(Hypertension. 1997;30:975-983.)
© 1997 American Heart Association, Inc.
Articles |
Dynamic Autoregulation and Renal Injury in Dahl Rats
From the Department of Medical Physiology, The Panum Institute, and Department of Pathology (C.B.A.), University of Copenhagen (Denmark).
Correspondence to Finn Michael Karlsen, MD, University of Copenhagen, Department of Medical Physiology, The Panum Institute, Bldg 10.5, 3 Blegdamsvej, DK-2200 Copenhagen, Denmark. E-mail fmk{at}mfi.ku.dk
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Abstract |
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Abstract The Dahl salt-sensitive (Dahl S) rat develops hypertension and renal injuries when challenged with a high salt diet and has been considered to be a model of chronic renal failure. Renal injuries appear very early in life compared with the spontaneously hypertensive rat (SHR). During the course of hypertension, a gradual impairment of autoregulatory control of renal blood flow might expose the glomerular circulation to periods of elevated pressure, resulting in renal injuries in Dahl S rats. Dynamic autoregulatory capacity was assessed in Dahl S and Dahl salt-resistant (Dahl R) rats, SHR, and Sprague-Dawley rats by inducing broad-band fluctuations in the arterial blood pressure and simultaneously measuring renal blood flow. Dynamic autoregulation was estimated by the transfer function using blood pressure as the input and renal blood flow as the output. Renal morphological injuries were evaluated in Dahl S rats and SHR and were scored semiquantitatively. Dynamic autoregulation was efficient and comparable in the low-frequency range (<0.015 Hz) in Dahl R rats, SHR, and Sprague-Dawley rats. The response in Dahl S rats depended strongly on the initiation time of the high salt diet. Autoregulation was preserved during a low salt diet and in rats exposed to a late-onset hypertension of short duration, only partly preserved if the late-onset hypertension was of a longer duration, and abolished in early-onset hypertension. All Dahl S rats on a high salt diet showed severe morphological changes in the kidney. In conclusion, autoregulatory capacity in the kidney of Dahl S rats is gradually impaired when rats are rendered hypertensive with a high salt diet. Renal morphological injuries develop before loss of dynamic autoregulation. Impaired autoregulation appears to be the result, not the cause, of the process that ultimately leads to renal failure in the Dahl S rat.
Key Words: renal blood flow • hypertension, sodium-dependent • kidney failure, chronic • homeostasis
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Introduction |
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The Dahl S rat quickly develops hypertension when fed a high salt diet, whereas hypertension in the SHR is independent of salt. Despite similar degrees of hypertension, these two genetic hypertensive models react differently to elevated blood pressure. Renal injuries appear in Dahl S rats after 2 to 3 weeks of a high salt diet; the lesions are of a focal nature1 and comparable to malignant hypertensive renal disease seen in humans.2 3 4 5 In contrast, SHR develop milder but otherwise similar lesions, and these appear later in life.6 7 The different response is also reflected in data on survival. When Dahl S rats are placed on a high salt diet early in life, they typically die after 4 to 8 weeks.1 2 Median survival time for SHR has been reported to be approximately 70 weeks.8 9
The reason for the rapid development of end-stage renal disease in the Dahl S rat is unknown. It has been suggested that the glomeruli of SHR are protected by preglomerular vasoconstriction,10 whereas those of Dahl S rats are exposed to the damaging effect of an elevated pressure caused by a decrease in afferent arteriolar resistance.11 However, most studies have not provided evidence for a decreased renal vascular resistance in Dahl S rats; rather, total renal vascular resistance appears to be elevated.12 13
The normal kidney shows very efficient autoregulation, and as a consequence RBF, GFR, and glomerular capillary pressure remain relatively constant despite wide variations in arterial pressure. If the autoregulatory efficiency were decreased in hypertensive Dahl S rats, this could play an important role in the development of end-stage renal disease by exposing the renal microvasculature to periods of increased pressure. Renal autoregulation is the result of the combined action of the myogenic mechanism and the TGF response,14 and a possible autoregulatory deficiency could be the result of changes in one or both of these mechanisms. In vitro studies using the isolated, perfused hydronephrotic kidney have shown that the myogenic component of autoregulation in Dahl S rats on high salt was abolished,15 whereas in SHR it was intact but shifted to higher renal arterial pressures.16 17 However, studies in the intact animal have found steady-state autoregulation of total RBF to be equally efficient in Dahl S rats on high salt12 and in SHR18 compared with their respective normotensive controls.
