AMERICAN
JOURNAL
OF PHYSIOLOGY
Vol. 228, No. f,January
1975.
Autoregulation WILLIAM
Printed
in U.S.A.
of blood Je ARENDSHORST,
flow in the rat kidney
WILLIAM
F. FINN,
AND
(With the Technical Assistance of H. K. Lucas) Departments of Medicine and Physiology, University of North Carolina Chapel Hill, North Carolina 27514
ARENDSHORST,
WILLIAM J., WILLIAM F. FINN, AND CARL W, Autoregulation of blood flow in the rat kidney. Am. J. Physiol. 228( 1) : 127-l 33. 1975.-We present evidence showing that renal blood flow (RBF) in the anesthetized nondiuretic rat can be measured reliably and accurately using a noncannuIating flow transducer and an electromagnetic flowmeter. In vitro calibration yielded a linear relationship (r = 0.998) between flowmeter output voltage and blood flow rates from 0.2 to 10.3 ml/min. Excellent agreement was observed between simultaneous determinations of RBF by the flowmeter and the PAH clearance technique. Glomerular filtration rate and RBF for a kidney with a flow transducer around its renal artery did not differ significantly from correspondink values for the undisturbed contralateral kidney. The relationship of mean RBF with steady-state variations in perfusion pressure was evaluated in 13 nondiuretic rats. RBF averaged 6 ml/min +g kidney wt at GOTTSCHALK.
arterial
pressures (AP) above 100 mmHg.
A high degree of auto-
regulatory eff%iency was observed when mean AP varied between 105 and 145 mmHg. Qver this pressure range RBF changed only 3 y0 as changes in intrarenal vascular resistance and AP were directly related (r = 0.994). Below 95-105 mmHg RBF decreased in a curvilinear fashion with the concavity toward the pressure
axis. renal blood flow; renal vascular resistance; flow-pressure relationship; electromagnetic PAH clearance
arterial pressure; flow transducer;
IS AN ABUNDANCE of evidence indicating that the mammalian kidney possesses the intrinsic ability to regulate its vascular resistance as perfusion pressure varies from about 80 to 200 mmHg (15, 24, 28). Consequently renal blood flow (RBF) and glomerular filtration rate (GFR) are maintained relatively constant over this pressure range importance in (9, 25, 2% a response of physiological maintenance of salt and water balance. In most in vivo studies of this intriguing phenomenon of autoregulation, the dog has been the laboratory animal of choice. This has been the case largely because of the availability of noncannulating flow transducers with large-diameter lumens which are capable of measuring accurately static and transient variations in RBF. The relatively long time period required to determine RBF by clearance methodology obviously renders it incapable of accurate detection of transient changes in blood flow. The unavailability of noncannulating flow transducers with the desired sensitivity and stability and which also have lumens small enough to fit snugly around the rat renal artery has reTHERE
CARL
W. GOTTSCHALK
School of Medicine,
stricted the use of the rat in such studies. Data obtained from this species are limited and conflicting. Our observations demonstrate for the first time that blood flow in the renal artery of an anesthetized nondiuretic rat can be measured accurately using a small noncannulating electromagnetic flow transducer without significant compromise in GFR and RBF for that kidney. From determinations of the relationship between mean RBF and arterial pressure, we show that the rat kidney prepared for micropuncture has the ability to autoregulate blood flow efficiently as a consequence of enhanced intrarenal vascular resistance when mean perfusion pressure varies from 105 to 145 mmHg. Below 95-105 mmHg RBF decreased in a curvilinear fashion with the concavity toward the pressure axis. METHODS
Observations are reported on a total of 17 male SpragueDawley rats, weighing 250 =t 34 (SD) g, which were deprived of food but allowed free access to water overnight prior to study. The rats were anesthetized by intraperitoneal injection of sodium pentobarbital, 50 mg/kg body wt, placed on a heating table that maintained body temperature between 37 and 38 C, and the left kidney was exposed incision for micropuncture as through an abdominal previously described (12) Saline (0.85 % NaCI) was infused continuously into a jugular vein at a rate of 40 pl/min. Both ureters were catheterized with PE-10 polyethylene tubing. Femoral arterial pressure (AP) was monitored with a Statham P23 Db pressure transducer connected to a Beckman Dynograph. Blood flow in the left renal artery1 was measured continuously by a small-diameter flow transducer (EP model, 40 1.5, 1.5 mm circum. lumen size) connected to a squarewave electromagnetic flowmeter (model 50 1, Carolina Medical Electronics, Inc.) and a Beckman Dynograph. The flowmeter system was calibrated in vitro and in vivo periodically throughout the study. In vitro calibration consisted of constant infusions of heparinized rat blood at known rates through a cannulated excised segment of a rat carotid artery placed in a container of saline. In addition, the central end of a carotid artery was cannulated 1 The probe was positioned around the renal artery proximal to any discernable branching as close as possible to its origin from the aorta in order to assure measurement of total flow. Transient occlusion of the renal artery distal to the probe always resulted in uniform bJanching of the kidney surface in acceptable preparations.
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128 in anesthetized rats and the outflow was timed; heparinized rat blood was also infused through the carotid. For the in vivo calibration, the flow transducer was placed around the left renal artery in six anesthetized rats whose left kidney was prepared for micropuncture, and the measurements by the flow probe (RBFdi&, hereafter termed “direct” in contrast with data obtained by clearance methodology, were compared to values determined by the PAH clearance technique [RBFpAH = CPAH/EPdH zero flow through the renal artery (1 - Hct)]. Absolute was determined on several occasions during an experiment by completely occluding the artery distal to the probe with a forceps for 2-3 s; base-line drift was negligible. Positioning of the noncannulating flow probe around the renal artery was judged acceptable and presumably optimal when there was a snug fit, pulsatile flow was recorded free of mechanical noise, intravenously injected dye (0.04 ml of 5 % buffered F, D & C no. 3 dye) had a uniform distribution in superficial cortical vessels and nephrons, and the kidney continued to form urine. In several instances animals had to be discarded because the artery was too close to the vein to be certain that the probe did not compress the renal vein and significantly restrict venous outflow. The relationship of renal blood flow to arterial pressure was studied over a pressure range from 40 to 150 mmHg in 13 rats. To provide elevated arterial pressure, the carotid arteries were occluded in most animals. Progressive decrein approximately 10 mmHg ments in arterial pressure stages were achieved and maintained at a stable level (& 3 mmHg) by employing an adjustable constrictor clamp around the abdominal aorta above the renal arteries. The degree of aortic constriction by mechanical compression was controlled by movement of a piston toward the baseplate of a modified aortic clamp connected to an electronic servo system designed to maintain femoral arterial pressure at a present level (D. Smith, unpublished data). To evaluate the possible influence of mechanical trauma due to having the flow probe in position, function of that kidney was compared to that of the undisturbed right Priming solutions of inulin-3H, 10 kidney in six animals. &X/100 g body wt, and PAH-14C, 2 &X/ 100 g body wt, were followed by an intravenous infusion of 0.85 % NaCl at 40 &min so that inulin and PAH were administered at 20 and 3 pCi/lOO g body wt per hour, respectively. Clearance periods of 20-30 min duration commenced 1 h after priming. Blood samples were taken periodically from the tip of the tail and the left renal vein. Samples of urine and plasma were discharged into counting vials containing 1 ml water and 10 ml PCS Solubilizer (Amersham/Searle), and radioactivity was measured in a threechannel liquid scintillation spectrometer. Intrarenal vascular resistance (RVR) was calculated from the arterial-venous pressure difference and blood flow per gram kidney wet weight (KW). Renal venous pressure was assumed to be constant and assigned a value of 5 mmHg in the calculations. Paired and unpaired i tests and linear regression by the least-squares method were performed for analysis of significance. A P value greater than 0.05 was considered to be not statistically significant (NS).
