Departments of Internal Medicine and Physiology and Biophysics, University of Iowa Carver College of Medicine, and Veterans Administration Medical Center, Iowa City, Iowa 52242
Submitted 13 January 2004 ; accepted in final form 12 February 2004
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ABSTRACT |
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autoregulation; renal sympathetic nerve activity; transfer function analysis
Steady-state or stepwise RBF autoregulation is also differentially affected by subvasoconstrictor vs. vasoconstrictor intensities of renal sympathetic nerve activity (RSNA) (4, 15). Subvasoconstrictor intensities of RNS have little effect on stepwise RBF autoregulation. However, vasoconstrictor intensities of RNS that decrease basal RBF by 1520% result in a progressive elevation of the autoregulatory threshold ("break point"), i.e., that level of renal arterial pressure (RAP) below which further reductions in RAP are associated with decreases in RBF. Correspondingly, basal RBF and stepwise and dynamic autoregulation of RBF are not affected by removal of basal RSNA by either renal denervation or ganglionic blockade with hexamethonium (1, 10, 14).
The in vitro physiological reality of RBF autoregulation is the moment-to-moment adjustments of the renal vasculature to the oscillations in RAP that are needed to maintain RBF constant. This spontaneous or dynamic RBF autoregulation spans the frequency range of RAP oscillations, which is 010 Hz.
Studies in normal dogs and rats indicate that neither basal RBF nor dynamic RBF autoregulation is affected by removal of basal RSNA by renal denervation or hexamethonium administration (1, 10, 14). However, it appears that the situation is different in the rabbit. In two studies, renal denervation resulted in increases in RBF of 42 (11) and 55% (12), suggesting that the basal level of RSNA in the rabbit is renal vasoconstrictor and significantly greater than that in the rat and dog. It was observed further that dynamic RBF autoregulation was impaired after renal denervation (transfer function gain was lower in innervated than denervated kidneys), suggesting that the vasoconstrictor intensity of basal RSNA was a factor contributing to the maintenance of normal dynamic autoregulation of RBF in the rabbit (12).
The current studies were undertaken to test the hypothesis that 1) removal of basal RSNA that is of subvasoconstrictor intensity does not affect dynamic RBF autoregulation; and 2) removal of basal RSNA that is of vasoconstrictor intensity improves dynamic RBF autoregulation. To test these hypotheses, dynamic RBF autoregulation was measured before and after acute renal denervation in rats where the basal level of RSNA is subvasoconstrictor, i.e., in control and Wistar-Kyoto (WKY), and in rats where the basal level of RSNA is vasoconstrictor, i.e. in congestive heart failure (CHF) (5, 7, 8) and spontaneous hypertension (SHR) (18).
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MATERIALS AND METHODS |
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CHF was produced in Sprague-Dawley rats by left coronary artery ligation with subsequent myocardial infarction using a method established and validated in our laboratory (5, 7, 8). Control (Control or sham CHF), CHF, WKY, and SHR rats were studied 46 wk later.
Rats were anesthetized with pentobarbital sodium (50 mg/kg ip); an oral endotracheal tube was inserted, and mechanical ventilation with room air was instituted. A jugular vein was catheterized for the administration of additional anesthetic (10 mg·kg1·h1 iv) and isotonic saline at 0.05 ml/min. A carotid artery was catheterized for the measurement of arterial pressure [AP; pulsatile or mean (MAP)] and heart rate (HR). Via a left-flank incision, the left renal nerve bundle was gently dissected free (but not transected) and loosely snared with a suture for future identification and retrieval. To measure RBF, a noncannulating electromagnetic flow probe (1.5-mm circumference) was placed around the left renal artery and connected to an electromagnetic flowmeter (Carolina Medical Electronics). The flow probe was calibrated in situ by pumping heparinized rat blood at known flow rates through the cannulated rat renal artery (with the flow probe in place) at the end of the experiment.
After surgery, a 30-min period was allowed for equilibration and stabilization. This experimental protocol resulted in four experimental groups: Control (n = 8), CHF (n = 7), WKY (n = 9), and SHR (n = 10).
The experimental protocol consisted of a 30-min control (intact renal innervation) period during which continuous recordings of AP and RBF were made. Then, the left kidney was denervated by transecting the left renal nerves. The experimental period began 30 min later and consisted of 30 min during which continuous recordings of AP and RBF were made. Thereafter, a bipolar stimulating electrode was placed on the left lumbar sympathetic chain above the left kidney. A Grass S9 stimulator delivered conventional rectangular pulses of 0.2-ms duration, 15-V amplitude, and 4-Hz frequency for a total stimulation period of 1 min. The absence (<5% change) of a decrease in RBF was taken as evidence of the completeness of left renal denervation. Then, the carotid artery catheter was advanced into the left ventricle for the measurement of left ventricular end-diastolic pressure. The rats were killed with an overdose of pentobarbital sodium, and a 20-min recording of postmortem signals was made. The heart was removed and weighed.
