Renal Modulation of Urinary Catecholamine
Excretion During Volume Expansion in the Dog
K E N N E T H R. B O R E N , M.D.,
D A V I D P. H E N R Y , M . D . ,
E W A L D E. SELKURT, P H . D . ,
AND M Y R O N H. WEINBERGER,
M.D.
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SUMMARY The role of the kidney in handling the biologically acthe, nnconjugated endogenous catecholamines epinephrine (E), noreptnephrine (NE), dopamlne (DA), and the O-methylated metabolite of NE,
normetanephrine (NM), was studied in the anesthetized dog before and after Toloroe expansion with iaotonk
saline by measuring renal arterial deUrery, urinary excretion rate, and renal Tenons return of these materials.
Net turnover in the renal metabolic compartment was estimated by comparing arterial delivery to urinary excretion and renal Tenons return. The kidney extracted E and produced NE, DA, and NM before and after
Tolume expansion. After Tolume expansion a significant decrease in the extraction of E, an Increase in the
production of DA, and no change in the production of NE or NM was seen. Clearance of catecbolamlnes and
NM through the kidneys appeared to increase with Tolume expansion. The clearance of DA exceeded the
clearance of creatinine (Cr) after volume expansion, while the clearance of NM exceeded that of Cr before and
after volume expansion, indicating that urinary DA and NM are derived, in part, from processes other than
glomerular filtration. These observations suggest an important role for the kidney in the modulation of the excretion of catecholamines and metabolites. This role must be considered before the urinary excretion rate of N,
E, DA, and NM can be related to generalized sympathetic function. The observed increases in renal DA
production after saline infusion suggest a possible natriuretic function of intrarenal DA.
(Hypertension 2: 383-389, 1980)
Q
KEYWORDS
• dopamlne
* epinephrine
• norepinephrine
UANTIFICATION of the urinary excretion
rate of the endogenous catecholamines norepinephrine (NE), epinephrinc (E), and dopamine (DA), and catecholamine metabolites such as
normetanephrine (NM) has been used to evaluate
sympathetic nervous system activity in experimental
animals and man.1 It is not clear, however, what factors regulate renal metabolism, renal clearance, and
urinary excretion of these substances.1 Recent studies
from our laboratory have suggested that urinary NE
may be derived, in part, from intrarenal sources in
man.1 In dogs, other investigators have found that the
kidney appears important in modulating the urinary
excretion of catecholamines.*"8 Little is known concerning the renal clearance of normetanephrine
(NM).8
Our present study examines the role of the kidney in
the metabolism and urinary clearance of NE, E, DA,
and NM. Experiments were performed in anesthetized
dogs before and after volume expansion with saline
•
normetanephrwe
since this maneuver has been widely used in human
subjects to evaluate sympathetic nervous system function9 and is known to increase urinary DA excretion.10
Net catecholamine production or extraction by the
kidney was evaluated by quantifying arterial delivery,
urinary excretion, and the venous return of these
catecholamines.
These studies disclose a major and divergent role
for the kidney in determining the urinary excretion
rate of the catecholamines and NM and further
demonstrate net production by the kidney of NE, DA,
and NM. The apparent renal clearance or net production of these substances was influenced by isotonic
sodium chloride volume expansion.
Methods
Surgical Procedure
Seven male mongrel dogs weighing 19.9 ± 0.7 kg
(SE) were maintained on Purina dog chow. The
animals were anesthetized with intravenous pcntobarbital (30 mg/kg) and maintained with supplemental doses as needed. All animals were intubated.
Venous catheters of PE 260 tubing were placed in both
femoral veins. The left femoral vein catheter was advanced to the right atrium and used for infusion of
maintenance solutions of creatinine and p-aminohippurate (PAH) (Sigma Chemical Company). The
right femoral vein catheter was advanced to the distal
inferior vena cava and used for all other infusions.
Bilateral femoral artery catheters of the same tubing
From the Departments of Medicine, Physiology, the Specialized
Center of Research (SCOR) in Hypertension, Indiana University
School of Medicine and the Lilly Laboratory for Clinical Research,
Indianapolis, Indiana.
Supported by grants from the USPHS, HL-14159, Specialized
Center of Research (SCOR) in Hypertension, HL-07181, Training
in Hypertension Research (KRB), and a grant from the American
Heart Association, Indiana Affiliate.
