Autoregulatory responses of superficial nephrons and their association with sodium excretion during arterial pressure alterations in the dog.

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Autoregulatory Responses of Superficial Nephrons and Their Association with Sodium Excretion during Arterial Pressure Alterations in the Dog L. GABRIEL NAVAR, P. DARWIN BELL, AND THOMAS J. BURKE

SUMMARY Micropuncture experiments were conducted to assess autoregulatory behavior of superficial nephrons and to evaluate the possible contribution of alterations in tubular and peritubular capillary pressures to the mechanism responsible for the arterial pressure effects on urine flow and sodium excretion. In response to decreases in renal arterial pressure (RAP), autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR) was highly efficient to blood pressures as low as 80 mm Hg. Proximal tubule pressure (PTP), peritubular capillary pressure (PCP), distal tubule pressure, and single nephron GFR (SNGFR) remained within 5% of control values with reductions in RAP to 90 mm Hg. Further decreases in RAP to the lower autoregulatory range caused significant decreases in PTP and PCP; the slight decreases in SNGFR were not significant. Within the autoregulatory range, GFR and filtered sodium load were not altered, and urine flow and sodium excretion responses were due to changes in net fractional sodium and water reabsorption. No quantitative relationships could be established between the magnitude of the changes in urine flow or sodium excretion and PTP, PCP, or proximal tubule fractional reabsorption. These experiments demonstrate that superficial nephrons autoregulate in association with the total nephron population except for a proportionately greater reduction when RAP is reduced to the lower autoregulatory range. Furthermore, the urinary responses to reduced arterial pressure occur even in the absence of quantitatively associated alterations in proximal tubule function or PCP.

THE EFFECTS of changes in arterial pressure on renal hemodynamics and on urine flow and sodium excretion have been of interest to many investigators. At the whole kidney level, the phenomenon of renal autoregulation has been explored by evaluating the responses to changes in renal perfusion pressure and it has been demonstrated that both renal blood flow (RBF) and glomerular filtration rate (GFR) can exhibit a remarkable stability over a wide range of perfusion pressure.1"1 In spite of this autoregulatory efficiency, many studies have demonstrated consistent relationships between arterial pressure and both urine flow and sodium excretion rate.1"9 The existence of these relationships has provided the basis for the suggestion that there exists a physiologically significant interaction between the mechanisms that control water and electrolyte excretion and those responsible for the control of arterial blood pressure;10"14 however, the specific intrarenal mechanisms have not been delineated clearly. A number of micropuncture studies have focused on the evaluation of the responses of individual superficial nephrons to changes in arterial pressure. Many of these studies have evaluated primarily the alterations in tubular pressures and function that may be associated with the arterial From the Departments of Physiology and Biophysics and of Medicine (Division of Nephrology), University of Alabama in Birmingham Medical Center, Birmingham, Alabama, and Department of Physiology and Biophysics, University of Colorado Medical Center, Denver, Colorado. Supported by grants from the National Heart and Lung Institute (HL 18426, 7K04 HL00143), from the National Institute of Arthritis and Metabolic Disease (AM 17646), and from the American, Alabama, and Colorado Heart Associations. Address for reprints: Dr. Luis Gabriel Navar, Department of Physiology and Biophysics, University of Alabama Medical Center, University Station, Birmingham, Alabama 35294. Original manuscript received March 29, 1976; accepted for publication February 16, 1977.

pressure-induced changes in sodium excretion and urine flow.15"22 These latter observations are of interest in that they have explored one possible means to explain the intrarenal mediation of the observed changes in urine output. Specifically, it has been suggested that the changes in sodium excretion and urine flow associated with changes in arterial pressure are mediated primarily by alterations in peritubular factors and associated changes in proximal tubule pressure and reabsorption.15"20 However, in many of these studies, clear evidence of autoregulatory capability has not been present. This makes it difficult to determine how the coexisting presence of this phenomenon would interact with the observed changes in intrarenal function. In fact, agreement has not been reached concerning the degree of autoregulatory ability of the superficial nephrons with respect to the total nephron population. For example, the pressure responses in the tubular and peritubular capillary structures to changes in arterial pressure have been variable, with some studies showing good autoregulatory behavior of proximal tubule pressure and peritubular capillary pressure,23"26 whereas other studies have suggested that these values can be altered substantially in response to blood pressure even in the absence of significant changes in renal blood flow or GFR.15"20 With respect to distal tubule pressure, there is little information even as to its normal value in the dog, although autoregulation of distal tubule pressure in the rat has been reported.26 The responses of single nephron GFR (SNGFR) of the outer cortical nephrons to changes in renal arterial pressure are even more controversial, since interpretation of SNGFR data is complicated by the current controversy regarding the role of the distal tubular feedback media-

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CIRCULATION RESEARCH

nism in mediating the autoregulatory responses.25'27 This problem notwithstanding, some investigators have reported highly efficient autoregulation of SNGFR in response to decreases in renal arterial pressure down to about 100 mm Hg.20-26> "• 28 However, the results of Robertson et al.24 indicated that SNGFR autoregulation was less complete than autoregulation of glomerular and proximal tubule pressures, whereas Kallskog et al.29 suggested that the outer cortical nephrons do not exhibit autoregulation following reductions in arterial pressure. Because of the degree of controversy that exists concerning the response of the superficial nephrons to changes in arterial pressure and the uncertainty concerning the interaction of the autoregulatory process with the process responsible for the arterial pressure induced changes in urine output and sodium excretion, we were prompted to evaluate this problem in a setting in which we could determine the effects of arterial pressure on both the autoregulatory process and the urinary excretion responses. Methods Experiments were performed on 43 mongrel dogs of both sexes, weighing 13 to 21 kg and anesthetized with sodium pentobarbital (30 mg/kg, iv).They were prepared for micropuncture and clearance studies as previously described.25-27 A tracheotomy was performed to maintain a patent airway, and a jugular vein was catheterized for the infusion of inulin. Following the priming dose, an infusion of 5% inulin was administered to maintain a plasma concentration of either 0.2 or 0.8 mg/ml, depending on whether or not tubular fluid samples were to be taken. The carotid arteries were isolated from the carotid sheath and ligatures were placed loosely around each vessel. A catheter was advanced into a foreleg vein for the administration of additional anesthetic as necessary, and of isotonic saline at a rate no greater than 1 ml/min. Systemic blood pressure was monitored continuously through a catheter placed in the femoral artery with a Statham pressure transducer (Statham Laboratories). This catheter also was used for collection of arterial blood samples. The kidney was exposed via a left flank incision, and the renal artery, vein, and ureter were freed of the surrounding tissue. An electromagnetic flow probe was placed around the renal artery near its base and connected to a square wave flowmeter (Carolina Medical Electronics). A 22-gauge curved needle was inserted into the renal artery to measure renal arterial pressure (RAP). The line was kept patent by a continuous infusion of heparinized saline solution at a rate of 0.2 ml/min, with a Harvard infusion pump (Harvard Apparatus). This system was mechanically zeroed before each experiment to assure that there was no overestimation of true renal arterial pressure. An adjustable plastic clamp was placed around the renal artery between the flow probe and the pressure-measuring needle in order to constrict the renal artery and cause a direct reduction in RAP. A catheter was inserted into the left ureter to allow collections of timed urine volumes. The kidney was placed on a Lucite holder, and a square section of renal capsule of approximately 2 cm2 was removed. This area was continuously bathed with either a heparinized