Data analysis. AP and MAP were recorded via an electronic pressure transducer (Statham). HR was determined via a tachometer (Grass 7P4) driven by the pulsatile AP waveform. RBF, both pulsatile and mean, was recorded via the electromagnetic flowmeter, the output of which was low-pass filtered below 10 Hz by the built-in analog filter. The outputs of the pressure transducer, the tachometer, and electromagnetic flowmeter were sent to a Grass model 7D polygraph recorder for graphic output and to VHS tape via a pulse code modulation adapter (Vetter, model 4000A PCM Recording Adapter) for later offline analysis.
Analog AP and RBF signals were sampled from the tape at 1,000 Hz. The postmortem signals were subtracted from the recorded control and experimental period data. For assessment of the effect of renal denervation on steady-state values of AP, RBF, and renal vascular resistance (RVR = AP/RBF), the averages of the values from the last 5 min of the control period were compared with the averages of the values from the first 5 min of the experimental period.
Subsequent processing of the data was performed with Matlab software (Matlab, release 12.1, MathWorks, Natick, MA). The 1,000-Hz data files were digitally low-pass filtered (3.5-Hz cutoff frequency, finite-impulse-response, order 50) and then decimated to a rate of 5 Hz. These 5-Hz data were split into blocks of 4,096 data points. The transfer function spectra were calculated from AP (input) and RBF (output) during both control and experimental periods. The transfer function was taken as the quotient of the cross spectrum of input and output divided by the power spectrum of the input. The algorithm involved mean detrending and a Hanning window with 50% overlap of the blocks. To permit comparison among rats, the transfer function gain (magnitude) values over the frequency range have been normalized to the value at 0-Hz frequency (direct current; i.e., to a value of 1). After conversion of the normalized transfer function gain values into decibels (20 log[gain]), a mean spectrum was calculated from the consecutive spectra in each rat, and these were subsequently averaged for all rats.
Coherence is a frequency domain estimate of a linear correlation (i.e., squared coherence, akin to coefficient of determination) between two signals indicating the degree to which the variance in one signal can be explained by a linear operation on the other signal. The coherence spectra were calculated from AP (input) and RBF (output) during both control and experimental periods. The coherence function was taken as the quotient of the square of the cross spectrum of input and output divided by the product of the power spectrum of the input times the power spectrum of the output. The algorithm involved mean detrending and a Hanning window with no overlap of blocks of 256 data points.
For assessment of RBF variability, the mean RBF was calculated for the entire 30-min control and experimental periods for each rat. Then, the change in RBF from this mean value was calculated for each RBF value within the respective control and experimental periods for each rat. The range of change in RBF from the mean value, 5 to + 5 ml/min, was divided into 0.1 ml/min intervals (bins), and a histogram of percent occurrence was calculated for the control and experimental periods in each rat.
Statistical analysis was performed with analysis of variance with the subsequent use of Scheffé's method for simultaneous comparisons within groups and the subsequent use of the F ratio and modified statistic for nonsimultaneous comparisons between groups (19). For a comparison of distributions, the Brandt-Snedecor 2 test for comparison of arbitrary distributions was used. A significance level of 5% was chosen. Data are expressed as means ± SE.
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RESULTS |
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DISCUSSION |
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These results indicate that intensities of RSNA that produce tonic and sustained renal vasoconstriction (i.e., SHR) impair dynamic autoregulation of RBF, whereas subvasoconstrictor intensities do not (i.e., Control, WKY). However, acute renal denervation in CHF rats, which produced a similar increase in basal RBF to that seen in SHR, had no effect on dynamic autoregulation of RBF rats. An important difference between SHR and CHF rats is that the maximum duration of the increase in RSNA in CHF rats can be only 46 wk, whereas the increase in RSNA in SHR is of lifelong duration. It is also likely that the influence of RSNA on dynamic autoregulation of RBF involves an interaction with structural as well as functional components of the renal vasculature. While the increase in RBF after acute renal denervation reflects the response of functional components of the renal vasculature (vascular smooth muscle relaxation after removal of sympathetic neural vasoconstrictor tone), it seems clear that prolonged increases in RSNA can affect structural components of the renal vasculature. Sympathetic vasoconstrictor tone is known to contribute to structural remodeling and hypertrophy of arterial resistance vasculature (9, 17). Another important difference between SHR and CHF rats is the level of AP with SHR having significant lifelong hypertension, whereas CHF rats are generally normotensive. Under the influence of chronic hypertension, the renal vasculature of SHR undergoes significant remodeling characterized by a marked increase in minimal renal vascular resistance, i.e., a decreased renal vasodilator capacity (9, 13). Thus it is likely that the longer duration of increased RSNA and hypertension in SHR compared with CHF rats contributes to the differences between SHR and CHF rats in the effect of acute renal denervation on dynamic autoregulation of RBF.