Address for reprints: Myron H. Weinberger, M.D., Indiana
University Medical Center, 1100 West Michigan Street, Clinical
Building Room 477, Indianapolis, Indiana 46223.
383
384
HYPERTENSION
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were inserted. The left femoral arterial catheter was
placed just distal to the renal arteries and used to
record distal aortic blood pressure (BP) via a Statham
P23De pressure transducer with recording via a
polygraph (Brush Instrument Company, Cleveland,
Ohio). The right femoral catheter was advanced to the
aortic bifurcation and used to sample aortic blood. A
right carotid artery catheter was placed in the intrathoracic descending aorta to measure proximal
blood pressure via a Statham P23De pressure
transducer and the Brush polygraph. A priming dose
of 0.5 g PAH and 4.30 g of creatinine (Cr) dissolved
in 30 ml normal saline was given intravenously. A
maintenance infusion of 5 g PAH and 14 g Cr dissolved in 150 ml of normal saline was given via the left
femoral vein at a rate of 0.2 ml/min.
The left kidney was exposed with a subcostal incision. The left ureter was catheterized with PE 160 tubing to record urineflowand collect urine samples from
the left kidney. The spermatic vein was freed of connective tissue and cannulated with PE 160 tubing and
advanced to the left renal vein. The renal artery was
freed of connective tissue and, with care to avoid injury to renal nerves, an electromagnetic flow probe
was placed on the proximal renal artery; renal blood
flow was then recorded using a CME flow meter
(Carolina Medical Electronics, Inc., King, North
Carolina) and a Brush polygraph. After the operation
had been completed, the animal was suspended by a
sling to provide access to the kidney, and a 1-hour
recovery period was begun.
Experimental Protocol
After the recovery period, urine and midpoint blood
samples obtained from the aorta and renal vein were
collected for two, 15-minute control periods for Cr,
PAH, sodium (Na), potassium (K), and osmolality.
Blood samples were collected in heparinized tubes and
placed on ice for catecholamines at the beginning,
middle, and end of each time period. The integral of
the plasma concentration curve was used for clearance
calculations. The samples were centrifuged in a
refrigerated centrifuge at 4°C for 15 minutes at 2000
rpm. The plasma samples for catecholamines and NM
were immediately frozen with dry ice. The remaining
samples were kept refrigerated prior to analysis. The
urine samples for catecholamine and NM determinations were acidified with glacial acetic acid and
immediately frozen for subsequent analysis. Creatinine and PAH were determined by an autoanalyzer technique,1'1 " Na and K were determined by
flame photometry (Corning Instrument Company).
Osmolality was determined by freezing point depression using an Advanced Instrument Osmometer.
After the two control periods, volume expansion
was induced with intravenous 0.9% saline to a total of
5% of body weight over 30 minutes. The urine volume
was replaced after each 15-minute period with additional 0.9% saline. After the 30 minutes of infusion,
three 15-minute urine collections were obtained
together with blood samples at the midpoint of each
VOL 2, No 4, JULY-AUGUST
1980
Abbreriadons Used
aortic concentration of catedndamlnes
AV arteriorenous
ce potassium clearance
COMT catedwJ-O-methyl-traiisferase
C M . clearance of sodium
cm osmolar clearance
Cr creatinine
DA dopamine
DBF diastoiic blood pressure
DTE dithioerjthritol
E epinephrine
EDTA ethylenedlamlnetetraacetic add
GFR glomerular filtration rate
NE norepinephrine
NM nonnetanepbrine
PAH p-amino-hlppurate
PNMT pbenyletnaoolamiDe-N-nietbyMraiuferase
RBF renal blood flow
RPF renal plasma flow
SBP systolic blood pressure
TES tris metnyl-2-amJnoethanesulfoaic add
Uc* urinary concentration of catechoUmlnes
V urine flow
ACA
period except for catecholamines, which were obtained at the beginning, middle, and end of each
period. Urine flow was determined by volumetric
collection of urine in calibrated centrifuge tubes.
Hematocrit was determined from arterial blood from
each period by the Wintrobe technique. After the end
of the protocol, both renalflowprobes were calibrated
in vivo and both kidneys removed, weighed, and
frozen. Pulse, systolic (SBP), diastoiic blood pressure
(DBP), mean BP, and renal blood flow (RBF) were
determined at the beginning, midpoint, and end of
each period. The weighted mean was calculated using
the trapezoid approximation and was used for
clearance determinations. Renal plasma flow was
calculated by (1-Hct) X (RBF). In one animal we
were unable to calibrate the flow probes, and in this
animal PAH extraction was used to estimate RBF.