VOL. 41, No. 4, OCTOBER

1977

saline solution warmed to 37°C or mineral oil dripped through a Lucite rod, also used to illuminate the surface of the kidney. The experiments were conducted either as "pressure experiments" or "collection experiments." Because the saline bathing the micropuncture surface is necessary for proper operation of the micropressure unit, pressure measurements were not taken in the collection experiments, in which the kidney was bathed with mineral oil to optimize tubular fluid collections. PRESSURE EXPERIMENTS Pressures in proximal tubules, peritubular capillaries, and distal tubules were measured by a micropressure servo null system (Instrumentation for Physiology and Medicine). To localize distal tubules and to measure tubular transit times, 0.5 ml of a 10% fast green dye solution (pH adjusted to 7.0) was injected directly into the renal artery through the renal arterial catheter. Upon injecting the dye, there was an initial green flush of the proximal tubules with a homogenous disappearance occurring within 20-30 seconds. Twenty to 40 seconds later, the dye would appear in the surface distal tubules. Superficial segments of distal tubules were selected for pressure measurements. After each distal tubule pressure measurement, the puncture site was checked for leaks by turning off the micropressure unit and injecting a small quantity of the stained solution from within the pressure-measuring pipette. First order peritubular capillaries were used for the measurement of peritubular capillary pressure, and proximal tubules were chosen at random. The time between the green flush on the surface of the kidney and its disappearance from the visible proximal tubule segments was designated as the proximal transit time, and the appearance of dye in the visible distal tubule segments was used to assess distal tubule transit times. These criteria were selected primarily because they allowed maximum consistency in the measurements. After the inulin infusion was initiated, a period of 50 minutes was allowed for stabilization of the preparation. Two to three timed volumes of urine (10-20 minutes) were collected and a plasma sample was taken at the midpoint of each urine collection. Systemic blood pressure, renal arterial pressure, and renal blood flow were continuously monitored throughout the experiment. Pressures in five to eight proximal tubules and three to four peritubular capillaries were measured. In seven experiments, the carotid arteries were constricted to elicit a baroreceptor response and allow the assessment of autoregulatory behavior in response to an elevation of arterial pressure. Renal arterial pressure was reduced in 16 experiments in steps of approximately 25 mm Hg by constriction of the renal artery clamp. After arterial pressure reduction, urine collections and pressure measurements were repeated. The renal arterial pressure was reduced to at least one level in 16 dogs, to two levels in 11 dogs, and to three levels in three dogs. Measurements were taken at renal arterial pressures down to and slightly below the lower limit for RBF autoregulation. The renal artery clamp was then released, allowing renal arterial pressure to return to control values. In 11 dogs, clearance measurements as previously described were repeated and, in addition, the


NEPHRON AUTOREGULATION AND PRESSURE DIVRESIS/Navar et al. fast green dye was injected into the renal artery to evaluate transit times and localize distal tubules. Transit times were measured and pressures determined in two to three distal tubules. Renal arterial pressure was reduced again, as explained, and measurements of transit times and distal tubular pressures repeated. Distal tubule pressures at reduced pressure were obtained in seven experiments. In three experiments, the two phases of the experimental protocol were combined. COLLECTION EXPERIMENTS

In 22 other experiments, a similar protocol was followed, except that the kidney surface was bathed with mineral oil and tubular fluid samples were collected. In 15 experiments, the SNGFR responses to variable decreases in RAP were assessed. Because of our previous experience that SNGFR based on proximal tubule fluid collections may not adequately reflect autoregulatory responsiveness,27 these SNGFR data were based only on distal tubule fluid collections. Distal tubules were identified, using intra-arterial injections of dye as described, and marked by the injection of a small quantity of nigrosin dye. Tubule fluid collections were made at control arterial pressure and after reduction of renal arterial pressure. In five of these experiments and seven additional experiments, the effects of decreases in arterial pressure on proximal fractional volume reabsorption were determined. To avoid obstruction of tubular fluid flow, samples were collected and recollected under free flow conditions without injection of an oil block. Individual tubule segments were identified with nigrosin dye as previously explained. The plasma inulin concentration in all the collection experiments was maintained between 0.7 and 0.9 mg/ml to allow a level of inulin in the tubule fluid samples that could be readily assessed by the microfluorometric method30 (Aminco). At the end of each experiment, the electromagnetic flow probe was calibrated in situ by catheterizing the renal artery and collecting timed blood samples in a graduated cylinder. The kidney was removed, stripped of all surrounding tissue, blotted dry, and weighed. This allowed RBF and GFR values to be expressed per gram of kidney

weight. Microhematocrit measurements were performed on all arterial blood samples. The anthrone colorimetric technique was used to determine inulin concentrations in both plasma and urine samples and glomerular filtration rate was calculated by the standard clearance formula. Routine measurements of plasma and urine composition were made. Sodium and potassium concentrations in plasma and urine samples were determined by flame photometry (Instrumentation Labs.) and osmolality was determined with an osmometer (Fiske Instruments). Protein concentration in each plasma sample was measured with a protein refractometer (American Optical Corp.), and plasma colloid osmotic pressure was measured directly with a membrane osmometer mounted on a pressure transducer (Statham). Data were analyzed by standard statistical techniques including paired analysis, linear regression analysis, and co-variance analysis. Unless noted otherwise, data are presented as the mean ± standard deviation (SD). When the standard error of the mean (SE) is used, this is designated. Results HEMODYNAMIC RESPONSES

Renal autoregulatory capability was assessed at the whole kidney level from the responses of RBF and GFR to change in renal arterial pressure. Table 1 compares the results obtained in the pressure and collection experiments. Renal blood flow was efficiently autoregulated over the entire range of arterial pressures studied and showed only a small decrease (less than 10%) over this pressure range in both groups. For the most part, GFR responses also showed a similar pattern of high efficiency autoregulation, with the exception that GFR at the lowest arterial pressure in the pressure group was decreased to a greater extent than RBF. This could be due to the inherent limitations of the clearance technique with low urine flows at lower arterial pressures. Overall, the experimental preparations set up for either the pressure or collection experiments exhibited whole kidney autoregulatory behavior down to renal arterial pressure levels similar to