The slower tubuloglomerular feedback and faster myogenic components of RBF autoregulation, readily evident in Control rats, were seen in CHF rats only after renal denervation. The slower tubuloglomerular feedback component of RBF autoregulation, readily evident in WKY, was seen in SHR only after renal denervation, which also decreased the gain of the myogenic component. These results indicate that the vasoconstrictor intensities of basal RSNA in CHF and SHR rats interacted with both the tubuloglomerular feedback and the myogenic mechanisms of RBF autoregulation.
The effect of renal denervation on the dynamic autoregulation of RBF in WKY and SHR rats has been previously examined (1). Acute renal denervation did not significantly affect basal RBF in either WKY or SHR rats; i.e., the intensity of basal RSNA in both WKY and SHR rats was subvasoconstrictor. Acute renal denervation significantly increased admittance gain in both WKY (0.1 ± 1.0 to 4.1 ± 0.9 dB) and SHR rats (from 2.9 ± 0.4 to 5.4 ± 0.3 dB) but only in the frequency range faster than autoregulation (0.250.7 Hz) and had no effect on the admittance phase. Withdrawal of the subvasoconstrictor intensities of RSNA did not affect admittance gain at frequencies 0.2 Hz, wherein the slower tubuloglomerular feedback and faster myogenic components of RBF autoregulation operate in either WKY or SHR rats. These previous finding agree with the current results that withdrawal of subvasoconstrictor intensities of RSNA does not affect dynamic autoregulation of RBF.
These results are in agreement with the observations made in studies of dynamic autoregulation of RBF in the rat and dog (1, 10). Removal of basal RSNA by acute or chronic renal denervation or by hexamethonium administration in otherwise normal rats or dogs does not produce sustained alterations in the basal level of RBF and does not alter dynamic autoregulation of RBF. Using graded intensities of renal nerve stimulation (4, 14), intensities that were subthreshold for producing decreases in the basal level of RBF, there was no major effect on stepwise autoregulation of RBF. Higher intensities of renal nerve stimulation that decreased the basal level of RBF by 1520% resulted in a progressive impairment of stepwise autoregulation of RBF, which was manifest as an elevation of the pressure threshold (break point). Similar results have been observed using carotid baroreflex activation (16) or renal arterial administration of methoxamine (15) to increase renal sympathetic adrenergic vasoconstrictor tone and decrease the basal level of RBF.
The new information from the current study concerns the influence of tonic vasoconstrictor intensities of RSNA on dynamic autoregulation of RBF. The tonic vasoconstrictor intensities of RSNA seen in CHF and SHR rats significantly worsened dynamic autoregulation of RBF in the frequency ranges of both the tubuloglomerular feedback component and the myogenic component. After renal denervation, the overall marked improvement in dynamic autoregulation of RBF was characterized by notable changes that occurred in the frequency ranges of these two components.
The similarity in results in the dog and the rat focuses on the different findings in the rabbit. In contrast to the dog and the rat where renal denervation has no sustained effect on basal RBF, the situation appears to be different in the rabbit. Compared with basal RBF in an innervated kidney, basal RBF in a denervated kidney was increased by 42 (11) and 55% (12) in two separate studies. These results suggest that, in these studies, basal RSNA in the rabbit was significantly increased, resulting in substantial tonic renal vasoconstrictor tone. Based on the results in rats and dogs, one would predict that assessment of dynamic autoregulation of RBF in these rabbit studies, wherein renal denervation markedly increased basal RBF, would show an improvement (i.e., change from a higher to a lower gain) after renal denervation. However, the opposite was observed, with gain being significantly greater in the denervated kidney than the innervated kidney across all frequencies (0.02.0 Hz) (12). These results would suggest that dynamic autoregulation of RBF in the rabbit is better in the presence of a tonic level of renal sympathetic neural vasoconstrictor tone. This might indicate that, in the rabbit, a tonic level of renal sympathetic neural vasoconstrictor tone, perhaps present throughout life, favorably impacts both the tubuloglomerular feedback and myogenic components of dynamic autoregulation of RBF. There is scant information on the effect of either acute or chronic renal denervation on the single-nephron chararacteristics of the tubuloglomerular feedback or myogenic mechanisms in the rabbit.