In conscious, freely moving animals, arterial pressure does not change slowly from one steady state to another but rather fluctuates over a wide range of times, ranging from a few seconds to several hours.19 20 Previous studies on autoregulation in Dahl S rats have focused only on its steady-state characteristics. However, since the arterial pressure has time-dependent variation, it is also necessary to determine the dynamic characteristics of autoregulation.21 A difference in the dynamics of autoregulation could allow more of the normally occurring fluctuations in the arterial pressure to reach the renal microcirculation and thus cause damage to the renal tissues. This could be the case even if steady-state autoregulation were unaffected.21
The purpose of the present study was to compare the efficiency of dynamic autoregulation of RBF in both Dahl S rats and SHR on a high salt and low salt diet and to relate changes in the dynamic autoregulatory efficiency to morphological changes in the glomeruli, tubuli, renal vessels, and renal interstitium.
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Methods |
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Experiments were performed on male Dahl S rats weighing 220 to 430 g, male Dahl R rats weighing 200 to 300 g, male Sprague-Dawley rats weighing 200 to 290 g, and male SHR weighing 210 to 300 g. The experimental protocol had been approved in advance by the institutional animal care committee. The animals were purchased from Moellegaard Breeding Centre, Lille Skensved, Denmark, or from Charles River, Sulzfeld, Germany. The Dahl rats were inbred strains derived from the strain developed by Rapp and Dene.1 The European colonies of Dahl rats have been maintained independently of those in the United States. The rats are routinely checked using genetic and biochemical markers, and there has been no evidence for genetic contamination of the Dahl S rats in Europe. All rats, except for the ones used in the histopathologic examinations, were fasted overnight before experiments. The diet was a wet-mash diet based on Altromin standard diet flour No. 1314 supplemented with NaCl to a final sodium content of 0.4%, 1%, or 8%. In series including clearance measurements, the diet was supplemented with 15 or 20 mmol lithium/kg dry weight 2 days before the experiments. This gave plasma lithium concentrations ranging from 0.15 to 0.30 mmol/L. The rats were allowed free access to tap water.
External Forcing
Dahl R and S rats, SHR, and Sprague-Dawley rats were divided
into different series according to the diet scheme shown in Fig 1
and
described in the legend. Note, that S-H2
was subjected to early-onset hypertension of short duration, and S-H2L
experienced early-onset hypertension of longer duration. Similarly,
S-H4 was subjected to late-onset hypertension of short duration, and in
S-H4L, the late-onset hypertension was of a long duration. We also
tested the effect of a high salt diet in SHR and Sprague-Dawley
rats.
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Anesthesia was induced by placing the rats in a chamber containing 5% halothane administered in a mixture of 35% oxygen and 65% nitrogen through a Fluotec Mark-3 vaporizer. Catheters were inserted into the left jugular vein for infusions and into the right carotid artery for continuous recording of arterial blood pressure. A tracheostomy was performed, and the rats were placed on a servo-controlled operating table that maintained their body temperature at 37°C. The rats were connected to a small-animal ventilator that was adjusted to maintain arterial plasma pH between 7.35 and 7.45 with a mixture of 35% oxygen and 65% nitrogen, tidal volume of 1.9 to 2.1 mL, and frequency of 55 to 57 breaths per minute. The final halothane concentration needed to maintain sufficient anesthesia was approximately 1%. An intravenous priming dose of 6 mg gallamine triethiodide (Relaxan, A/S GEA) in 0.6 mL of 0.9% saline was given, followed by a continuous intravenous infusion of 24 mg/mL gallamine triethiodide in 0.9% saline at 10 µL/min.