The NE, E, and DA were quantified by their conversion to tritiated O-methyl metabolites using highly
purified catechol-O-methyl-transferase (COMT) and
tritiated S-adenosyl methionine, with a modification
of our previous methods.1*"16 The COMT was purified
from fresh rat liver; 100 g of tissue was homogenized
in 4 volumes of cold 0.25 M sucrose using a polytron
homogenizer. The homogenate was centrifuged for 15
minutes at 18,000 rpm in a Sorval centrifuge, 0-4°C.
The supernatant was filtered through gauze and then
centrifuged for 1 hour using a Beckman 45 Ti rotor at
186,000 g. The supernatant was adjusted to pH 4.9
with 2 N acetic acid and stirred for 15 minutes. The
precipitate was removed by centrifugation for 15
minutes at 18,000 rpm. The supernatant was readjusted to pH 4.9, and ammonium sulfate was added to
give a final concentration of 1.2 M. After stirring for
20 minutes at 0-3°C, the ammonium sulfate
precipitate was removed by centrifugation for 15
RENAL CATECHOLAMINE HANDLING IN THE DOG/Boren et al.
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minutes at 18,000 rpm. Ammonium sulfate was added
again to the supernatant to yield a final concentration
of 2.0 M. After stirring 20 minutes at 0-3°C, the
precipitate was removed as before. The pellet was
resuspended to approximately 20 ml with the following buffer: 0.1 M TES, pH 8.0, 5 mM EDTA, 20%
glycerol, and 1 mM dithioerythritol (DTE). The
resuspended pellet was applied to a 5 X 85 cm
Sephadex G-50 column, which had been previously
equilibrated with 25 mM BIS-TRIS, pH 6.0 1 mM
EDTA, 20% glycerol, and 1 mM DTE. The active
fractions were pooled and concentrated to a volume of
10 ml using an Amicon PM 10filter.The concentrated
preparation was applied to a 1 X 16 cm DEAESephacel column which had been equilibrated with the
BIS-TRIS buffer described above. The enzyme was
eluted with 0.15 M NaCl. The active fractions were
pooled and concentrated to a final volume of 25 ml
using an Amicon filter as before.
The enzyme was aliquoted into plastic test tubes
and frozen at -20°C. The COMT was completely
devoid of 1-aromatic ammo acid decarboxylase activity. The O-methylated products were formed by incubation as previously described14' 15 for 60 minutes.
They were then extracted from potassium borate at
pH 10.0 into toluene-isoamyl alcohol (3.2, v/v) and
back-extracted into 0.2 M formic acid, which was then
evaporated to dryness in a vacuum centrifuge. The Omethylated products and added cold carrier were
separated by thin layer chromatography on silica
gel plates developed in chloroform, methanol,
ethylamine, ammonium hydroxide (80:18:3:3, v/v).
Each spot was visualized under ultraviolet light and
extracted with 2% bisdiethylhexyl phosphoric acid in
toluene, and quantified by liquid scintillation spectrpphotometry. Cross contamination of catecholamine and other catechol containing compounds
was less than 2.0%. Interassay variability of internal
standards was 4.0%. The sensitivity of the assay was
approximately 2.0 pg for NE, 1.5 pg for E, and 2.0 pg
for DA, but varied slightly depending upon the specific
activity of the tritiated S-adenosyl-methionine used.
Internal standards for NE, E, DA were included for
each sample; volume used in assays was 25 nl or less.
The NM was extracted from 250 pi of plasma using
2.5 ml of isoamyl alcohol. After mixing and centrifugation, 2.2 ml was transferred to tubes containing 200 fil of 0.1 M formic acid and 2.0 n\ of toluene.
The samples were shaken and centrifuged, and the
organic phase was discarded. The aqueous phase was
evaporated to dryness under vacuum and then resuspended in 25 n\ of 1 mM HC1. The NM was then converted to tritiated metanephrine using partially
purified bovine adrenal phenylethanolamine-Nmethyl-transferase (PNMT) with previously reported
incubation techniques.14- " Metanephrine was then extracted and separated by the same techniques used to
quantify NE, E, DA. Urine was diluted and assayed
without the initial solvent extraction. Crosscontamination by other substrates for PNMT was less
than 1%. Internal standards for each sample were included. Interassay variation was 5.2%, and the sen-
385
sitivity of the assay was 10.2 pg for standards added
after the initial solvent extraction step; recovery of
NM for the initial solvent extraction step was 48%.