While the removal of a tonic renal vasoconstrictor level of RSNA had different effects on dynamic autoregulation of RBF in the rabbit (worsened) compared with the rat (improved), the effects on coherence were similar in the two species in that coherence increased significantly. This indicates that there was a tighter coupling between AP and RBF after renal denervation and that the presence of a tonic renal vasoconstrictor level of RSNA impaired the coupling (e.g., autoregulation) between AP and RBF.
In dynamic or spontaneous autoregulation, it is apparent that vasoconstrictor intensities of RSNA also interfere with the moment-to-moment adjustments of the renal vasculature that keep RBF nearly constant during the moment-to-moment changes in AP. This is reflected in the finding that, for the same change in AP from the mean value, the range of change in RBF from the mean value is far greater after acute renal denervation. It is important to note that this loss of fine adjustment is the same over the entire range of change in AP from the mean value, i.e., both during diastole when the changes in AP and RBF from their mean values are negative and in systole when they are positive. Therefore, the moment-to-moment changes in the vasoconstrictor intensity of RSNA appear to be involved both when RVR is rising (when change of AP from mean is positive) and falling (when change of AP from mean is negative).
Both the oscillations in AP as well as the rhythmic bursting discharge in RSNA contribute to the oscillations in RBF. In the rabbit, modeling the RBF oscillations as a single output with AP and RSNA as inputs indicated that 80% of the variation in RBF could be accounted for by the variations in AP and RSNA (3). With the use of a single-input model, it appeared that the RSNA signal had a somewhat more dominant effect. In that regard, it was reasoned that plotting the simultaneous individual changes from the mean of RBF vs. the individual changes from the mean of AP before and after renal denervation would unmask the contribution of RSNA to RBF variability. In Control and WKY rats, wherein basal RSNA was subvasoconstrictor, acute renal denervation did not affect RBF variability. However, in CHF and SHR rats, wherein basal RSNA was vasoconstrictor, acute renal denervation significantly increased RBF variability. These results indicate that the oscillations in RBF, derived mainly from the oscillations in AP, are constrained in magnitude by the presence of vasoconstrictor but not subvasoconstrictor intensities of RSNA. As both AP and RBF are important determinants of glomerular filtration, it can be speculated that the ability of vasoconstrictor intensities of RSNA to limit RBF variability may contribute to optimization of the glomerular filtration process.
These studies on the effect of acute renal denervation on dynamic autoregulation of RBF support the conclusions drawn from previous studies on the effects of graded intensity renal sympathetic nerve stimulation and renal denervation on overall renal function (6). In normal rats and dogs with subvasoconstrictor intensities of RSNA, acute renal denervation has no effect on basal RBF, dynamic autoregulation of RBF, or RBF variability but results in decreased renal tubular sodium reabsorption and renin secretion rate. Similarly, low-intensity renal sympathetic nerve stimulation that does affect RBF or GFR increases renal tubular sodium reabsorption and renin secretion rate. In the presence of vaosonstrictor intensities of RSNA, acute renal denervation increases basal RBF, improves dynamic autoregulation of RBF, and increases RBF variability. Similarly, high-intensity renal sympathetic nerve stimulation decreases RBF and GFR, further increasing renal tubular sodium reabsorption and renin secretion rate.
This analysis receives support from simultaneous measurements of RSNA and RBF made in conscious, freely moving rats (20). Compared with non-rapid eye movement sleep, rapid eye movement sleep decreased RSNA by 39.0% and increased RBF by 4.8%. In contrast, moving and grooming increased RSNA by 29.4 and 65.3%, respectively, while decreasing RBF by only 5.4 and 6.6%, respectively. It is apparent that large increases in RSNA are required to elicit relatively small decreases in RBF in vivo. The slope of the relationship between percent change in RBF and percent change in RSNA was 0.079, indicating that RBF decreased only 0.79% for every 10% increase in RSNA. Thus this is consistent with the view that subvasoconstrictor intensities of RSNA are present in vivo; while these do not appear to affect RBF, it appears they are capable of influencing renal tubular sodium reabsorption and renin secretion rate.
In summary, these studies have shown that, before renal denervation 1) basal RBF was lower in CHF compared with Control rats and basal renal vascular resistance (RVR) was higher in SHR compared with WKY rats; and 2) dynamic RBF autoregulation was impaired in CHF and SHR rats compared with Control and WKY rats. In rats where basal RSNA was subvasoconstrictor (Control, WKY), renal denervation did not affect basal RBF, dynamic autoregulation of RBF, or RBF variability. In rats where basal RSNA was vasoconstrictor (CHF, SHR), renal denervation significantly increased basal RBF, improved dynamic autoregulation of RBF, and increased RBF variability.
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GRANTS |
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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REFERENCES |
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