The abdomen was opened through a midline incision extended to the left flank. The distal aorta was dissected free and cannulated at the bifurcation with a polyethylene tube (PE-90) and filled with blood freshly obtained from a donor animal, which had been on the same diet. Heparin was added to the blood in a concentration of 20 U/mL. The blood-filled tube led to a small Plexiglas chamber where low-viscosity silicone oil interfaced with the blood. Another polyethylene tube filled with silicone oil connected the chamber to a stainless steel bellows 3 cm in diameter and 5 cm long. The bellows was filled with the low-viscosity oil and connected to a linear motor (Ling Dynamic Systems) controlled by an 80286 IBM-compatible computer.
The left kidney was denervated by dissecting the renal artery, carefully stripping away all visible nerves, and wiping the artery with a solution of 5% phenol dissolved in ethanol. The ureter was cannulated to ensure the free flow of urine. Six percent bovine serum albumin (Sigma Chemical Co) in saline was infused at a rate of 20 µL/min for the duration of the surgery, and 1% bovine serum albumin was continued throughout the experiment at the same infusion rate. The left kidney was superfused with saline preheated to 37°C.
Measurements
A catheter (PE-50) filled with heparinized saline was inserted
into the superior mesenteric artery, and blood pressure
recording was shifted from the carotid artery to this catheter.
It was connected to a Statham P23-dB pressure transducer (Gould
Instruments). RBF was measured continuously with an electromagnetic
blood flowmeter (Scalar Medical, model 1402). A perivascular flow
sensor (lumen diameter, 0.7 or 0.8 mm, depending on the size of
the rat) was placed around the left renal artery.
Arterial blood pressure and RBF were recorded while broad-band fluctuations were induced in the arterial blood pressure. The fluctuations were generated by the bellows pump and resulted in blood pressure fluctuations with the spectral properties of band-limited white noise.22 The duration of the forcing was approximately 30 minutes.
Data Collection and Handling
Data acquisition has previously been described in
detail.22 Briefly, the two signals were passed through an
anti-aliasing filter and sampled simultaneously (off-line),
each at a frequency of 20 Hz for approximately 25 minutes. Each forcing
period generated 28 672 points for each of the two measured
variables; these time series were then subdivided into 14 segments,
each 2048 points long. The data were transformed using the fast Fourier
transform. The transfer functions and admittance magnitudes were
calculated as previously described in detail.22
The magnitude is a measure of the dynamic autoregulatory efficiency. It
corresponds to the ratio of the fractional variations in flow and
pressure (Fig 2). If, at a given
frequency f, the fractional variations in
arterial pressure and blood flow are identical, the
magnitude is 1 at that frequency, and dynamic autoregulation is absent.
A magnitude less than 1 indicates that the fractional variation in flow
is smaller than the fractional variation in pressure—the hallmark of
autoregulation. When the magnitude is zero, a fluctuation in pressure
is perfectly attenuated and produces no change in flow,
consistent with perfect autoregulation. A value greater than 1
signifies that the fractional variation in flow exceeds that of the
arterial pressure, indicative of passive vasodilation or
autonomous oscillatory activity.22
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Clearance Studies
Clearance studies were performed only in the series S-L, S-H2L,
R-L, and R-H2L (Fig 1
). Anesthesia and catheter placement
in the jugular vein and carotid artery were identical to that described
for the external forcing experiments. An intravenous
priming dose of 6 mg gallamine triethiodide (Relaxan, A/S GEA) in 0.6
mL of 0.9% saline was given, followed by a continuous
intravenous infusion of 12 mg/mL gallamine
triethiodide in 0.9% saline at 20 µL/min.