TABLE 1 Renal Blood Flow and Glomerular Filtration Rate (GFR) Responses to Changes in Renal Arterial Pressure for both "Pressure" (P) and "Collection" (C) Experiments Renal arterial pressure

Renal blood flow (ml/min g)

(mm Hg)

GFR (ml/min g)

P

C

P

148 ± 10 n = 11 118 ± 6 n = 16 101 ± 4 n = 14 82 ± 5 n = 12 69 ± 5 n =7

142 ± 12 n = 12 117 ± 5 n = 13 102 ± 4 n = 10 83 ± 6 n = 9

4.05 ± 0.75

4.05 ± 1.,15

0.71 ± 0.17

0.67 ± 0. 16

4.2 ± 1.0

3.93 ± 1 .13

0.75 ± 0.19

0.64 ± 0 .15

3.93 ± 0.89

3.9 ± 0 .95

0.73 ± 0.23

0.69 ± 0 .18

3.75 ± 0.92

3.83 ± 1 .01

0.67 ± 0.2

0.64 ± 0 .22

3.7 ± 0.9

489

C

C

P

0.40 ± 0.23

Results are presented as the mean values ± SD. The individual values were grouped intofivearterial pressure ranges: above 130 mm Hg, 110-129 mm Hg, 90-109 mm Hg, 75-89 mm Hg, and below 75 mm Hg (last level reached only in the pressure dogs). The number of experimental values contributing to the average value at each arterial pressure category is designated by n.


490


those previously obtained.2'3> 31~33 The close association of RBF and GFR was reflected in the measured values for filtration fraction. At the control arterial pressure range, filtration fraction was 0.31 ± 0.07 and was not significantly different from this value at any of the other pressure ranges studied except the lowest arterial pressure category in the "pressure" dogs. Plasma protein concentration averaged 5.9 ± .61 g/dl and colloid osmotic pressure was 15.2 ± 2 mm Hg. In some previous studies, overall intrarenal resistance (IRR) has been calculated by using either intrarenal venous pressure32'33 as an estimate of overall peritubular capillary pressure (PCP) or by using measurements of proximal tubule pressure to estimate PCP.'9 To compare the results of the present experiments with those reported by Kaloyanides et al.19 for the isolated kidney, direct measurements of peritubular capillary pressure were used to estimate overall IRR. The assumption that measurements of superficial PCP reflect overall PCP values seems valid in view of the similarity between intrarenal venous pressure measurements and micropressure measurements of pep. 32 - 33 - 34 Figure 1 shows the average responses of PCP and IRR to changes in renal arterial pressure. PCP was maintained reasonably constant over the arterial pressure range studied; it decreased slightly from 14 ± 5 mm Hg at a mean RAP of 147 mm Hg, and to 10 ± 2 mm Hg at a RAP of 70 mm Hg. This relative constancy of PCP was reflected in the calculated values for IRR which was 35 ± 7 units at the highest arterial pressure and approached a minimal intrarenal resistance of 17 ± 5 units at 70 mm Hg.32'33 Thus, the major changes in intrarenal resistance in response to changes in RAP appeared to be localized to segments proximal to the peritubular capillaries. A crude estimate of venous resistance could also be obtained from the quotient of PCP and RBF. With these calculations, it appeared that the changes in venous resistance with changes in RAP were very slight and we found no consistent or significant trend in venous resistance as a function of arterial pressure. At the highest arterial pressure, venous resistance was 3.5 resistance units and was not altered significantly at reduced arterial pressures. TUBULAR PRESSURES AND TRANSIT TIMES

Measurements of proximal tubular pressures (PTP) were made in 17 of the experiments, whereas distal tubule pressures (DTP) were obtained in only 12 of the dogs and were more limited. Thus for Figure 2 the proximal tubule pressures were grouped into five arterial pressure ranges as for Figure 1, but the distal tubule pressures were grouped into only three arterial pressure ranges. At the highest arterial pressures, proximal tubule pressure was 24 ± 5 mm Hg and distal tubule pressure averaged 13 ± 2 mm Hg. Both exhibited high efficiency autoregulation down to a RAP of 100 mm Hg, with PTP decreasing very slightly to 22.5 and DTP remaining unchanged. With further decreases in renal arterial pressure, the decreases in PTP were greater; PTP fell to 18 mm Hg at an average RAP of 70 mm Hg. These decreases in PTP were consistent and statistically significant. The limited number of DTP measurements at these lower pressures was not suffi-


1977

40 _ 30 -

> E

I20 RBF 10

4 60

100 120 140 160 80 Renal Arterial Pressure (mm Hg)

FIGURE 1 The average relationships obtained in 14 dogs between renal arterial pressure (RAP) and peritubular capillary pressure (PCP) (lower curve) and calculated intrarenal vascular resistance (IRR) (upper curve). The individual values obtained at the various arterial pressures were grouped into five arterial pressure ranges [above 130 mm Hg (n = 9), 110-130 mm Hg (n = 11), 90-109 mm Hg (n = 14), 75- 89 mm Hg (n = 11), and below 75 mm Hg (n = 7)]. The average values ± one standard deviation are demarcated. This technique for grouping the data is used in order to reflect any nonlinearities in the relationships. RBF = renal blood flow.

cient to establish any significant changes as a function of RAP. Overall responses of the superficial nephrons were assessed from measurements of transit times of lissamine green through visible segments of proximal and distal tubules in 12 experiments. These data are limited due to the fact that only a few injections of dye can be given before accumulation of dye interferes with precise determination of end points. At a control RAP (122 ± 20 mm Hg), proximal transit time averaged 26 ± 5.7 sec and 00

i

30

E E 20 -

o- 10

•ih

60

80 100 120 140 Renal Arterial Pressure (mm Hg)

160

FIGURE 2 Responses of proximal tubule pressure (O) and distal tubule pressure (0) to reductions in renal arterial pressure (RAP). Proximal tubule pressures were obtained in 17 experiments and are grouped in the five arterial pressure ranges as for Figure 1, with n = 7 at RAP above 130, n = 12 at RAP of 110-129, n = 15 at RAP of 90-109, n = 12 at RAP of 75-89, and n = 8 at RAP below 75 mm Hg. Distal tubule pressures were obtained in 12 dogs al control arterial pressures and in seven dogs at reduced arterial pressure and therefore are grouped into only three arterial pressure ranges [above 130 mm Hg (n = 4), 100-120 mm Hg (n = 10), and below 100 mm Hg (n = 7)[.