ARENDSHORST,
BLOOD
FINN,
AND
GOTTSCHALK
FLOW (ml/min)
1. In vitro calibration curve. Relationship between output voltage (mV) from electromagnetic flowmeter and constant rates of flow of rat blood with a mean hematocrit of 49(%. Means & I SD. Best-fit regression line : y = 54,7x + 10.7, r = 0.998. FIG.
RESULTS
Calibration of nancannulating flow trunsducer and squaraz.vaue electromagnetic jlowmeter. The flow transducers were calibrated several times in vitro and in situ by perfusing rat blood with an average hematocrit of 49 % at constant rates through carotid arteries as described in METHODS. The calibration curves obtained using these two methods were almost identical and have thus been pooled. Shown in Fig. 1 is the calibration curve for a flow transducer. The data points deviated very little from the calculated best-fit regression line (y = 54.7x + 11, r = 0.998) as the mean flow rates varied from 0.2 to lOa3 ml/min. Clearly the output voltage from the flowmeter system is proportional to flow rate and is sensitive enough to measure the relatively low rates of blood flow in the kidney of a nondiuretic rat. The results from the above means of calibration were also verified in vivo in the physiological range of flow. In Fig. 2 renal blood flow monitored by the flow probe is compared to values determined simul(RBFdircct) taneously by the PAH clearance method (RBFPAH) in six nondiuretic animals whose left kidney was prepared for micropuncture in the usual fashion in our laboratory. After the initial observations in one animal, AP was lowered by aortic compression to reduce RBF for two clearance periods. Excellent agreement (r = 0.997) was found between the two methods in each animal. Mean RBF ranged from 2.5 to 7.4 ml/min-g KW, and the ratio of RBFdir,,J different RBFPAH, 1.01 =t 0.02 (SD), is not statistically from unity. Glomerular filtration rate was also determined by the clearance of inulin in these animals in order to evaluate the possible influence of trauma resulting from clearing of the renal artery and having the flow transducer in position for up to 5 h. The undisturbed right kidney served as the control. The data in Table 1 indicate that GFR and RBF for the experimental and control kidneys were not significantly different. These results show that when properly applied, the Aow transducer does not significantly impede RBF or otherwise disturb renal function as reflected by alterations in GFR. Although the paired difference of urine flow rates for the two kidneys was only
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AUTOREGULATION
OF
RENAL
BLOOD
129
FLOW
of borderline statistical significance (0.05 < P < 0. lo), urine flow from the left was about twofold greater than that from the right in three of the six experiments, indicating that these kidneys were probably at least partially denervated. Relationship of renal blood flow to steady-state changes in perfusion pressure. To characterize the relationship of mean renal blood flow to perfusion pressure, mean arterial pressure was varied from 40 to 150 mmHg in 13 nondiuretic rats. Elevated AP was provided by occlusion of one or both carotid arteries in most animals. Following carotid occlusion, AP increased to variable extents (O-30 mmHg) without any consistent changes in RBF. Each different pressure level was maintained stable for at least 10 s. Holding AP stable at a given level for up to 10 min did not reveal further changes in RBF. AP was decreased from the highest to the lowest and usually allowed to return rapidly to the original level; in several instances when AP was increased in progressive stages from the lowest pressure, a slight but consistent hysteresis was observed. Nevertheless, the shapes of the autoregulation curves were similar. The responses of mean RBF to step decrements of about 10 mmHg in perfusion pressure in one experiment are
shown in Fig. 3. During the steady-state reductions in AP from 133 to 99 mmHg, RBF was practically unchanged ; however, after AP was dropped below 90 mmHg, obvious decreases in blood flow were evident. Note that the intrarenal circulatory adjustments occurred rapidly so that RBF reached a new steady-state level in less than 3 s. The capability of the flowmeter system to respond to ‘pulsatile flow is shown in Fig. 4. During each cardiac cycle there are large fluctuations in renal blood flow synchronous with arterial pressure changes. It seems probable that these transient variations occurred too rapidly to elicit an autoregulatory response. For example, at a systolic arterial pressure of 155 mmHg, RBF was 7.84 ml/min .g KW; blood flow was 4.89 at a diastolic pressure of 110 mmHg. In this particular case mean RBF and AP were 6.17 ml/ min. g KW and 130 mmHg. At least five separate autoregulation curves were obtained in each of the 13 rats studied. The summarized results for RBF and RVR at each pressure level are presented in Table 2. Renal biood flow in these anesthetized nondiuretic rats was approximately 6 ml/min .g KW when AP was above 100 mmHg. In order to compare results in different animals, RBF and RVR were normalized to their respective values at the pressure of 114 mmHg for each experiment. The percent changes of RBF to variations in TABLE 1. Clearance data from six nondiuretic rats comparing GFR and v for the left kidney with a noncannulating J~OW transducer around its renal artery and undisturbed right kidney Left Kidney Right Kidney V, p1jmin.g
GFR,
ml/min.g
RBFr*n,
v I
I
I
I
I
I
2
3
I
4
5
6
,
7
8
RBF,,,
(ml/mln~gKW)
FID. 2. In vivo verification of in vitro calibration. Comparison of renal blood flow measured simultaneously by a noncannulating flow transducer (RBFdi,,,t) with that by PAH clearance technique (RBF,,). Individual paired determinations in 6 nondiuretic rats show very little deviation from indicated line of identity. Best-fit regression line: y = 1.007x + 0.030, r = 0.997.
KW
RBFdirect,
ml/min.g
ml/min.g
RBFdi.eet/RBFrAn, Values averaged taneously technique
4.07 f2.76
KW
1.08
KW
KW
ratio
2.70 fl.66
1.09
f0.24
f0.18
6.10 fl.03
5.91 fl.13
P
0.05-0.10
NS NS
6.15 Yk1.07 1.01 f0.02
are means f 1 SD. V = urine flow rate. Body weight 245 g. Comparison of renal blood flow measured simulusing a flow probe (RBF d,reot) and by the PAH clearance (RBFr&. FID. 3. A representative autoregulation curve. Tracing showing response of renal blood flow monitored by an electromagnetic flow transducer to step decreases in femoral arterial pressure achieved and maintained by compression of abdominal aorta above renal arteries using an electrical-mechanical servo-control system. Note characteristic transient overshoot in flow after relief of temporary occlusion of renal artery distal to flow probe (mechanical zero).
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130
ARENDSHORST,
perfusion pressure are graphed in Fig. 5. Mean RBF was quite stable when mean perfusion pressure ranged from 105 to 145 mmHg (r = 0.08x + 91.2, r = 0.844), increasing only 3 % in response to an overall pressure increase of 40 mmHg. Below 105 mmHg RBF decreased progressively from 95 % of control flow at 95 mmHg to 44 % at 45 mmHg. In Fig. 6 are shown the percent changes of RVR to alterations in arterial pressure. When AP was decreased from 145 to 105 mmHg, there were proportional reductions of RVR from 127 to 92 % of the resistance at 114 mmHg (r = 0.83x + 4.91, r = 0.994). Below 95 mmHg RVR gradually decreased to the lowest percent at 65 mmHg. At the two lowest pressures studied, there was a tendency for RVR to increase again ; RVR increased significantly from 65 to 45 mmHg (P < 0.05). Although this response was not evident when considering the mean absolute values of RVR for all animals (Table 2), paired analysis of the data from the nine experiments in which RBF was measured at 45 mmHg revealed that RVR expressed either in absolute terms or as percent change. did increase significantly (P < 0.05) at each of the two pressure levels below 65 mmHg. Thus, it is clear that RBF in the rat under these experimental conditions is autoregulated eihciently above a mean AP of 105 mmHg as RVR increased in proportion to changes in AP. Below 95-105 mmHg RBF decreased in a curvilinear fashion with the concavity toward the pressure axis.