The abdomen was opened through a midline incision extended to the left flank, and the left ureter was cannulated for urine collection. For clearance determinations, 51Cr-EDTA (Hoechst-Behring) was given intravenously as a priming dose of 13.8 µCi (0.51 MBq) in 1.75 mL saline followed by a continuous infusion of 0.16 µCi (0.0058 MBq)/min at 20 µL/min. The experiment started after an equilibration period of 45 to 60 minutes.
A blood sample was collected from the carotid artery; then three serial 10-minute urine samples were collected into preweighed vials; and finally a second arterial blood sample was drawn. After the experiment, the left kidney was removed, drained, and weighed.
Analytic Methods
51Cr-EDTA activity was measured by a Selectronic
well scintillation counter (model 54-23, Moellsgaard Medical). Lithium
was measured by atomic absorption spectrophotometry (Perkin-Elmer
2380). Urine flow was measured gravimetrically.
Calculations
The clearance of 51Cr-EDTA was taken as a measure of
GFR23 and CLi as a measure of the output of
fluid from the proximal tubule.24 25 A prerequisite for
the latter is that lithium is not reabsorbed beyond the proximal tubule
in any significant amount. In rats, distal lithium reabsorption has
been shown to occur when the sodium content of the diet falls below
50 mmol/kg dry weight.26 In this study, all
animals were on a sodium intake well above this value. It can therefore
be assumed that CLi gives a reliable estimate of the flow
rate at the end of the proximal straight segment. Fractional
reabsorption of fluid in the proximal tubule may then be calculated as
1-CLi/GFR, and absolute proximal fluid reabsorption rate
as GFR-CLi.
Histological Examination
The kidneys were examined histologically in all
Dahl S series and in the two SHR series, ie, in a total of seven series
(Fig 1). The kidneys were perfusion-fixated at a servo-controlled
pressure of 100 mm Hg by means of the whole-body
perfusion-fixation technique as described in detail by Rostgaard et
al.27 Briefly, anesthesia was induced by
halothane and sustained by methohexital (Brietal, Eli Lilly) (15 to 18
mg IP). The abdominal and thoracic cavities were opened, and a
double-barreled cannula was advanced into the left ventricle of the
heart. The fixative (10% buffered formalin) was delivered through the
outer barrel of the cannula, and the perfusion pressure was monitored
through the inner barrel. The right atrium was cut open, and the
perfusion was started and sustained for approximately 2 minutes. Both
kidneys were then excised and cut into halves.
The halved kidneys were routinely processed for paraffin embedding, and 3- to 5-µm-thick serial sections were stained with hematoxylin and eosin, periodic acid–Schiff, periodic acid–silver–methanamine, and Masson's trichrome. Histological evaluations of the kidneys were performed by one of the authors (C.B.A.) in a blind fashion. Glomerular, arterial, and tubular lesions as well as interstitial changes were evaluated by light microscopy using semiquantitative scoring methods. The degree of damage was quantified on a scale from 0 to 3 (0=no changes, 1=slight damage, 2=moderate damage, 3=severe damage). For each kidney, the dominating degree of severity was scored. For each parameter evaluated, the mean score in a given series was calculated, and the total score in the series was obtained by summing the mean scores for each of the different parameters.
The following parameters were evaluated: (1) Glomeruli: hypercellularity; crescent formation, which was divided into cellular and fibrotic crescents; glomerular necrosis, including microthrombosis; and glomerular sclerosis. (2) Tubuli: degeneration/atrophy. (3) Interstitial tissue: inflammatory changes as evidenced by cellular infiltration and fibrosis. (4) Vessels: intimal thickening, including endothelial hyperplasia and fibrosis; hypertrophy and hyperplasia of the media; necrosis (fibrinoid necrosis) of the entire arterial vessel wall. All parameters were weighted equally.