NEPHRON AUTOREGULATION AND PRESSURE DIUKESIS/Navar et al. distal transit time was 56 ± 12 sec. When RAP was reduced to 89 ± 12 mm Hg, proximal transit time was not altered significantly [A = +2.4 ± 2.0 (SE) sec], although the increase in distal transit time of +7.4 ± 2.6 (SE) sec was statistically significant. Further reductions in RAP to 68 ± 5 mm Hg resulted in significant increases in both proximal transit time [A +5.5 ± 2.0 (SE) sec] and distal transit time [A = +15.8 ± 5.2 (SE) sec]. To assess the association between autoregulation of pressure in the superficial nephrons and whole kidney autoregulatory responses, the relative changes in RBF were compared to the relative changes in PTP and PCP in response to reduction in renal arterial pressure. Data obtained during RAP reduction were grouped into three pressure ranges: control values, values during the first constriction, and values during the second constriction. The relative responses are shown in Figure 3. With the first constriction, RAP decreased from 122 ± 22 mm Hg to 94 ± 12 mm Hg. Average RBF remained within 1 % of control. Average decreases in PTP and PCP were slight, being 4.4% and 5.2%. The consistency of the PTP responses was greater than that of PCP responses and the small decrease in PTP was statistically significant, although it amounts to a change of about 1 mm Hg. With the second constriction, RAP was decreased to 73 ± 11 mm Hg; there appeared to be small but inconsistent decreases in RBF which did not achieve statistical significance. In contrast, both PTP and PCP demonstrated substantial and highly significant decreases of 18% and 23%. In the seven experiments in which pressure measurements were obtained before and after carotid artery occlusion, arterial pressure was increased from an average of 113 ± 11 mm Hg to 141 ± 15 mm Hg. There were no significant changes in RBF (-8%), GFR ( + 7%), PTP (+11%), and PCP ( + 16%) in response to the carotid occlusion, although the variability in these measurements seemed substantially greater than could be explained on the basis of just the elevated arterial pressure.

Constriction #1

491 Constriction #2

+5 0

RBF

-5

PTP

y

PCP

10 -

RBF

PTP

T

15

T

20 25 30

-0.7 -4.4 -5.2 ±2.2 ±1.8 ±3.1 N.S. N.S

PC

1 1 !

- 6 . 8 - 1 8 -22.9 ±3.4 ±4.1 ±3.8 N.S.

FIGURE 3 Comparison of the percent changes in renal blood flow (RBF), proximal tubular pressure (PTP), and peritubular capillary pressure (PCP) after reduction in renal arterial pressure by constriction of the renal artery. The average control renal arterial pressure was 122 ±22 mm Hg and was decreased to 94 ±12 mm Hg during the first constriction. In 10 experiments, the renal arterial pressure was reduced further to 73 ± 11 mm Hg, and the data shown for the second constriction no. 2 are averaged from these 10 experiments. The mean differences ± SEare shown and * designates significance at the 5% level, whereas ** designates significance at the 1 % level.

tionship obtained for SNGFR and distal volume flow rate when plotted against arterial pressure. Average SNGFR at the highest RAP was 45 nl/min and remained within 1 nl/ min of this value at the next two pressure levels. This average value was decreased to 36 nl/min at the lowest pressure level; however, this decrease did not achieve statistical validity even when evaluated on a paired basis with the values at control pressure. Because of this degree of variability, one cannot be confident whether this decrease is real or only apparent. Nevertheless, essentially complete autoregulation of SNGFR was observed down to a RAP of 100 mm Hg. Likewise, distal tubular volume flow rate was not altered significantly and the tubular fluid-to-plasma inulin ratios were not modified to a significant extent. Because both SNGFR and whole kidney GFR

SNGFR RESPONSES

In a separate group of 15 dogs, we determined the changes in SNGFR in response to decreases in arterial pressure. As explained, the requisite total tubular fluid collections require the interposition of an oil block in the tubule and consequently interrupt volume flow to the remaining part of the nephron. To avoid or at least minimize changes in volume flow past the macula densa segments of the distal tubule and thereby prevent interference with the postulated mechanism thought to be involved in feedback regulation of SNGFR,27 all estimates of SNGFR were based on distal tubule fluid collections. The disadvantages of this approach, however, are the limited number of samples that can be obtained and the greater technical difficulty in these collections compared to proximal collections. These factors probably contribute, at least partially, to the observed degree of variation between dogs. From these 15 experiments, a total of 56 collections were made from 30 experimental periods. These experimental periods were divided into the four arterial pressure ranges designated in Table 1. Figure 4 describes the rela-

•

•I

60-

50

S E c 40-

^ o §30 JS •o

20

100 •/H-

70 90 110 Renal Arterial Pressure

130 mm Hg

150

FIGURE 4 Relationships between renal arterial pressure (RAP) vs. single nephron glomerular filtration rate (SNGFR) (0) and RAP vs. distal volume flow rate (M). Data are grouped as described for Table. 1 All values are expressed as the mean ± SD and the numbers in parenthesis refer to the number of dogs per number of tubules used to obtain the designated average point. No significant changes in TF/P inulin ratios could be demonstrated, the values being 3.39 ± 0.79, 4.43 ± 1.85, 3.68 ± 0.77, and 4.01 ± 1.29 from the highest to the lowest pressure ranges.


492



1977

exhibited autoregulatory behavior, the ratio of these two measurements (SNGFR/GFR) was not altered significantly by reductions in renal arterial pressure. The control value was 68 ± 6 (nl/min)/ml/min • g) at the higher pressures and 76 ± 9 at the lower pressures. URINARY OUTPUT AND FRACTIONAL REABSORPTION RESPONSES TO REDUCED ARTERIAL PRESSURE

As already mentioned, there were no significant alterations in GFR in response to reductions in RAP down to about 80 mm Hg. To evaluate possible trends in the filtered sodium load, we analyzed the relationship between filtered sodium load (FNa) and RAP by linear regression. This failed to provide any statistical evidence supporting the possibility that FNa changed with RAP. Average FNa was 101 /xEq/(min-g) and the regression coefficient was 0.26 /u,Eq/(min • g) per mm Hg; this value is not statistically different from zero. Analysis of data points above a RAP of 80 mm Hg yielded a regression coefficient almost equal to zero (—0.09). In contrast, the urinary excretion responses showed consistent changes with RAP. Figure 5 shows the urine flow responses to changes in renal arterial pressure. The line representing the regression equation is also shown; a highly significant and consistent relationship is evident. The relationship obtained between RAP and fractional water reabsorption is shown in the top portion of Figure 5. The "hydropenic" status of the dogs is evident and at pressures below the overall mean of 110 mm Hg, almost all fractional reabsorption values are greater than 99 %. Net fractional reabsorption decreased progressively at the higher arterial pressures and the variability also increased somewhat. Nevertheless, the responses were sufficiently consistent to yield a regression coefficient of — 0.013%/mm Hg that was highly significant. Similar patterns were obtained when the relationships between RAP and urinary sodium excretion or fractional sodium reabsorption were evaluated. These are shown in Figure 6. At the average RAP, sodium excretion was 1.06 ^Eq/(min-g). The regression coefficient for this relationship is 0.021 /xEq/(min • g) per mm Hg. Fractional Na reabsorption averaged 98.8% with a regression coefficient of — 0.02%/mm Hg. Both regression coefficients were highly significant at the 0.001 level. To determine possible relationships between proximal tubule or peritubular capillary pressures and the urinary responses to changes in arterial pressure, a comparison of the magnitude of the responses in these variables was performed for each experiment. The data were grouped into control data, responses during the first renal arterial constriction, and responses during the second renal arterial constriction. The percent changes from control were calculated for proximal tubular pressure, peritubular capillary pressure, sodium excretion, and urine flow during the two constriction periods. Similar relative responses were calculated for the seven dogs in which data were obtained before and after elevation of systemic arterial pressure by constriction of the carotids, although these responses showed substantially greater variability. Presumably, this could be due to the nonspecific effects of the baroreceptor reflex.15-16>18iZ3 The relative responses in