FINN,
AND
GOTTSCHALK
able of quantitating blood flow in a renal artery of the rat without detectably altering GFR and blood flow in the kidney under investigation. It is probable, however, that in some experiments an unpredictable consequence of the preparation is at least partial renal denervation. In addition, we have demonstrated excellent agreement between RBF measured by the electromagnetic flowmeter and by the PAH clearance technique (Table 1, Fig. 2). Renal blood flow in these 17 anesthetized nondiuretic rats averaged 6 ml/min *g KW at arterial pressures between 100 and 150 mmHg. This mean flow is compared in Table 3 with values published by other investigators employing a variety of techniques. It is readily apparent that some of the flow rates per kidney are comparable to ours and others are appreciably different. In general, values reported from studies using PAH clearances, excluding those of Eisenbach et al. (8) and Girndt and Ochwadt (1 l), or radioactive microspheres (30) agree well with that found in the present study. The antiglomerular basement membrane antibody technique (2, 30) has yielded mean flow rates slightly higher than ours, while RBF in the preparations emplo ying the methods of *6Rb uptake (1 l), 13SXe washout (1, 14), high-frequency microcinematography 110 r
+
100 ----_______---1 ____-____--
T I
f
DISCUSSION
Our observations show magnetic flow transducer,
that a noncannulating when properly applied,
electrois cap-
45
55
65
75 ARTERIAL
FID. 4. A typical tracing showing pressure and renal blood flow.
pulsatile
and
mean
105
115
125
135
145
( m m Hg I
FIG. 5. Autoregulation of renal blood flow during graded constriction of aorta above renal arteries. Relationship between percent change in renal blood flow normalized to mean value at 114 mmHg for each experiment and arterial pressure. Means f 1 SE of 13 experiments. Best-fit regression line for flow between 105 and 145 mmHg: y = 0.08x + 91.16, r = 0.844. Data points below 105 mmHg are joined by a curve drawn freehand.
arterial
Pressure
mm
Hg
RBF , ml/min
* g KW
RVR, mmHg/ml. min*g KW No.
95
PRESSURE
2. Effect of variations in AP on RBF and RVR in the rat
TABLE
AP,
85
of animals Values
are means
f
Range
40-50
SO-60
60-70
70-80
So-90
90-100
100-110
110-120
120-130
130-140
140-150
45.3 f3.2
54.5 f2.0
64.6 f0.9
75.2 fl.O
84.9 zkl.1
94.5 fl.2
104.7 Al.3
114.0 f1.4
123.1 4x2.9
132.7 f1.5
145.4 f2.2
2.82 f0.69
3.39 zkO.65
4.29 ho.82
4.76 f1.05
5.24 fl.05
5.66 Ylzo.91
5.90 rko.93
5.95 f0.82
6.05 f0.88
6.13 f0.80
6.14 f0.86
14.9 f2.9
15.1 f2.9
14.4 f2.9
15.5 f3.6
15.9 Yk3.7
16.2 zk2.8
17.3 f3.1
18.8 +3.1
20.1 h3.2
21.2 52.9
23.2 rt3.2
9
12
12
13
13
13
13
13
13
12
5
1 SD.
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AUTOREGULATION
OF
R ENAL
BLOOD
131
FLOW
I 7" c L
I
,
45
55
65
c,
/
/ I
75
6. Intrarenal
FIG.
autoregulation renal vascular for each periments. mmHg:
vascular
of renal resistance
experiment Best-fit y = 0.83x
TABLE
3.
d$erent ~~-
1
I
85
ARTERIAL
( mm
resistance
blood flow. normalized
105
95
PRESSURE
I
I
I
I
115
125
135
145
Hq)
changes
Correlation to mean
associated of percent
vahe
and arterial pressure. Means =t 1 SE of 13 exregression line for resistance between 105 and 145 + 4.91, r = 0.994.
of meanuahes
&npurison
techniques in nondiuretic -.--.-
for tutu/ blood j?ow
flow
RBF, ml/min transducer
6.0/g
clearance”
6.2/g
5.4/g 6.5/g 6.4/250 6.2/250 5*5 6.6 3.9/250 8.5/g Microsphere
86Rb
Renal
membrane
uptake
%% a
KW” KWd
114 112
SD SD
13 6
KWd KW
112 112
SD W
SD SD SD W W
6 6 28 124 7 9 8 10 10
SD
11
Wallin
et al.
SD SD
14 6 6
Barratt Wallin Wallin
et al. (2) et al. (30) et al. (30)
17 23
Sapirstein Sapirstein
KW’~ g g BWgr g BW
h, i 112 120
g BWj*
k
KW
3.6/g of superficial
washout
venous
Eligh-frequency
6.8/g 7.5 6.3
KW 127
4.4/250 7.1/250
uptake
Micropuncture
133Xe
basement
nephrons’
outflow
microcinematographic
SD
g BW g BW KW
Of
W
4.8 4.8 3.9 4.2/g 3,4/g
No. of Animals
Mean AP, mmHg”
5.8
Antiglomerular antibody
42K
in one kidney in uiuo by
~.
6.1/g PAH
(RBF)
rats
Technique
Electromagnetic
at
with
change in 114 mmHg
(27), and renal venous outflow (8, 27) is distinctly lower. However, for a more critical analysis of the reproducibility and accuracy of the different techniques, the measurements of blood flow obviously should be performed simultaneously, as we have done using the square-wave electromagnetic Aowmeter and PAH clearance methods. Our observations also demonstrate the presence of autoregulation of blood flow in in situ kidneys of anesthetized to the present study several nondiuretic rats. Previous groups of investigators have examined the integrity of the autoregulatory phenomenon in isolated-perfused rat kidneys (3, 16, 18, 31). W eiss et al. (31) found that the autoregulation of RBF occurred when perfusion pressures were elevated above 100 mmHg 30-60 min after initiation of perfusion with a 5 % dextran solution, then became progressively less efficient and eventually disappeared as a function of time. More recently in rat kidneys perfused with oxygenated isotonic cell-free electrolyte solution or artificial blood, the characteristic transient overshoots of
KW KW
Reference
Table Table
Table 1 Unpublished data Dicker and Heller (7) Peters (19) Lewy and Windhager (17) Daugherty et al. (5) Daugherty et al. (5) Eisenbach et al. (8) Girndt and Ochwadt (11)
Girndt
(30)
(22) (22) and
Ochwadt
(11)
130 122 109
MW MW MW
7 8 18
Deen et al. (6) Robertson et al. (20) Brenner et al. (4)
114
W
28 15
Grandchamp Ayer et al.
3.5/250 3.9/250
g BWk g BWk
W W
20 16
Eisenbach Steinhausen
3.1/250
g BWk
W
9
Steinhausen
& AP = arterial pressure. b Strain of rat: SD = Sprague-Dawley, c KW = kidney weight. d Simultaneous determinations in the same animals. 0.85 (0.5). g Unanesthetized rats. h Assuming EPAH = 0.85 and Hct k Acute contralateral nephrectomy. 1 RBF = (single-nephron GFR), (1-Hct), assuming Hct = 0.5.
2 1
W
=
et al,
(14)
(1) et al. (8) et al. (27)
Wistar, MW = mutant Wistar with e RBFPAH = CPAH/EPAH (1 Hct). = 0.5. i BW = body weight. j RBF in nl/min (single-nephron filtration fraction)
et al.
(27)
surface f RBF
glomeruli. =
Cdiodrast
= Cp&O.7
(0.58).