Statistics
Data were analyzed using ANOVA for repeated or
nonrepeated measures as appropriate. Statistical differences between
individual means were assessed using a post hoc test (least significant
differences) for planned comparisons. A value of P<.05 was
considered significant. To compare the individual transfer functions,
the magnitudes were averaged over three different frequency ranges: a
low-frequency range (f<0.015 Hz), a medium-frequency range
(0.03<f<0.045 Hz), and a high-frequency range
(0.1<f<0.3 Hz). All values are given as mean±SE.
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Results |
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External Forcing
Mean values for body weight, arterial blood pressure, RBF, and total renal vascular resistance are listed in Table 1
. As expected, a high salt diet
increased arterial blood pressure in Dahl S rats. The
increase was statistically significant in the series S-H2L, S-H4, and
S-H4L but failed to reach significance in S-H2 (P=.07). In
Dahl S rats, one of the series, R-H4, also showed a significant rise in
blood pressure in response to a high salt diet but only to a level
comparable to that in Dahl S rats on a low salt diet (S-L). Neither SHR
nor Sprague-Dawley rats responded to a high salt diet with changes in
blood pressure.
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A total of 137 forcings were performed in 57 Dahl S rats, 34 Dahl R rats, 22 SHR, and 24 Sprague-Dawley rats. The external forcing provided uniform blood pressure fluctuations at frequencies less than 0.5 Hz, and fluctuations did not differ significantly between the different series.
The transfer function of the Dahl S rats on a low salt diet (S-L)
displayed two distinct resonance peaks—one at approximately 0.04 Hz
and another at approximately 0.2 Hz (Fig 3A). The magnitude decreased below 1 at
approximately 0.08 Hz, indicating autoregulation at frequencies lower
than this value. In the low-frequency range, the average magnitude was
0.48±0.19. This indicates that the autoregulatory activity resulted in
flow oscillations that were half the size of what would be
expected in a passive vascular bed. Series S-H4 had a transfer function
that was not significantly different from that of the S-L rats. In
contrast, the S-H2 and S-H2L rats had a significantly reduced
autoregulatory efficiency in both the middle- and low-frequency ranges
compared with both the S-L and S-H4 rats. Early-onset hypertension of
either short or long duration therefore seems to be associated with a
severely reduced ability of the kidney to autoregulate RBF. In the low-
and middle-frequency ranges, the transfer function of the S-H4L rats
seemed to be located more or less in between the transfer functions of
the S-L, S-H4 and the S-H2, S-H2L rats, respectively. There were no
significant differences between the average magnitudes in the
high-frequency range between any of the Dahl S groups (see Table 2).
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The transfer functions in the Dahl S rats (Fig 3B) only showed minor
changes in response to the different salt contents of the diets. All
three groups showed efficient dynamic autoregulation in the
low-frequency range (see Table 2).
In Sprague-Dawley rats on a normal salt diet (SPD-N), the magnitude had
a maximum at approximately 0.2 Hz and reached a value close to 1 at
approximately 0.09 Hz (Fig 3C). A secondary shoulder was seen at
approximately 0.05 Hz, and the magnitude then declined, reaching a
value of 0.49±0.07 in the low-frequency range. Autoregulation was
evident at frequencies less than 0.09 Hz, with the strongest degree of
autoregulation at low frequencies. Dynamic autoregulation was
maintained in the low-frequency range when challenged with a high salt
diet (SPD-H4).
SHR on a normal salt diet (SHR-N) showed two peaks in the magnitude
curve (Fig 3D), and the magnitude reached a value of 0.78±0.26 in the
low-frequency range. No statistical difference could be detected
between the transfer functions for SHR-N and SHR-H4 series, indicating
that the salt content of the diet did not influence the dynamic
autoregulatory behavior in SHR.
Inspection of the transfer functions in all four strains suggested that the average magnitude in the high-frequency range was lower in the Dahl S rats than the three other groups. Since the average magnitudes did not differ between salt diets within the different rat strains, all values for the average magnitudes in the high-frequency range for a given strain were pooled. The pooled average magnitudes were significantly lower in the Dahl S rats (1.44±0.06) than the Dahl R rats, the Sprague-Dawley rats, and the SHR (2.05±0.14, 2.05±0.14, and 2.05±0.16, respectively).