80 100 120 140 160~ RENAL ARTERIAL PRESSURE mm Hg

FIGURE 5 Alterations in urine flow (UF) and fractional water reabsorption in response to changes in renal arterial pressure (RAP). RAP vs. urine flow is plotted in the lower portion. Data points from individual pressure experiments are connected and the line of the regression equation is drawn. The regression equation is UF = 0.09-(RAP) - 4.8. The standard error of the regression coefficient (Sb) was 0.014, and the t value of 6.59 is highly significant. In the upper portion of the figure, fractional water reabsorption is plotted against RAP for all data points. The regression equation for these data is FRV = -0.013(RAP) + 100.7. Sb was 0.0024 and the t value of 5.46 is also highly significant.

PTP and PCP were compared to the relative changes in both urine flow and sodium excretion. During the first constriction, RAP decreased from an average of 123 mm Hg to 94 mm Hg. The decrease in PTP averaged 5.21 ± 1.77 (SE) %, and PCP decreased by 2.55 ± 3.16% compared to decreases in urine.flow and sodium excretion of 34 ± 5% and 44 ± 7%. During the second constriction, RAP was decreased to 77 mm Hg; PTP decreased by 19.42 ± 4.4% and PCP decreased by 22 ± 4%. Urine £ IOO jg

99

g |

98

O

W

*£ z

Q7

96 95

60

80

100

120

140

160

RENAL ARTERIAL PRESSURE mm Hg

FIGURE 6 Responses of sodium excretion C£,Vo and fractional sodium reabsorption (FRXa) to changes in renal arterial pressure (RAP). For lower graph, data points from individual dogs are joined. The regression equations are EXa = 0.021 (RAP) - 1.2; Sb = 0.0046, and FRSa = -0.020 • (RAP) + 101; Sb = 0.004. The regression coefficients for both relationships are highly significant at the 0.001 level.


NEPHRON AUTOREGULATION AND PRESSURE DIVSRESIS/Navar et al.

493

flow and sodium excretion decreased by 64 ± 6% and 78 ± 7%. During carotid constriction, arterial pressure increased to 141 mm Hg. The PTP and PCP responses were more variable and showed average increases of 11 ± 7 % and 16 ± 16%. The urine flow and sodium excretion responses were 89 ± 32% and 134 ± 52%. In Figure 7, the individual PTP responses are compared to the respective urine flow changes for each experimental period. Decreases in PTP during the first constriction were usually quite small and, in a number of the experiments, PTP was essentially unchanged. However, the responses in urine flow and sodium excretion were of greater magnitude and consistent. Analysis of covariance failed to establish evidence suggestive of a correlation between the magnitude of the PTP changes and the magnitude of the urinary excretion responses. The correlation coefficient was not significantly different from zero regardless of whether all data points were evaluated as one group or as subgroups according to the experimental periods. Similarly, there was no evidence of a significant correlation between the PTP changes and the changes in sodium excretion. Analysis of the degree of correlation between the percent changes in PCP and the percent changes in either urine flow or sodium excretion also failed to indicate a significant degree of correlation. Therefore, while PTP and PCP, as well as urine flow and sodium excretion, were coincidently related to changes in renal arterial pressure, these pressure responses could not be associated quantitatively with the alterations in urine flow and sodium excretion.

To evaluate the possibility that the changes in renal arterial pressure were altering fractional reabsorption at the level of the proximal tubule, 12 dogs were subjected to a similar procedure of renal arterial constriction during which free flow collections from proximal tubules were taken at control and reduced RAP. RAP was reduced from a control of 125 ± 16 (SD) to 94 ± 13 (SD) mm Hg. Both RBF and GFR exhibited satisfactory autoregulation, and values at reduced RAP were not significantly different from control values (ARBF = - 7 ± 4% and AGFR = - 6 ± 4 % ) . Urine flow and sodium excretion decreased to about the same levels as in the other series of experiments, the urine flow decreasing by 44 ± 6% and the sodium excretion by 70 ± 5%. The average TF/P(inulin) ratio based on 53 tubules was 1.69 ± 0.08 (SE) at control pressure and 1.53 ± 0.08 at reduced arterial pressure. Paired analysis of data failed to reveal a significant difference in these values; furthermore, it should be noted that the direction of the change, even though not significant statistically, is opposite to that expected for a proximal contribution to the increased net fractional reabsorption. For control purposes, collections and recollections were taken from 12 tubules with the arterial pressure maintained at control levels. Control TF/PIn averaged 1.69 ± 0.09 and recollections TF/PIn was 1.68 ± 0.18.

% A urine flow = 0.12; and % A peritubular capillary pressure vs. % A sodium excretion = 0.05. All of these values were below that necessary to establish significance at 5% level (0.396).

There are numerous differences between the present results and those of previous studies. In particular, Kallskog et al.29 suggested that the outer cortical nephrons did

Discussion

The results of the present investigation extend previous findings concerning single nephron autoregulatory behavior in the dog19-20'23>" by evaluating autoregulatory behavior at the lower arterial pressures. The measurements of proximal and distal tubule pressures, peritubular capillary pressures, tubular transit times, and SNGFR indicate 80 that, for the most part, the autoregulatory behavior of the 60 entire kidney is closely reflected by the behavior of the 40 superficial nephrons. Only at renal arterial pressures in the lower range for autoregulation was there an indication of •20 % CHANGE IN PTP 10 20 A30 proportionately greater decreases in superficial nephron -50 -30 -10 function. As blood pressure was reduced to the lower limit -20 of the autoregulatory range, there was a small decrease in RBF which was not significant; yet both proximal tubule and peritubular capillary pressures decreased substan• Constriction # 1 -60 o Constriction #2 tially. There was also an indication of a decrease in -80 • Carotid occlusion SNGFR at the lowest RAP although the decrease failed to % CHANGE achieve statistical significance. The proportionately IN UF greater decreases in single nephron function compared to FIGURE 7 Analysis of the association between proximal tubular whole kidney RBF and GFR at the lower pressures still pressure (PTP) and urine flow (UF) responses to changes in renal arterial pressure. Data are expressed as percent change from con- within the autoregulatory range suggest that the lower limit of the autoregulatory range of the superficial cortical trol values; • = responses observed during thefirstconstriction with reduction in renal arterial pressure (RAP) to a mean of 94 mm nephrons is higher than for the total nephron population, and that, in the dog, decreases in superficial nephron Hg; O = those obtained during the second constriction to a mean function occur as the renal arterial pressure is reduced RAP of 77 mm Hg; A = the responses obtained in the seven experiments in which measurements were taken before and after below approximately 90 mm Hg. Although no estimates of occlusion of the carotid arteries. Thefigureshows only the results outer cortical nephron plasma flow were made, the results for PTP vs. UF, but similar analyses were performed for PTP vs. of the present study are consistent with the previously E a and for PCP versus both UF and E o . The following correlareported changes in the distribution of intrarenal blood tion coefficients were obtained: % A proximal tubule pressure vs. flow with decreases in renal arterial pressure below 100 % A urine flow = 0.107; % A proximal tubule pressure vs. % A mm Hg reported by Abe35 and McNay and Abe.38 sodium excretion = 0.188; % A peritubular capillary pressure vs.