(30,000)
(10-G)/
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132
ARENDSHORST,
renal venous flow monitored by a cannulating flowmeter were proportional to step increases in perfusion pressure above 100 mmHg (3, 16). These authors also observed that the myogenic responses could be abolished by hypoxia, papaverine, and dibenamine. In contrast, Ohler et al. (18) were unable to demonstrate autoregulation in perfused rat kidneys; RBF was directly related to perfusion pressures between 50 and 150 mmHg. Despite the conflicting data and lack of direct evidence of autoregulation in vivo, it has generally been inferred from data obtained in the dog that rat renal hemodynamics are also regulated by some intrinsic mechanism(s) to maintain blood flow (and GFR) relatively constant at normal and elevated arterial pressures. Evidence obtained in the rat suggests that appropriate adjustments in preglomerular vascular resistance occur in response to changes in perfusion pressure within the autoregulatory range. Hydrostatic pressures in proximal and distal tubules and peritubular capillaries are unrelated to spontaneous changes of AP (12, 13) and between 90 and 190 mmHg (29). Glomerular capillary pressures (GCP) estimated by Gertz the stop-flow technique were th ought et al. (10) employing to plateau at about 90 mmHg when arterial pressure was greater than 105 mmHg. Although we know now that their estimates are artifactually high, their results might as suggesting autoregulation of GCP. still be interpreted Recently, Robertson et al. (20) observed rather poor autoregulatory efficiency of directly measured GCP and singleglomerular filtration rate (SNGFR) in hydronephron penic mu tant Wistar rats compared with those expanded by 2.5 % body weight with plasma. These authors found significant decreases in GCP and ST\JGFR while glomerular plasma flow was statistically unchanged after AP was reduced from 122 to 100 mmHg in the hydropenic rats. When comparing the nonlinear perfusion pressure/renal blood flow relationships observed in the rat and dog, the characteristic plateau of flow seen at the higher pressures and the concave portion at the lower pressures are generally similar, but the points of inflection differ. It is clear from the normalized pressure/flow graph shown in Fig. 5 that a high degree of autoregulatory eficiency obtains in the rat above an arterial pressure of 105 mmHg; RBF increased only 3 4/o when AP increased 42 YL At a pressure between 95 and 105 mmHg, blood flow began to decline with reductions in AP. This point of inflection is appreci-
FINN,
AND
GUTTSCHALK
ably higher than that usually found in the dog, 70-80 mmHg (15, 21, 24). Th e reason for this apparent species difference is not evident. One possible explana .tion is that RVR in the rat is set at a lower level during basal condi tions a.nd thus reaches a condition of maximal dilatation at a higher AP than in the dog. It is worthy of note that RBF-per gram kidney at normal AP is higher in the rat than that generally reported for the dog, 3.5-4 ml/min . g KW (2 1, 24) . As shown in Fig. 6, RVR was directly and linearly related to AP between 105 and 145 mmHg, but below an AP of 95 mmHg it reached a low value that was almost independent of AP. RVR increased from a minimal resistance when AP was further decreased from 65 to 45, mmHg (Fig. 6). A similar response was observed by Selkurt (23) in the dog at perfusion pressures less than 30 mmHg, The apparent tendency, however, was abolished when he made appropriate corrections for “yield pressure.” The almost instantaneous response time in which the overall intrarenal adjustments in vascular resistance took place after changes in AP in the autoregulatory range (Fig. 3) suggests the involvement of a highly eficient control mechanism(s). Whether these alterations are mediated bvd a primary myogenic response, the participation of a rapidly acting tubulovascular feedback loop at the individual nephron level or some other mechanism(s) awaits further investigation. With the availability of a reliable and accurate noncannulating electromagnetic flow transducer with a small-lumen diameter, it is now possible to monitor blood flow in the rat kidney continuously while employing micropuncture techniques to investigate individual nephron functionThe electrical-mechanical servo-control system for regulation of arterial pressure was designed and constructed by Mr. David Smith, Department of Physiology, University of North Carolina School of Medicine. This study was supported by a grant from the ,\merican Heart Association, by Nationai Institutes of Health Grants HE-02334 and NS 11132, by National Institutes of Health Training Grant 2 TOI A01 AM05054, and by Grant 1973-74-A-36 from the North Carolina Heart Association. W. J. Arendshorst and W. F. Finn are Postdoctoral Fellows on a National Institutes of Health Training Grant. C. MT. Gottschalk is a Career Investigator of the American Heart Association. Received
for publication
15 May
1974.
REFERENCES G., A. GRANDCHAMP, T. WYLER, AND B. TRUNIGER. Intrahemodynamics in glycerol-induced myohemoglobinuric renal failure in the rat. Circulation Res. 29 : 128-135, 1971_ BARRATT, L. J., J. D. WALLIN, F. C. RECTOR, JR., AND D. W. SELDIN. Influence of volume expansion on single-nephron filtration rate and plasma flow in the rat. *Am. J. Physiol. 224: 643650, 1973. BASAR, E., H. TISCHNER, AN? C. WEISS. Untersuchungen zur Dynamik druckinduzierter Anderungen des Strbmungswiderstandes der autoregulierenden, isolierten Rattenniere. P’uegers Arch. 299: 191-213, 1968. BRENNER, B. M-, J. L. TROY, T. hl. DAUGHARTY, W. M. DEEN, AND C. R. ROBERTSON. Dynamics of glomerular ultrafiltration in the rat. II. Plasma-flow dependence of GFR. Am. J. Physiol. 223: 1184-l 190, 1972. DAUGX-ZARTY, T. AI., I. F. UEKI, D. P. NICF-IOLAS, AND B. M.
1. AYER,
renal acute
2.
3.
4.
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6.
7.
8.
9.
BRENNER. Comparative renal effects of isoncotic and colloidfree volume expansion in the rat. Am. J. PhysioZ. 222: 225-235, 1972. DEEN, W. hl., J. L. TROY, C. K. ROBERTSON, AND B. M. BRENNER. Dynamics of glomerular ultranltration in the rat. IV. Determination of the ultrafiltration coefficient. J. Clin. best. 52 : 15001508, 1973. DICKER, S. E., AND H. HELLER. The renal action of posterior pituitary extract and its fractions as analyzed by clearance experiments on rats. J. physiol., London 104 : 353-360, 1946. EISENBACH, G. M., B. KITZLINGER, AND M. STEINHATJSEN. Renal blood flow after temporary ischemia of rat kidneys. Pfluegers Arch. 347: 223-234, 1974-b FORSTER R. P., AND J. P. MAES. Effect of experimental neurogenie hypertension on renal blood flow and glomerular filtration rates in intact denervated kidneys of unanesthetized rabbits
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AUTOREGULATION with
adrenal
OF glands
RENAL
demedullated.
BLOOD
133
FLOW
Am. J. Physiol.
150:
534-540,
1947. 10.
GERTZ, Pressure relation
K. H., J. A. MANGOS, G. BROWN, AND H. D. PAGEL. in the glomerular capillaries of the rat kidney and its to arterial blood pressure. Arch. Ges. Physiol. 288: 369-
21.
374, 1966. 11.
GIRNDT, J., AND B. OCHWADT. Durchblutung des Nierenmarkes, Gesamtnierendurchblutung and cortico-medullgre Gradienten beim experimentellen renalen Hochdruck der Ratte. P’uegers Arch. 313: 30-42, 1969. 12. GOTTSCHALK, C. W., AND M. MYLLE. Micropuncture study of pressures in proximal tubules and peritubular capillaries of the rat kidney and their relation to ureteral and renal venous pressures. Am. J. Physiol. 185 : 430-439, 1956. 13. GOTTSCHALK, C. W., AND M. MYLLE. Micropuncture study of pressures in proximal and distal tubules and peritubular capillaries of the rat kidney during osmotic diuresis. Am. J. Physiol.
189 : 323-328, 14.
15. 16.
17.
18,
19,
20.
22. 23.
24.
25.
1957.