Clearance Studies
The results from the clearance experiments are given in Table 3. The Dahl S rats on a high salt diet
(S-H2L) had an increased arterial pressure compared with
the rats that remained on a low salt diet (S-L). The
arterial pressures in the two series of Dahl R rats did not
differ significantly. There was a general tendency for GFR,
CLi, and absolute proximal reabsorption to be lower in Dahl
S rats than Dahl R rats. However, only a few of these differences
reached statistical significance (GFR and CLi between the
R-H2L and S-H2L series). There were no significant differences between
the groups that had been exposed to a high salt diet and those that
remained on a low salt diet. Thus, no drastic decline in renal function
was observed in the S-H2L rats on a high salt diet.
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Histological Examination
Table 4 summarizes the results of
the morphological evaluations of the kidneys in the various groups. All
Dahl S rats exposed to a high salt diet (S-H2, S-H2L, S-H4, and S-H4L)
had severe tissue changes, as indicated by high total scores. The
typical features of malignant hypertension were seen; that is, the
intrarenal arteries exhibited myointimal thickening, fibrinoid
necrosis, and the classic "onion skin" appearance (Fig 4A, 4C, and 4D). Thrombosis in larger vessels was
not present in any case. The glomeruli were sclerotic, with
hypercellularity, necrotic foci, and/or crescent formation (Fig 4B and 4C). Furthermore, tubular atrophy, interstitial
inflammation, or both were apparent (Fig 4A). The lesions were
focal—normal areas existed next to areas with lesions. In contrast,
the Dahl S rats that were kept on a low salt diet had either no (Fig 4E and 4F) or only minor lesions (slight glomerular sclerosis)
in the kidney.
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The kidneys of the SHR had a fairly normal morphological appearance irrespective of the salt content of the diet, with the dominant finding being focal mononuclear inflammatory infiltrates in the interstitium.
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Discussion |
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The results of the present study demonstrate a loss of dynamic autoregulation in hypertensive Dahl S rats that is uncorrelated with the morphological changes in the kidney. The morphological changes precede the functional changes, and it is concluded that the loss of dynamic autoregulation after exposure to a high salt diet is not the cause but rather is the result of the renal pathology.
Renal Injury, Blood Pressure Response, and Survival
Dahl S rats develop a severe degree of renal injury when
challenged with a high salt diet.1 2 3 4 5 A characteristic
feature of the renal injuries is its focal nature.1 In the
present study, the focal character of the lesions is reflected in
the total injury scores, which failed to reach maximal values because
of the mixture of normal and pathological areas within the kidney.
Although a comparison between different studies is difficult because of
differences in the amount of salt given, the time of initiation of the
high salt diet,2 4 and the time the rats are kept on the
high salt diet,3 4 it appears that the present
morphological findings in the Dahl S rats closely agree with those
previously reported.2 3 4 5
The time of initiation and duration of the high salt diet determine the magnitude of the blood pressure response28 and the survival of Dahl S rats. In the present study, the mean arterial blood pressures in the different series of Dahl S rats on a high salt diet ranged from 134 to 149 mm Hg. The lowest values were recorded in the S-H2 series and were measured only 4 weeks after the rats were put on a high salt diet. Since the mean arterial pressures were obtained in halothane-anesthetized, surgically prepared rats, they cannot be directly compared with the pressures reported in the majority of studies on Dahl S rats. In general, most reported values are SBP measured by the tail-cuff method in conscious animals. The present values appear to be within the range of pressures measured in chronically instrumented rats. Thus, Brown et al29 found a mean arterial pressure of 131 mm Hg after 5 to 8 weeks of high salt feeding. In anesthetized rats, more marked responses have been recorded, with mean arterial pressures from 153 to 160 mm Hg.2 12 30
Previous studies showed that when 3-week-old Dahl S rats were placed on a high salt diet, they were either dead or dying after 4 weeks of the diet.2 If the high salt diet was postponed until 4 weeks of age, all rats died by 8 weeks of the high salt diet1 ; when the diet was further postponed until 5 to 6 weeks of age, 30% of the rats survived a high salt diet for more than 10 weeks.31 In the present study, none of the Dahl S rats survived for more than 8 weeks when put on an early-onset (5 weeks) high salt diet for 4 weeks, and high salt feeding for more than 4 weeks was abandoned because of either premature death of the animals or severe vascular complications (eg, strokes) that required euthanasia. In contrast, no deaths caused by the high salt diet occurred in the Dahl R rats, SHR, or Sprague-Dawley rats. Thus, it appears that the Dahl S rats in the present study showed a natural course of the disease similar to what has been observed in previous studies.