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not exhibit autoregulation. However, these conclusions were based in large part on estimates of glomerular pressure as determined by the stop-flow pressure technique which may actually interfere with autoregulatory behavior.25 Also, their SNGFR data are based on indirect estimates and were evaluated only at two pressure levels with the lower one possibly below the autoregulatory limits for the outer cortical nephrons in the rat. The present results also differ from those reported by Kaloyanides et al.19 for the isolated perfused dog kidney, since it was suggested that as much as one-third of the total resistance adjustments in response to changes in arterial pressure may occur at venular sites distal to the peritubular capillary network. In our study, calculations of intrarenal resistance indicated that most of the autoregulatory adjustments in renal vascular resistance occurred proximal to the peritubular capillary network. In this regard, it is clear that the autoregulatory responses of the kidney, when prepared as in the present study, are distinct from those obtained in the isolated kidney preparation. The reasons for these differences are not clear, but the findings of the present study are consistent with results of similar micropuncture studies tested over a more limited pressure range.23'26 In addition, the SNGFR responses observed in this study are somewhat different from those reported by Robertson et al.24 in that they suggested a small decrease in SNGFR even though there was little change in proximal tubule pressure and peritubular capillary pressure. One possible means to explain this apparent difference is that the SNGFR data of Robertson et al. are based only on proximal tubule fluid collections which may not adequately reflect autoregulatory behavior.25'27 However, it is fully recognized that this interpretation is not shared by all investigators.26 To our knowledge, this is the first report of distal tubule pressure in the dog. Average DTP at control arterial pressure was 13 ± 1.4 mm Hg compared to 21.7 ± 3.1 mm Hg for proximal tubule pressure in the same 12 dogs. The average proximal to distal tubule pressure gradient was 8.6 ± 2.9 mm Hg. Thus the absolute levels of DTP in the hydropenic dog seem to be higher than those reported for the hydropenic rat,37' 38 but because of the correspondingly greater proximal tubule pressures, the proximal to distal tubule pressure gradient is similar or perhaps slightly greater in the dog than in the rat. In recent years, a substantial degree of emphasis has been given to the potentially important role of the relationship between blood pressure and sodium excretion in the control of blood pressure and in the pathogenesis of various forms of hypertension."^13 Paramount to a further understanding of the subtle means by which this relationship is altered in various types of hypertensive conditions, is the need for a more complete understanding of the intrarenal mechanism or mechanisms that mediate this interaction between blood pressure and urinary excretion of sodium and water. One conclusion appears to be evident from the present study. The effects of arterial pressure on urinary excretion rates are still present under conditions in which there is no evidence of significant alterations in renal blood flow or GFR. Coincident with


1977

this high efficiency autoregulation, proximal tubule pressure and peritubular capillary pressure as well as SNGFR are also autoregulated with a high degree of efficiency, at least at blood pressures above 90 mm Hg. Although alterations in hemodynamics or SNGFR of deep nephrons could explain the responses to RAP, studies using other techniques to estimate renal function in deep nephrons and medullary areas seem to suggest that the deep nephrons also exhibit autoregulation that is at least as good as that of the superficial nephrons.28'29' 3S-36' 39 Thus, the effect of blood pressure in a setting where GFR and various indices of intrarenal function are autoregulated with a high degree of efficiency, poses a dilemma. On the one hand, there is the consistent effect of RAP on urine flow and sodium excretion. On the other, there is a highly efficient autoregulatory mechanism that prevents the transmission of altered pressure to the intrarenal elements. While a number of potential solutions to this dilemma exist, most of these appear to have questionable validity for conditions in which high autoregulatory efficiency is present. The most obvious possible explanation is that small, perhaps unmeasurable, changes in glomerular function lead to slight changes in glomerular filtration rate that are accompanied by a slight degree of incomplete glomerular tubular balance and this leads to proportionately larger changes in urine flow and sodium excretion.6'7> l4 Although this mechanism may be a satisfactory explanation when there is some evidence that changes in glomerular filtration rate do occur in response to changes in arterial pressure, one should be hesitant to use this explanation when there is no statistical evidence indicative of a real change in GFR in response to changes in RAP, as was true for the present study over the arterial pressure range of approximately 90-140 mm Hg. In addition, no significant alteration in filtered sodium load occurred in response to changes in RAP. However, one cannot completely exclude the possibility that physiologically significant changes occur but are not within our capability for measurement. Because urine osmolality is decreased when RAP is increased, it has also been suggested that the effects of RAP on urine flow could be mediated by a type of medullary washout phenomenon dependent on the arterial pressure level.3'5'40 It was suggested that as arterial blood pressure is altered, the blood flow to the medullary areas is changed proportionately such that the hypertonicity of the medullary environment is reciprocally related to the arterial pressure.3 To a large extent, this hypothesis was based on data indicating that medullary renal blood flow was not autoregulated with the same degree of efficiency as whole kidney renal blood flow.5 More recent studies have indicated that this is not the case and that medullary blood flow is autoregulated at least as well as cortical blood flow.39 In addition, it is doubtful that the medullary washout phenomenon could also explain the concomitant changes in sodium excretion that occur in response to changes in arterial pressure. Furthermore, it has been demonstrated that arterial pressure-mediated effects on urine flow and sodium excretion continue to occur in