GRANDCHAMP, A*, G. AYER, J. R. SCHERRER, AND B. TRUNIGER. Intrarenal hemodynamics of the rat kidney determined by the xenon washout technique. Nephron 8: 33-45, 197 1. JOHNSON, P. C. (editor). Autoregulation of renal blood flow. Circulation Res. 15, Suppl. 1 : 103-198, 1964. LEICHTWEISS, H. P., H. SCHRODER, AND C. WEISS. Die Beziehung zwischen Perfusionsdruck und Per&isionsstromstarke an der mit Paraffin61 perfundierten isolierten Rattenniere. Arch. Ges. Physiol. 293 : 303-309, 1967 LEWY, J. E., AND E. E. WIN~DHAGER. Peritubular control of proximal tubular fluid reabsorption in the rat kidney. Am. J. Physiol. 2 14 : 943-954, 1968. OHLER, W., 0. HARTH, AND W. KREIENBERG. Die Abhgngigkeit der Nierendurchblutung vom arteriellen Blutdruck bei der Ratte. Arch. Ges. Physiol. 269 : 274-281, 1959. PETERS, G. GlomerulZre Clearancen, j+AminohippursZureClearance und Diurese bei verschiedenen Diureseformen der nicht narkolisierten Ratte. Arch. Exptl. Pathol. Pharmakol. 235: 113-142, 1959. ROBERTSON, C. R., W. M. DEEN, J- L. TROY, AND B. M. BRENNER.
26.
27.
28. 29.
Dynamics of glomerular uhrafiltration in the rat. III. Hemodynamics and autoregulation. Am. J. Physiol. 223: 1191-1200, 1972. ROTHE, C. F,, F. D. NASH, AND D. E. THOMPSON. Patterns in autoregulation of renal blood flow in the dog. Am. J. Physiol. 220: 1621-1626, 1971. SAPIRSTEIN, L. A. Fractionation of the cardiac output of rats with isotopic potassium. Circtilation Res. 4: 689-692, 1956. SELKURT, E. E. The relation of renal blood flow to effective arterial pressure in the intact kidney of the dog. Am. .I. Physiol. 147 : 537-549, 1946. SELKURT, E. E. The renal circulation. In: Handbook uf Physiology. Circulation. Washington, D. C. : Am. Physiol. Sot., 1963, sect. 2, vol. IT, chapt. 43, p. 1457-1516. SELKURT, E. E., P. W. HALL, AND M. P. SPENCER. Influence of graded arterial decrement on renal clearance of creatinine, p-aminohippurate and sodium. Am. J. Physiol. 159 : 369-378, 1949. SHIPLEY, R. E., AND R. S. STUDY. Changes in renal blood flow, extraction of inulin, glomerular filtration rate, tissue pressure, and urine flow with acute alterations of renal arterial blood pressure. Am. J. PhysioZ. 167 : 676-688, 1951. STETNHAUSEN, M., G. M. EISENBACH, AND W. B~TTCHER. Highfrequency microcinematographic measurements of peritubular blood flow under control conditions and after temporary ischemia of rat kidneys. PJuepers Arch. 339: 273-288, 1973. THURAU, K. Renal hemodynamics. Am. J. Med. 36 : 698-719, 1964. THURAU, K., AND E. WOBER. Zur Lokalisation der autoregulativen Widerstandsgnderungen in der Niere. Arch. Ges. Physiol.
274 : 553-566, 1962. 30. WALLTN, J, D., R. C. BLANTZ, M. A. KATZ, V. E. ANDREUCCT, F. C. RECTOR, JR., AND D. W. SELDIN. Effect of saline diuresis on intrarenal blood flow in the rat. Am. J, Physiol. 221: 129731.
1304, 1971. WEISS, C., H. PASSOW, AND A. ROTHSTEIN. flow in isolated rat kidney in the absence Physiol. 196: 1115-l 118, 1959.
Autoregulation of red cells.
of
Am.
J.
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JOURNAL
OF PHYSIOLOGY
Vol. 228, No. f,January
1975.
Autoregulation WILLIAM
Printed
in U.S.A.
of blood Je ARENDSHORST,
flow in the rat kidney
WILLIAM
F. FINN,
AND
(With the Technical Assistance of H. K. Lucas) Departments of Medicine and Physiology, University of North Carolina Chapel Hill, North Carolina 27514
ARENDSHORST,
WILLIAM J., WILLIAM F. FINN, AND CARL W, Autoregulation of blood flow in the rat kidney. Am. J. Physiol. 228( 1) : 127-l 33. 1975.-We present evidence showing that renal blood flow (RBF) in the anesthetized nondiuretic rat can be measured reliably and accurately using a noncannuIating flow transducer and an electromagnetic flowmeter. In vitro calibration yielded a linear relationship (r = 0.998) between flowmeter output voltage and blood flow rates from 0.2 to 10.3 ml/min. Excellent agreement was observed between simultaneous determinations of RBF by the flowmeter and the PAH clearance technique. Glomerular filtration rate and RBF for a kidney with a flow transducer around its renal artery did not differ significantly from correspondink values for the undisturbed contralateral kidney. The relationship of mean RBF with steady-state variations in perfusion pressure was evaluated in 13 nondiuretic rats. RBF averaged 6 ml/min +g kidney wt at GOTTSCHALK.
arterial
pressures (AP) above 100 mmHg.
A high degree of auto-
regulatory eff%iency was observed when mean AP varied between 105 and 145 mmHg. Qver this pressure range RBF changed only 3 y0 as changes in intrarenal vascular resistance and AP were directly related (r = 0.994). Below 95-105 mmHg RBF decreased in a curvilinear fashion with the concavity toward the pressure
axis. renal blood flow; renal vascular resistance; flow-pressure relationship; electromagnetic PAH clearance
arterial pressure; flow transducer;
IS AN ABUNDANCE of evidence indicating that the mammalian kidney possesses the intrinsic ability to regulate its vascular resistance as perfusion pressure varies from about 80 to 200 mmHg (15, 24, 28). Consequently renal blood flow (RBF) and glomerular filtration rate (GFR) are maintained relatively constant over this pressure range importance in (9, 25, 2% a response of physiological maintenance of salt and water balance. In most in vivo studies of this intriguing phenomenon of autoregulation, the dog has been the laboratory animal of choice. This has been the case largely because of the availability of noncannulating flow transducers with large-diameter lumens which are capable of measuring accurately static and transient variations in RBF. The relatively long time period required to determine RBF by clearance methodology obviously renders it incapable of accurate detection of transient changes in blood flow. The unavailability of noncannulating flow transducers with the desired sensitivity and stability and which also have lumens small enough to fit snugly around the rat renal artery has reTHERE
CARL
W. GOTTSCHALK
School of Medicine,
stricted the use of the rat in such studies. Data obtained from this species are limited and conflicting. Our observations demonstrate for the first time that blood flow in the renal artery of an anesthetized nondiuretic rat can be measured accurately using a small noncannulating electromagnetic flow transducer without significant compromise in GFR and RBF for that kidney. From determinations of the relationship between mean RBF and arterial pressure, we show that the rat kidney prepared for micropuncture has the ability to autoregulate blood flow efficiently as a consequence of enhanced intrarenal vascular resistance when mean perfusion pressure varies from 105 to 145 mmHg. Below 95-105 mmHg RBF decreased in a curvilinear fashion with the concavity toward the pressure axis. METHODS
Observations are reported on a total of 17 male SpragueDawley rats, weighing 250 =t 34 (SD) g, which were deprived of food but allowed free access to water overnight prior to study. The rats were anesthetized by intraperitoneal injection of sodium pentobarbital, 50 mg/kg body wt, placed on a heating table that maintained body temperature between 37 and 38 C, and the left kidney was exposed incision for micropuncture as through an abdominal previously described (12) Saline (0.85 % NaCI) was infused continuously into a jugular vein at a rate of 40 pl/min. Both ureters were catheterized with PE-10 polyethylene tubing. Femoral arterial pressure (AP) was monitored with a Statham P23 Db pressure transducer connected to a Beckman Dynograph. Blood flow in the left renal artery1 was measured continuously by a small-diameter flow transducer (EP model, 40 1.5, 1.5 mm circum. lumen size) connected to a squarewave electromagnetic flowmeter (model 50 1, Carolina Medical Electronics, Inc.) and a Beckman Dynograph. The flowmeter system was calibrated in vitro and in vivo periodically throughout the study. In vitro calibration consisted of constant infusions of heparinized rat blood at known rates through a cannulated excised segment of a rat carotid artery placed in a container of saline. In addition, the central end of a carotid artery was cannulated 1 The probe was positioned around the renal artery proximal to any discernable branching as close as possible to its origin from the aorta in order to assure measurement of total flow. Transient occlusion of the renal artery distal to the probe always resulted in uniform bJanching of the kidney surface in acceptable preparations.