Dynamic Autoregulation
The dynamic characteristics of renal autoregulation have been
studied extensively in normotensive rats and SHR.21 Most
of the studies have used linear techniques for systems identification.
The justification for taking a linear approach is that nonlinear
behavior can be described with a linear approximation, provided that
inputs with sufficiently small amplitudes are used. This was the case
in the present study, since the forcing amplitude was only a small
percentage of the mean arterial pressure.
One of the advantages of the dynamic approach is that it allows a
partial characterization of the two mechanisms involved in renal
autoregulation—the myogenic response and the TGF mechanism—based on
differences in the rapidity of the responses of the two systems. The
myogenic response is a fast mechanism that attenuates the flow response
to pressure fluctuations at frequencies below approximately 0.2 Hz. TGF
is a slow-reacting system that contributes to autoregulation only at
lower frequencies, ie, below approximately 0.03 Hz. The presence of
these two systems is reflected by the presence of two resonance peaks
in the admittance magnitude (Fig 3)—one around approximately 0.2 Hz
and another around 0.03 to 0.045 Hz. The first peak is the result of
the interaction between the myogenic mechanism and the passive elastic
properties of the vessel, whereas the second peak is due to the
intrinsic oscillation of the TGF system.21
Because of the intrinsic oscillation, there will be
significant power in the RBF at this frequency independent of the
fluctuations in the arterial blood pressure. Since the
magnitude is defined as the ratio of the powers in the blood flow and
the arterial pressure, the result will be a high value for
the magnitude at this frequency, a resonance. Thus, there will be no
autoregulation in the narrow frequency band around the TGF-mediated
peak. In the low-frequency range, the frequency-response curves are
characterized by a plateau region where the admittance magnitude is
low, showing significant attenuation of the fluctuations in RBF. This
is the frequency range where significant dynamic autoregulation occurs,
and it is the result of the combined action of the myogenic and TGF
mechanisms.
The magnitude curves in Dahl R rats closely paralleled the ones
seen in SHR and Sprague-Dawley rats, and efficient autoregulation was
present in the low-frequency range in Sprague-Dawley rats, SHR, and
Dahl R rats. Autoregulation in Dahl S rats, on the other hand, depended
strongly on the diet scheme (Fig 3A). It was intact in Dahl S rats on
low salt diet and in rats with late-onset hypertension of short
duration. Rats with late-onset hypertension of a longer duration
demonstrated an impaired autoregulation, whereas the latter was
abolished in Dahl S rats with early-onset hypertension. The
autoregulatory capacity of the kidney is thus more susceptible early in
life to the deleterious effects of a high salt diet and
hypertension.
Besides the abolishment of autoregulation in the Dahl S-H2 and S-H2L series, a marked amplification of the resonance peak in the medium-frequency range was evident. A similar amplification was noted in Dahl R rats that were fed a high salt diet early in life (R-H2L). In fact, the common denominator of all three series showing an amplified resonance peak was early high salt feeding. We therefore suggest that a high salt diet begun early in life increases the reactivity and amplifies the oscillations of the TGF mechanism in Dahl rats. However, the exact mechanism behind this phenomenon remains to be established.