NEPHRON AUTOREGULATION AND PRESSURE DIURESIS/Navar et al. diabetes insipidus dogs in which the changes in urine osmolality in response to changes in arterial pressure are very slight.7 Therefore, while medullary washout may account, in part, for changes in urine flow and urine osmolality that occur with changes in blood pressure, it is unlikely that it serves as a major mediator of the phenomenon of pressure diuresis and natriuresis. Finally, the observation that changes in the net fractional reabsorption of sodium and water occur in response to changes in arterial pressure even in the absence of changes in filtered loads, could be interpreted as showing that this is mediated by changes in peritubular physical forces2'9> "•19> 41 or "tubular driving force."42 This hypothesis has been suggested to explain the results of several previous studies in which there were substantial alterations in proximal tubule pressure or peritubule capillary pressure or both in response to changes in RAP.15"20 This hypothesis may be of questionable validity in explaining the alterations in fractional reabsorption of sodium and water that occurred in the present study. There was no significant quantitative association that could be established between the pressure responses in the proximal tubules and peritubular capillaries and the excretory responses. However, these studies cannot rule out the possibility that very minute alterations in PCP, not within the limits of our measurement capability, could be the significant factor responsible for altering the net tubular fractional reabsorption in response to alterations in renal arterial pressure. Admittedly, this is a difficult and controversial area and definitive conclusions could be made only with knowledge of factors such as the interstitial fluid hydrostatic and oncotic pressures as well as the oncotic pressure profile within the peritubular capillaries. In the absence of such information that could allow a complete analysis, one can only say that these studies fail to provide evidence that changes in either proximal tubule pressure or peritubular capillary pressure are quantitatively associated with the urinary excretion responses. In addition, decreases in renal arterial pressure did not cause significant alterations in fractional reabsorption from the accessible portion of the proximal tubule, at least over the pressure range evaluated. Collectively, these data suggest that the decreased urinary sodium and water excretion observed during reduction in renal arterial pressure most likely reflects an increase in reabsorption of filtrate in some portion of the distal nephron through a mechanism that remains unknown.43 Acknowledgments We acknowledge the excellent technical assistance of Jan Ranellone and Charles Thomas, the secretarial assistance of Allwyn Brown, and the graphical assistance of Lynne Cohen. Assistance in statistical evaluation of the data was provided by K. Kirk of the Department of Biostatistics, University of Alabama.

References 1. Selkurt EE, Hall PW, Spencer MP: Influence of graded arterial pressure decrement on renal clearance of creatinine, p-amino hippurate, and sodium. Am J Physiol 159: 369-378, 1949 2. Shipley RE, Study RS: Changes in renal blood flow, extraction of inulin, GFR, tissue pressure and urine flow with acute alterations of renal artery blood pressure. Am J Physiol 167: 676-688, 1951 3. Thurau K: Renal Hemodynamics. Am J Med 36: 698-719, 1964

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4. Selkurt EE: The renal circulation. In Handbook of Physiology, sec. 2, vol. II, Washington, D.C. American Physiological Society, 1963, pp 1457-1516 5. Thurau K, Deetjen P: Die Diurese bein arteriellen Drucksteigerungen. Pfluegers Arch 274: 567-580, 1962 6. Thompson DD, Pitts RF: Effects of alterations of renal arterial pressure on sodium and water excretion. Am J Physiol 168: 490-499, 1952 7. Navar LG, Uther JB, Baer PG: Pressure diuresis in dogs with diabetes insipidus. Nephron 8: 97-102, 1971 8. Navar LG: Distal nephron diluting segment responses to altered arterial pressure and solute loading. Am J Physiol 222: 945-952, 1972 9. Aperia AC, Broberger CG, Soderlund S: Relationship between renal artery perfusion pressure and tubular sodium reabsorption. Am J Physiol 220: 1205-1212, 1971 10. Guyton AC, Coleman TG, Fourcade JC, Navar LG: Physiologic control of arterial pressure. Bull NY Acad Med 45: 811-830,1969 11. Ledingham JM: Experimental renal hypertension. Clin Nephrol 4: 127-137, 1975 12. Guyton AC, Coleman TG, Cowley AW, Sheel KW, Manning RD, Norman RA: Arterial pressure regulation. Am J Med 52: 584-594, 1972 13. Thompson, JMA, Dickinson CJ: Relation between pressure and sodium excretion in perfused kidneys from rabbits with experimental hypertension. Lancet 2: 1362-1363, 1973 14. Navar LG, Guyton AC: Intrarenal mechanisms for regulating body fluid volume. In Circulatory Physiology II: Dynamics and Control of the Body Fluids, edited by AC Guyton, AE Taylor, HJ Granger. Philadelphia, Saunders, 1975, pp 243-261 15. Koch KM, Aynedjian HS, Bank N: Effect of acute hypertension on sodium reabsorption by the proximal tubule. J Clin Invest 47: 16961709, 1968 16. Bank N, Aynedjian HS, Bansal VK, Goldman DM: Effect of acute hypertension on sodium transport by the distal nephron. Am J Physiol 219: 275-280, 1970 17. Bank N: The renal regulation of sodium transport. Bull NY Acad Med 46: 818-829, 1970 18. Dresser TP, Lynch RE, Schneider EG, Knox FG: Effect of increases in blood pressure on pressure and reabsorption in the proximal tubule. Am J Physiol 220: 444-447, 1971 19. Kaloyanides GJ, DiBona GF, Raskin P: Pressure natriuresis in the isolated kidney. Am J Physiol 220: 1660-1666, 1971 20. DiBona GF, Kaloyanides GJ, Bastron RD: Effect of increased perfusion pressure on proximal tubular reabsorption in the isolated kidney. Proc Soc Exp Biol Med 143: 830-834, 1973 21. Landwehr DM, Schnermann J, Klose RM, Giebisch G: The effect of acute reductions in glomerular filtration rate on renal tubular sodium and water reabsorption. Am J Physiol 215: 687-697, 1968 22. Stumpe KO, Lowitz HD, Ochwadt B: Fluid reabsorption in Henle's loop and urinary excretion of sodium and water in normal rats and rats with chronic hypertension. J Clin Invest 49: 1200-1212, 1970 23. Liebau G, Levine DZ, Thurau K: Micropuncture studies on the dog kidney. I. The response of the proximal tubule to changes in systemic blood pressure within and below the autoregulatory range. Pfluegers Arch 304: 57-68, 1968 24. Robertson CR, Deen WM, Troy JL, Brenner BM: Dynamics of glomerular ullrafiltration in the rat. III. Hemodynamics and autoregulation. Am J Physiol 223: 1191-1200, 1972 25. Navar LG, Chomdej B, Bell PD: Absence of estimated glomerular capillary pressure autoregulation during interrupted distal delivery. Am J Physiol 229: 1596-1603, 1975 26. Knox FG, Ott C, Cuche JL, Gasser J, Hass J: Autoregulation of single nephron filtration rate in the presence and absence of flow to the macula densa. Circ Res 34: 836-842, 1974 27. Navar LG, Burke TJ, Robinson RR, Clapp JR: Distal tubular feedback in the autoregulation of single nephron glomerular filtration rate. J Clin Invest 53: 516-525, 1974 28. Bonvalet JP, Bencsath P, De Rouffignac C: Glomerular filtration rate of superficial and deep nephrons during aortic constriction. Am J Physiol 222: 599-606, 1972 29. Kallskog O, Linbom LO, Ulfendah; HR, Wolgast M: The pressure flow relationship of different nephron populations in the rat. Acta Physiol Scand 94: 289-300, 1975 30. Vurek G, Pegram S: Fluorometric method for the determination of nanogram quantities of inulin. Anal Biochem 16: 409-419, 1966 31. Baer PG, Navar LG, Guyton AC: Renal autoregulation, filtration rate, and electrolyte excretion during vasodilation. Am J Physiol 219: 619-625, 1970 32. Navar LG: Minimal preglomerular resistance and calculation of normal glomerular pressure. Am J Physiol 219: 1658-1664, 1970 33. Baer PG, Navar LG: Renal vasodilation and uncoupling of blood flow and filtration rate autoregulation. Kidney Int 4: 12-21, 1973 34. Knox FG, Willis LR, Strandhoy JW, Schneider EG: Hydrostatic