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128 in anesthetized rats and the outflow was timed; heparinized rat blood was also infused through the carotid. For the in vivo calibration, the flow transducer was placed around the left renal artery in six anesthetized rats whose left kidney was prepared for micropuncture, and the measurements by the flow probe (RBFdi&, hereafter termed “direct” in contrast with data obtained by clearance methodology, were compared to values determined by the PAH clearance technique [RBFpAH = CPAH/EPdH zero flow through the renal artery (1 - Hct)]. Absolute was determined on several occasions during an experiment by completely occluding the artery distal to the probe with a forceps for 2-3 s; base-line drift was negligible. Positioning of the noncannulating flow probe around the renal artery was judged acceptable and presumably optimal when there was a snug fit, pulsatile flow was recorded free of mechanical noise, intravenously injected dye (0.04 ml of 5 % buffered F, D & C no. 3 dye) had a uniform distribution in superficial cortical vessels and nephrons, and the kidney continued to form urine. In several instances animals had to be discarded because the artery was too close to the vein to be certain that the probe did not compress the renal vein and significantly restrict venous outflow. The relationship of renal blood flow to arterial pressure was studied over a pressure range from 40 to 150 mmHg in 13 rats. To provide elevated arterial pressure, the carotid arteries were occluded in most animals. Progressive decrein approximately 10 mmHg ments in arterial pressure stages were achieved and maintained at a stable level (& 3 mmHg) by employing an adjustable constrictor clamp around the abdominal aorta above the renal arteries. The degree of aortic constriction by mechanical compression was controlled by movement of a piston toward the baseplate of a modified aortic clamp connected to an electronic servo system designed to maintain femoral arterial pressure at a present level (D. Smith, unpublished data). To evaluate the possible influence of mechanical trauma due to having the flow probe in position, function of that kidney was compared to that of the undisturbed right Priming solutions of inulin-3H, 10 kidney in six animals. &X/100 g body wt, and PAH-14C, 2 &X/ 100 g body wt, were followed by an intravenous infusion of 0.85 % NaCl at 40 &min so that inulin and PAH were administered at 20 and 3 pCi/lOO g body wt per hour, respectively. Clearance periods of 20-30 min duration commenced 1 h after priming. Blood samples were taken periodically from the tip of the tail and the left renal vein. Samples of urine and plasma were discharged into counting vials containing 1 ml water and 10 ml PCS Solubilizer (Amersham/Searle), and radioactivity was measured in a threechannel liquid scintillation spectrometer. Intrarenal vascular resistance (RVR) was calculated from the arterial-venous pressure difference and blood flow per gram kidney wet weight (KW). Renal venous pressure was assumed to be constant and assigned a value of 5 mmHg in the calculations. Paired and unpaired i tests and linear regression by the least-squares method were performed for analysis of significance. A P value greater than 0.05 was considered to be not statistically significant (NS).
ARENDSHORST,
BLOOD
FINN,
AND
GOTTSCHALK
FLOW (ml/min)
1. In vitro calibration curve. Relationship between output voltage (mV) from electromagnetic flowmeter and constant rates of flow of rat blood with a mean hematocrit of 49(%. Means & I SD. Best-fit regression line : y = 54,7x + 10.7, r = 0.998. FIG.
RESULTS
Calibration of nancannulating flow trunsducer and squaraz.vaue electromagnetic jlowmeter. The flow transducers were calibrated several times in vitro and in situ by perfusing rat blood with an average hematocrit of 49 % at constant rates through carotid arteries as described in METHODS. The calibration curves obtained using these two methods were almost identical and have thus been pooled. Shown in Fig. 1 is the calibration curve for a flow transducer. The data points deviated very little from the calculated best-fit regression line (y = 54.7x + 11, r = 0.998) as the mean flow rates varied from 0.2 to lOa3 ml/min. Clearly the output voltage from the flowmeter system is proportional to flow rate and is sensitive enough to measure the relatively low rates of blood flow in the kidney of a nondiuretic rat. The results from the above means of calibration were also verified in vivo in the physiological range of flow. In Fig. 2 renal blood flow monitored by the flow probe is compared to values determined simul(RBFdircct) taneously by the PAH clearance method (RBFPAH) in six nondiuretic animals whose left kidney was prepared for micropuncture in the usual fashion in our laboratory. After the initial observations in one animal, AP was lowered by aortic compression to reduce RBF for two clearance periods. Excellent agreement (r = 0.997) was found between the two methods in each animal. Mean RBF ranged from 2.5 to 7.4 ml/min-g KW, and the ratio of RBFdir,,J different RBFPAH, 1.01 =t 0.02 (SD), is not statistically from unity. Glomerular filtration rate was also determined by the clearance of inulin in these animals in order to evaluate the possible influence of trauma resulting from clearing of the renal artery and having the flow transducer in position for up to 5 h. The undisturbed right kidney served as the control. The data in Table 1 indicate that GFR and RBF for the experimental and control kidneys were not significantly different. These results show that when properly applied, the Aow transducer does not significantly impede RBF or otherwise disturb renal function as reflected by alterations in GFR. Although the paired difference of urine flow rates for the two kidneys was only
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AUTOREGULATION
OF
RENAL
BLOOD
129
FLOW
of borderline statistical significance (0.05 < P < 0. lo), urine flow from the left was about twofold greater than that from the right in three of the six experiments, indicating that these kidneys were probably at least partially denervated. Relationship of renal blood flow to steady-state changes in perfusion pressure. To characterize the relationship of mean renal blood flow to perfusion pressure, mean arterial pressure was varied from 40 to 150 mmHg in 13 nondiuretic rats. Elevated AP was provided by occlusion of one or both carotid arteries in most animals. Following carotid occlusion, AP increased to variable extents (O-30 mmHg) without any consistent changes in RBF. Each different pressure level was maintained stable for at least 10 s. Holding AP stable at a given level for up to 10 min did not reveal further changes in RBF. AP was decreased from the highest to the lowest and usually allowed to return rapidly to the original level; in several instances when AP was increased in progressive stages from the lowest pressure, a slight but consistent hysteresis was observed. Nevertheless, the shapes of the autoregulation curves were similar. The responses of mean RBF to step decrements of about 10 mmHg in perfusion pressure in one experiment are
shown in Fig. 3. During the steady-state reductions in AP from 133 to 99 mmHg, RBF was practically unchanged ; however, after AP was dropped below 90 mmHg, obvious decreases in blood flow were evident. Note that the intrarenal circulatory adjustments occurred rapidly so that RBF reached a new steady-state level in less than 3 s. The capability of the flowmeter system to respond to ‘pulsatile flow is shown in Fig. 4. During each cardiac cycle there are large fluctuations in renal blood flow synchronous with arterial pressure changes. It seems probable that these transient variations occurred too rapidly to elicit an autoregulatory response. For example, at a systolic arterial pressure of 155 mmHg, RBF was 7.84 ml/min .g KW; blood flow was 4.89 at a diastolic pressure of 110 mmHg. In this particular case mean RBF and AP were 6.17 ml/ min. g KW and 130 mmHg. At least five separate autoregulation curves were obtained in each of the 13 rats studied. The summarized results for RBF and RVR at each pressure level are presented in Table 2. Renal biood flow in these anesthetized nondiuretic rats was approximately 6 ml/min .g KW when AP was above 100 mmHg. In order to compare results in different animals, RBF and RVR were normalized to their respective values at the pressure of 114 mmHg for each experiment. The percent changes of RBF to variations in TABLE 1. Clearance data from six nondiuretic rats comparing GFR and v for the left kidney with a noncannulating J~OW transducer around its renal artery and undisturbed right kidney Left Kidney Right Kidney V, p1jmin.g
GFR,
ml/min.g
RBFr*n,
v I
I
I
I
I
I
2
3
I
4
5
6
,
7
8
RBF,,,
(ml/mln~gKW)
FID. 2. In vivo verification of in vitro calibration. Comparison of renal blood flow measured simultaneously by a noncannulating flow transducer (RBFdi,,,t) with that by PAH clearance technique (RBF,,). Individual paired determinations in 6 nondiuretic rats show very little deviation from indicated line of identity. Best-fit regression line: y = 1.007x + 0.030, r = 0.997.
