSNA. Baroreceptor sensitivity was not different between patients and control subjects (Table 1).
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Effects of Changes in Fluid Status
Seven patients (diagnosis: nephrosclerosis (n = 2), polycystic kidney disease (n = 2), Alport disease (n = 2), and chronic pyelonephritis (n = 1); age: 49 ± 7 yr; four men; creatinine clearance: 39 ± 18 ml/min) were examined twice, while clinically normovolemic and after stopping diuretic therapy. After stopping the diuretics, body weight increased by 1.6 ± 0.6 kg. ECV and BP increased, and MSNA and PRA were significantly suppressed in the hypervolemic compared with the normovolemic condition (Table 2, Figure 1).
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Kidney Donors
Creatinine clearance decreased, but BP, PRA, and MSNA remained unchanged (Table 4).
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| Discussion |
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Fluid overload is considered to play a major role in the pathogenesis of nephrogenic hypertension. It has long been recognized that the renin-angiotensin system is "inappropriately" activated in relation to the fluid status (16). This contributes to the hypertension and explains why hypertension may persist even after correction of the fluid overload. The role of the sympathetic nervous system in renal hypertension is less clear. Whereas the renin system is especially involved in the long-term regulation of BP, the sympathetic nervous system is particularly relevant for the short-term regulation. This is apparent from the quick baroreflex-mediated changes in sympathetic activity, confirmed also in the present study. Arterial baroreceptor sensitivity in patientsi.e., the ability of the sympathetic nervous system to respond to acute changes in BPwas not different from control subjects. Changes in volume status have a reciprocal effect on plasma noradrenaline (17) and, as is shown in our study, on MSNA. MSNA is also suppressed in subjects with mineralocorticoid excess (18). However, in clinical situations with sustained low BP, such as heart failure, resting sympathetic activity is elevated (19). A recent study showed that MSNA remained consistently elevated over 8 wk in hypertensive subjects who switched to a low-salt diet (20). Clearly, changes in volume status and BP have a sustained effect on sympathetic activity.
The present study shows that resting sympathetic activity is, on average, elevated in hypertensive patients with renal parenchymal disease compared with control subjects. The physiologic increase with age is still present (8,9). Although the number of studied subjects was limited, the present data clearly show that changes in the volume status induced comparable changes in sympathetic activity as observed in the healthy subjects, however, at a higher level of sympathetic activity. Therefore, we conclude that in hypertensive patients with renal parenchymal disease, sympathetic activity is inappropriately high for the volume status, similar as has been described for the activity of the renin angiotensin system (16). Animal experiments have shown that intrarenal ischemia by arterial constriction (21) or by intrarenal phenol injection can cause sympathetic activation by stimulation of renal afferents (4). That indeed the diseased kidney induces the sympathetic activation is confirmed by the observation that sympathetic activity is not increased in patients who underwent bilateral nephrectomy because of hypertensive end-stage renal disease (5). Our hypothesis is that renal parenchymal disease, by causing local or diffuse compromised perfusion, leads to stimulation of both the renin angiotensin system and the sympathetic nervous system. This suggests that not the renal failure itself but the renal structural changes are crucial. Indeed, we have shown that hypertensive subjects with adult polycystic kidney disease show increased MSNA before developing loss of GFR (7). In the present study, we found no independent correlation between the severity of renal failure and sympathetic activity. We also observed that unilateral nephrectomy does not increase MSNA. Even though this was based on a few subjects, it seems that a major reduction in the number of functioning nephrons without damage to the residual parenchyma will by itself not induce significant sympathetic activation.
The relation between PRA and MSNA may indicate a causeeffect relation or a common origin. Angiotensin II can increase central sympathetic output (22). Renal ischemia will stimulate the renin-angiotensin system. Indeed, the patients of this study had increased PRA. Previously, we demonstrated that an ACE inhibitor and an angiotensin II receptor antagonist reduce sympathetic overactivity, strongly suggesting that angiotensin II importantly contributes in the pathogenesis of the sympathetic hyperactivity (5,23).
The present study was limited to hypertensive patients with renal parenchymal disease, starting from the idea that it is in these patients that renin activity is increased (16). In these patients, sympathetic activity is apparently also high. We have shown that MSNA is normal in normotensive patients with polycystic kidney disease (7). It remains unclear whether sympathetic activity is lower or normal in normotensive patients with other types of renal disease. Furthermore, many of the present subjects had polycystic kidney disease, a condition associated with pronounced parenchymal structural changes. The numbers of patients with other conditions are too small to allow separate evaluation of the data.
In conclusion, the present large study shows definitely that sympathetic activity is increased in hypertensive patients with renal parenchymal disease. Sympathetic activity is inappropriately increased for the volume status. It is not increased in subjects with a reduced number of nephrons in the absence of renal parenchymal disease. Recent evidence indicates that sympathetic activity is associated with mortality and cardiovascular outcomes in patients with chronic renal failure (24). Although not specifically addressed by this study, we suggest that normalization of sympathetic overactivity should be considered a specific goal of treatment of hypertensive patients with renal parenchymal disease.
| Acknowledgments |
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| References |
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