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pressures in proximal tubules and peritubule capillaries in the dog. Kidney Int 2: 11-16, 1972 Abe Y: Intrarenal blood flow distribution and autoregulation of renal blood flow and glomerular filtration rate. Jap Circ J 35: 1163-1173, 1971 McNay JL, Abe Y: Pressure dependent heterogeneity of renal cortical blood flow in dogs. Circ Res 27: 571-587, 1970 Gottschalk CW, Mylle M: 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, 1957 Falchuk KH, Berliner RW: Hydrostatic pressures in peritubular capillaries and tubules in the rat kidney. Am J Physiol 220: 1422-1426, 1971 Gransjo G, Wolgast M: The pressure-flow relationship in renal corti-


1977

cal and medullary circulation. Ada Physiol Scand 85: 228-236, 1972 40. Selkurt EE, Womack I, Dailey WN: Mechanism of natriuresis and diuresis during elevated renal arterial pressure. Am J Physiol 209: 9599,1965 41. Earley LE, Schrier RW: Intrarenal control of sodium excretion by hemodynamic and physical factors. In Handbook of Physiology, sec. 8, chapter 22. Washington, D C , American Physiological Society, 1973, pp 721-762 42. Omvik P, Raeder M, Kiil F: Relationship between tubular driving force and urine flow. Am J Physiol 226: 982-988, 1974 43. Schnermann J: Physical forces and transtubular movement of solutes and water. In Kidney arfU Urinary Tract Physiology (Physiology, series one, vol. 6) (MTP International Review of Science), edited by KT Thurau. London, Butterworths, 1974, pp 157-198

Anomalous Responses of Tumor Vasculature to Norepinephrine and Prostaglandin E2 in the Rabbit JOHN H. G.

RANKIN, RANDY JIRTLE, AND TERRANCE M.

PHERNETTON

SUMMARY We used 25-/im microspheres to compare blood flow to the V-2 carcinoma in the awake, unanesthetized rabbit with blood flow to other organs. Injection of norepinephrine (50 /ig) into the left ventricle caused a 41fold [95% confidence interval = (25-69)] increase in tumor vascular resistance (P < 0.01). This was more than one order of magnitude greater than the increase in resistance in any other organ. Prostaglandin E2 (50 /ig) injected into the left ventricle caused a 7-fold (4-13) increase in tumor vascular resistance (P < 0.01) and no significant increase of the vascular resistance of other organs. The change in tumor vascular resistance was not completely due to an increased level of circulating catecholamines because a 2-fold (1.6-3.4) increase in the resistance (P < 0.01) was seen when prostaglandin E, was injected into the left ventricle of animals pretreated with phenoxybenzamine. The prostaglandin E2-induced tumor vasoconstriction was not due to an increased level of circulating angiotensin II because in animals in which a and angiotensin receptors were blocked, prostaglandin E 2 increased the tumor vascular resistance by a factor of 3 (2.3-5.5) (P < 0.01). The tumor vasculature appears to be hypersensitive to a-receptor activation and responds to prostaglandin E2 with vasoconstriction which cannot be accounted for by an increased level of circulating catecholamines or angiotensin II. In these experiments, the vasculature of the tumor responded to pharmacological agents in a manner that was not displayed by the vasculature of other organs. It may be possible to selectively control tumor blood flow without adversely affecting the blood flow to other organs of the host.

MOST CANCERS are found as solid tumors, and much research in the field of cancer is concerned with a description of the basic causes of tumor growth at the cellular level. A unique approach has been taken by Folkman1 who has described the concept of angiogenesis in which the tumor forces the host to develop a new vasculature in order to supply nutrients to the tumor. Folkman has pointed out that solid tumors cannot grow unless the angiogenic responses are elicited. In view of the great interest in cancer it is curious that fundamental information regarding the cardiovascular mechanisms responsible for the regulation of the tumor From the Departments of Physiology and Gynecology-Obstetrics, University of Wisconsin Medical School, and Wisconsin Perinatal Center, Madison General Hospital. Madison, Wisconsin. Supported by Grant CA18756, awarded by the National Cancer Institute, Department of Health, Education and Welfare. Address for reprints: John Rankin. Ph.D.. Madison General Hospital, 722E. 202 South Park Street, Madison, Wisconsin 53715. Abstract presented at Spring meeting of American Physiological Society, 1977. Received November 19. 1976; accepted for publication March 23, 1977.

blood flow has not been obtained .In 1975, Gullino2 stated that "Control of circulation in tumors is an open question, at present." There is undoubtedly a practical reason for this neglect. The vasculature of the tumor is ever changing and does not lend itself to precise anatomical description. The traditional methods used to measure blood flow to an organ are difficult to apply to tissues such as tumors which have a variable size and location. In recent years the microsphere method for the determination of regional blood flows has been validated in many laboratories.3'4 We have performed a series of experiments designed to explore the use of radioactive microspheres to measure tumor blood flow, and we have used this technique to test the hypothesis that the regulation of the tumor blood flow does not differ from that of normal tissue. This hypothesis was tested by observing the responses of the vasculature of the tumor and other organs of the rabbit to norepinephrine which is known to cause vasoconstriction. Prostaglandin E2 also was used because it is known to have vasodilator activity in many organs5 and to modify the vasoconstrictor actions of norepinephrine.6


Autoregulatory responses of superficial nephrons and their association with sodium excretion during arterial pressure alterations in the dog. L G Navar, P D Bell and T J Burke Circ Res. 1977;41:487-496 doi: 10.1161/01.RES.41.4.487 Circulation Research is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231 Copyright © 1977 American Heart Association, Inc. All rights reserved. Print ISSN: 0009-7330. Online ISSN: 1524-4571

The online version of this article, along with updated information and services, is located on the World Wide Web at: http://circres.ahajournals.org/content/41/4/487.citation

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