KW
RBFdirect,
ml/min.g
ml/min.g
RBFdi.eet/RBFrAn, Values averaged taneously technique
4.07 f2.76
KW
1.08
KW
KW
ratio
2.70 fl.66
1.09
f0.24
f0.18
6.10 fl.03
5.91 fl.13
P
0.05-0.10
NS NS
6.15 Yk1.07 1.01 f0.02
are means f 1 SD. V = urine flow rate. Body weight 245 g. Comparison of renal blood flow measured simulusing a flow probe (RBF d,reot) and by the PAH clearance (RBFr&. FID. 3. A representative autoregulation curve. Tracing showing response of renal blood flow monitored by an electromagnetic flow transducer to step decreases in femoral arterial pressure achieved and maintained by compression of abdominal aorta above renal arteries using an electrical-mechanical servo-control system. Note characteristic transient overshoot in flow after relief of temporary occlusion of renal artery distal to flow probe (mechanical zero).
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130
ARENDSHORST,
perfusion pressure are graphed in Fig. 5. Mean RBF was quite stable when mean perfusion pressure ranged from 105 to 145 mmHg (r = 0.08x + 91.2, r = 0.844), increasing only 3 % in response to an overall pressure increase of 40 mmHg. Below 105 mmHg RBF decreased progressively from 95 % of control flow at 95 mmHg to 44 % at 45 mmHg. In Fig. 6 are shown the percent changes of RVR to alterations in arterial pressure. When AP was decreased from 145 to 105 mmHg, there were proportional reductions of RVR from 127 to 92 % of the resistance at 114 mmHg (r = 0.83x + 4.91, r = 0.994). Below 95 mmHg RVR gradually decreased to the lowest percent at 65 mmHg. At the two lowest pressures studied, there was a tendency for RVR to increase again ; RVR increased significantly from 65 to 45 mmHg (P < 0.05). Although this response was not evident when considering the mean absolute values of RVR for all animals (Table 2), paired analysis of the data from the nine experiments in which RBF was measured at 45 mmHg revealed that RVR expressed either in absolute terms or as percent change. did increase significantly (P < 0.05) at each of the two pressure levels below 65 mmHg. Thus, it is clear that RBF in the rat under these experimental conditions is autoregulated eihciently above a mean AP of 105 mmHg as RVR increased in proportion to changes in AP. Below 95-105 mmHg RBF decreased in a curvilinear fashion with the concavity toward the pressure axis.
FINN,
AND
GOTTSCHALK
able of quantitating blood flow in a renal artery of the rat without detectably altering GFR and blood flow in the kidney under investigation. It is probable, however, that in some experiments an unpredictable consequence of the preparation is at least partial renal denervation. In addition, we have demonstrated excellent agreement between RBF measured by the electromagnetic flowmeter and by the PAH clearance technique (Table 1, Fig. 2). Renal blood flow in these 17 anesthetized nondiuretic rats averaged 6 ml/min *g KW at arterial pressures between 100 and 150 mmHg. This mean flow is compared in Table 3 with values published by other investigators employing a variety of techniques. It is readily apparent that some of the flow rates per kidney are comparable to ours and others are appreciably different. In general, values reported from studies using PAH clearances, excluding those of Eisenbach et al. (8) and Girndt and Ochwadt (1 l), or radioactive microspheres (30) agree well with that found in the present study. The antiglomerular basement membrane antibody technique (2, 30) has yielded mean flow rates slightly higher than ours, while RBF in the preparations emplo ying the methods of *6Rb uptake (1 l), 13SXe washout (1, 14), high-frequency microcinematography 110 r
+
100 ----_______---1 ____-____--
T I
f
DISCUSSION
Our observations show magnetic flow transducer,
that a noncannulating when properly applied,
electrois cap-
45
55
65
75 ARTERIAL
FID. 4. A typical tracing showing pressure and renal blood flow.
pulsatile
and
mean
105
115
125
135
145
( m m Hg I
FIG. 5. Autoregulation of renal blood flow during graded constriction of aorta above renal arteries. Relationship between percent change in renal blood flow normalized to mean value at 114 mmHg for each experiment and arterial pressure. Means f 1 SE of 13 experiments. Best-fit regression line for flow between 105 and 145 mmHg: y = 0.08x + 91.16, r = 0.844. Data points below 105 mmHg are joined by a curve drawn freehand.
arterial
Pressure
mm
Hg
RBF , ml/min
* g KW
RVR, mmHg/ml. min*g KW No.
95
PRESSURE
2. Effect of variations in AP on RBF and RVR in the rat
TABLE
AP,
85
of animals Values
are means
f
Range
40-50
SO-60
60-70
70-80
So-90
90-100
100-110
110-120
120-130
130-140
140-150
45.3 f3.2
54.5 f2.0
64.6 f0.9
75.2 fl.O
84.9 zkl.1
94.5 fl.2
104.7 Al.3
114.0 f1.4
123.1 4x2.9
132.7 f1.5
145.4 f2.2
2.82 f0.69
3.39 zkO.65
4.29 ho.82
4.76 f1.05
5.24 fl.05
5.66 Ylzo.91
5.90 rko.93
5.95 f0.82
6.05 f0.88
6.13 f0.80
6.14 f0.86
14.9 f2.9
15.1 f2.9
14.4 f2.9
15.5 f3.6
15.9 Yk3.7
16.2 zk2.8
17.3 f3.1
18.8 +3.1
20.1 h3.2
21.2 52.9
23.2 rt3.2
9
12
12
13
13
13
13
13
13
12
5
1 SD.
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AUTOREGULATION
OF
R ENAL
BLOOD
131
FLOW
I 7" c L
I
,
45
55
65
c,
/
/ I
75
6. Intrarenal
FIG.
autoregulation renal vascular for each periments. mmHg:
vascular
of renal resistance
experiment Best-fit y = 0.83x
TABLE
3.
d$erent ~~-
1
I
85
ARTERIAL
( mm
resistance
blood flow. normalized
105
95
PRESSURE
I
I
I
I
115
125
135
145
Hq)
changes
Correlation to mean
associated of percent
vahe
and arterial pressure. Means =t 1 SE of 13 exregression line for resistance between 105 and 145 + 4.91, r = 0.994.
of meanuahes
&npurison
techniques in nondiuretic -.--.-
for tutu/ blood j?ow
flow
RBF, ml/min transducer
6.0/g
clearance”
6.2/g
5.4/g 6.5/g 6.4/250 6.2/250 5*5 6.6 3.9/250 8.5/g Microsphere
86Rb
Renal
membrane
uptake
%% a
KW” KWd
114 112
SD SD
13 6
KWd KW
112 112
SD W
SD SD SD W W
6 6 28 124 7 9 8 10 10
SD
11
Wallin
et al.