The combination of an abolished low-frequency autoregulation and a preserved TGF resonance peak in Dahl S rats fed a high salt diet early in life leads us to hypothesize that the decreased efficiency of RBF autoregulation is due to a defect in the myogenic response. This is in accordance with the results of Takenaka et al15 obtained in the isolated, perfused hydronephrotic kidney preparation. However, it is clear that direct studies of the myogenic response in isolated vessels is needed to resolve this question.
Autoregulation and Renal Injuries
When the functional and morphological changes were correlated in
the present study, an unexpected finding appeared. The Dahl S rats
that were exposed to late-onset hypertension of either short (S-H4) or
long (S-H4L) duration demonstrated autoregulation of RBF in the
low-frequency range (Fig 3A). Their total injury scores were, however,
just as high as in the Dahl S rats with early-onset hypertension (Table 4). The present finding of preserved autoregulation and severe
morphological lesions in the same series suggests that salt-induced
hypertension in Dahl S rats results in severe renal injuries that are
followed by a loss of dynamic autoregulation of RBF. The present
study does not permit any conclusions as to the mechanism or mechanisms
underlying the renal injuries. Possible mechanisms could be primary
damage of larger vessels exposed to the elevated blood pressure,
increased blood pressure variability exceeding the dynamic
autoregulatory capacity, or mechanisms not directly linked to the high
blood pressure, such as autoimmune processes7 or
endothelial dysfunction.2 32
It is interesting to note that the causal relationship seems to be reversed in another rat model of chronic renal failure, the 5/6 remnant kidney model. This model is characterized by hypertension, loss of autoregulation, and a progressive damage to the kidneys morphologically similar to that in the Dahl S rats.33 In this model, a low protein diet prevents the development of glomerulosclerosis and preserves autoregulation.33 An intact autoregulation appears to be critical because, as shown by Bidani et al34 and Griffin et al,35 administration of calcium channel blockers causes autoregulatory impairment and a reversal of the glomeruloprotective effect of the low protein diet.
Independent of the salt diet, the Dahl S rats showed a second peculiarity in the frequency-response curves, namely, a reduced admittance magnitude in the high-frequency range. Mathematical models suggest that the size of the admittance magnitude in this region is a consequence of the interaction between passive changes in the hemodynamic resistance in response to pressure changes and the onset of the myogenic response.36 A reduced magnitude either could result from a reduced compliance, yielding a reduced passive vasodilation in response to pressure increases, or could be due to an increase in the upper frequency limit of the myogenic response. However, considering the fact that the decreased magnitude was insensitive to the salt diet, it appears most likely that it represents a decreased renal vascular compliance in the Dahl S rat.
In conclusion, efficient dynamic autoregulation of RBF was demonstrated in Dahl R rats, Dahl S rats on a low salt diet, Sprague-Dawley rats, and SHR, but Dahl S rats lost this capacity when challenged with a high salt diet early in life. Morphological changes in the kidney seemed to precede the functional changes; hence, the primary consequence of a salt-induced hypertension in Dahl S rats is renal pathological lesions, which are followed by impairment of dynamic blood flow autoregulation.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This work was supported by grants from the Danish Heart Association, the Danish Medical Research Council, the University of Copenhagen Medical Faculty Foundation, the Novo-Nordisk Foundation, the Oester-Joergensen Medical Research Foundation, the Koenig-Petersen Foundation, and the Engineer of Frederikssund, Søren Alfred Andersens Legacy. The authors thank Drs Jørgen Rostgaard and Klaus Qvortrup for access to the perfusion-fixation equipment. The excellent technical assistance of Eva Christensen Heins, Ian Godfrey, and Anni Salomonsson is gratefully appreciated.
Received March 10, 1996; first decision March 31, 1997; accepted March 31, 1997.
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