SD SD
14 6 6
Barratt Wallin Wallin
et al. (2) et al. (30) et al. (30)
17 23
Sapirstein Sapirstein
KW’~ g g BWgr g BW
h, i 112 120
g BWj*
k
KW
3.6/g of superficial
washout
venous
Eligh-frequency
6.8/g 7.5 6.3
KW 127
4.4/250 7.1/250
uptake
Micropuncture
133Xe
basement
nephrons’
outflow
microcinematographic
SD
g BW g BW KW
Of
W
4.8 4.8 3.9 4.2/g 3,4/g
No. of Animals
Mean AP, mmHg”
5.8
Antiglomerular antibody
42K
in one kidney in uiuo by
~.
6.1/g PAH
(RBF)
rats
Technique
Electromagnetic
at
with
change in 114 mmHg
(27), and renal venous outflow (8, 27) is distinctly lower. However, for a more critical analysis of the reproducibility and accuracy of the different techniques, the measurements of blood flow obviously should be performed simultaneously, as we have done using the square-wave electromagnetic Aowmeter and PAH clearance methods. Our observations also demonstrate the presence of autoregulation of blood flow in in situ kidneys of anesthetized to the present study several nondiuretic rats. Previous groups of investigators have examined the integrity of the autoregulatory phenomenon in isolated-perfused rat kidneys (3, 16, 18, 31). W eiss et al. (31) found that the autoregulation of RBF occurred when perfusion pressures were elevated above 100 mmHg 30-60 min after initiation of perfusion with a 5 % dextran solution, then became progressively less efficient and eventually disappeared as a function of time. More recently in rat kidneys perfused with oxygenated isotonic cell-free electrolyte solution or artificial blood, the characteristic transient overshoots of
KW KW
Reference
Table Table
Table 1 Unpublished data Dicker and Heller (7) Peters (19) Lewy and Windhager (17) Daugherty et al. (5) Daugherty et al. (5) Eisenbach et al. (8) Girndt and Ochwadt (11)
Girndt
(30)
(22) (22) and
Ochwadt
(11)
130 122 109
MW MW MW
7 8 18
Deen et al. (6) Robertson et al. (20) Brenner et al. (4)
114
W
28 15
Grandchamp Ayer et al.
3.5/250 3.9/250
g BWk g BWk
W W
20 16
Eisenbach Steinhausen
3.1/250
g BWk
W
9
Steinhausen
& AP = arterial pressure. b Strain of rat: SD = Sprague-Dawley, c KW = kidney weight. d Simultaneous determinations in the same animals. 0.85 (0.5). g Unanesthetized rats. h Assuming EPAH = 0.85 and Hct k Acute contralateral nephrectomy. 1 RBF = (single-nephron GFR), (1-Hct), assuming Hct = 0.5.
2 1
W
=
et al,
(14)
(1) et al. (8) et al. (27)
Wistar, MW = mutant Wistar with e RBFPAH = CPAH/EPAH (1 Hct). = 0.5. i BW = body weight. j RBF in nl/min (single-nephron filtration fraction)
et al.
(27)
surface f RBF
glomeruli. =
Cdiodrast
= Cp&O.7
(0.58).
(30,000)
(10-G)/
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132
ARENDSHORST,
renal venous flow monitored by a cannulating flowmeter were proportional to step increases in perfusion pressure above 100 mmHg (3, 16). These authors also observed that the myogenic responses could be abolished by hypoxia, papaverine, and dibenamine. In contrast, Ohler et al. (18) were unable to demonstrate autoregulation in perfused rat kidneys; RBF was directly related to perfusion pressures between 50 and 150 mmHg. Despite the conflicting data and lack of direct evidence of autoregulation in vivo, it has generally been inferred from data obtained in the dog that rat renal hemodynamics are also regulated by some intrinsic mechanism(s) to maintain blood flow (and GFR) relatively constant at normal and elevated arterial pressures. Evidence obtained in the rat suggests that appropriate adjustments in preglomerular vascular resistance occur in response to changes in perfusion pressure within the autoregulatory range. Hydrostatic pressures in proximal and distal tubules and peritubular capillaries are unrelated to spontaneous changes of AP (12, 13) and between 90 and 190 mmHg (29). Glomerular capillary pressures (GCP) estimated by Gertz the stop-flow technique were th ought et al. (10) employing to plateau at about 90 mmHg when arterial pressure was greater than 105 mmHg. Although we know now that their estimates are artifactually high, their results might as suggesting autoregulation of GCP. still be interpreted Recently, Robertson et al. (20) observed rather poor autoregulatory efficiency of directly measured GCP and singleglomerular filtration rate (SNGFR) in hydronephron penic mu tant Wistar rats compared with those expanded by 2.5 % body weight with plasma. These authors found significant decreases in GCP and ST\JGFR while glomerular plasma flow was statistically unchanged after AP was reduced from 122 to 100 mmHg in the hydropenic rats. When comparing the nonlinear perfusion pressure/renal blood flow relationships observed in the rat and dog, the characteristic plateau of flow seen at the higher pressures and the concave portion at the lower pressures are generally similar, but the points of inflection differ. It is clear from the normalized pressure/flow graph shown in Fig. 5 that a high degree of autoregulatory eficiency obtains in the rat above an arterial pressure of 105 mmHg; RBF increased only 3 4/o when AP increased 42 YL At a pressure between 95 and 105 mmHg, blood flow began to decline with reductions in AP. This point of inflection is appreci-
FINN,
AND
GUTTSCHALK
ably higher than that usually found in the dog, 70-80 mmHg (15, 21, 24). Th e reason for this apparent species difference is not evident. One possible explana .tion is that RVR in the rat is set at a lower level during basal condi tions a.nd thus reaches a condition of maximal dilatation at a higher AP than in the dog. It is worthy of note that RBF-per gram kidney at normal AP is higher in the rat than that generally reported for the dog, 3.5-4 ml/min . g KW (2 1, 24) . As shown in Fig. 6, RVR was directly and linearly related to AP between 105 and 145 mmHg, but below an AP of 95 mmHg it reached a low value that was almost independent of AP. RVR increased from a minimal resistance when AP was further decreased from 65 to 45, mmHg (Fig. 6). A similar response was observed by Selkurt (23) in the dog at perfusion pressures less than 30 mmHg, The apparent tendency, however, was abolished when he made appropriate corrections for “yield pressure.” The almost instantaneous response time in which the overall intrarenal adjustments in vascular resistance took place after changes in AP in the autoregulatory range (Fig. 3) suggests the involvement of a highly eficient control mechanism(s). Whether these alterations are mediated bvd a primary myogenic response, the participation of a rapidly acting tubulovascular feedback loop at the individual nephron level or some other mechanism(s) awaits further investigation. With the availability of a reliable and accurate noncannulating electromagnetic flow transducer with a small-lumen diameter, it is now possible to monitor blood flow in the rat kidney continuously while employing micropuncture techniques to investigate individual nephron functionThe electrical-mechanical servo-control system for regulation of arterial pressure was designed and constructed by Mr. David Smith, Department of Physiology, University of North Carolina School of Medicine. This study was supported by a grant from the ,\merican Heart Association, by Nationai Institutes of Health Grants HE-02334 and NS 11132, by National Institutes of Health Training Grant 2 TOI A01 AM05054, and by Grant 1973-74-A-36 from the North Carolina Heart Association. W. J. Arendshorst and W. F. Finn are Postdoctoral Fellows on a National Institutes of Health Training Grant. C. MT. Gottschalk is a Career Investigator of the American Heart Association. Received
for publication
15 May
1974.
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AUTOREGULATION with
adrenal
OF glands
RENAL
demedullated.
BLOOD
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Autoregulation of red cells.
of
Am.
J.
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