Statistical analysis was performed by the Student t
test for paired data or by least-squares linear regression analysis. The 95% limits of probability were
accepted as significant using a two-tailed test except
for analysis of data for which previous studies have
reported an increase in values with volume expansion,
where a one-tailed test was applied.
Results
Volume expansion produced a significant increase
in urine flow rate (V), systolic blood pressure (SBP),
clearance of sodfum (CN«); clearance of potassium
(CK) osmolar clearance (Co.m), and the excretion of'
Na and K. A significant decrease in hematocrit was
observed (tables 1 and 2).
TABLE 1. Hemodynamic Effects of Saline Infusion
Measure
Urine flow rate
(ml/min)
Mean arterial blood
pressure (mm Hg)
SBP (mm Hg)
DBP (mm Hg)
Heart rate
(beats/min)
Renal blood flow
(ml/min)
Renal resistance units
(mm Hg/ml/min)
Hematocrit (%)
Control
mean *• SEM
Experimental
mean •*= 8EM
0.24 =f» 0.04
1.07 *t 0.28*
118.3 =i» 7.9
156.0 ±9= 9.9
94.0 =f» 9.6
125.3 * 7.3
168.9 =•» 8.6*
94.9 •»= 9.9
= 13.0
148.6 =f
152.1 *= 9.3
303.9 =>' 28.6
322.1 ± 37.1
0.43 "*» 0.08
50.9 ±2= 2.2
0.44 ± 0.007
41.1 * 1.8t
*p < 0.05.
tp < O.QOl.
TABLE 2.
Renal Function Before
Function
Cor (ml/min)
CPAB (ml/min)
CN» (ml/min)
CK (ml/min)
Co«m (ml/min)
Na excretion
(MEq/min)
K excretion
(/lEq/min)
OBmolar excretion
Giosm/min)
*p < 0.05.
tp < 0.01.
and After Saline Infusion
Control
mean •± SBM
Experimental
33.7 •*= 3.2
102.7 *. 12.8
0.091 *= 0.022
6.25 =*= 0.77
0.76 *= 0.08
39.1 ± 4.5
109.1 *•• 15.5
1.004 ± 0.268*
mean = B8EM
11.05 *• 1.28t
1.75 ± 3.07t
14.11 ±. 3.40
145.73 ± 40.11*
21.66 ±3= 3.25
35.00 * 3.22t
218^9 *. 23.2
516.3 * 86.5t
386
HYPERTENSION
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The arterial and venous levels of E, NE, DA, and
NM in the control and experimental periods are
presented in figure 1. During the control period, renal
venous E was less than arterial E (p < 0.001). No
arteriovenous (AV) difference was observed for NE,
DA, or NM. Volume expansion resulted in a significant decrease in both arterial and venous levels of E.
The arterial level decreased from a control value of
188.3 ± 35 (SE) pg/ml to 56.7 ± 21.9 {p < 0.001).
The renal venous level of E decreased from 77.8 ±
27.9 pg/ml to 36.8 ± 18.1 (p < 0.01). In contrast, no
significant changes were noted in NE, DA, or NM
levels in the aortic or renal venous blood after saline
infusion, and no significant AV differences were noted
for DA, NE, or NM.
The amount of each substance filtered was assumed
to be equal to the arterial concentration of the substance times the glomerular filtration rate (GFR X
P C A). Using this formula, we found that the amount of
E filtered fell significantly from 0.112 ± 0.023 to 0.035
± 0 . 1 3 ng/min/g KW (p < 0.002) with saline administration. However, filtered NE, DA, and NM did
not change significantly. The urinary excretion of E,
NE, DA, and NM before and after volume expansion
are presented in figure 2. There was a significant
decrease in the excretion of E after volume expansion
from 0.084 ±0.019 to 0.030 ± 0.011 ng/min/g KW (p
< 0.025). The excretion of NE did not change
significantly. DA excretion increased significantly
with volume expansion (0.050 ± 0.012 ng/min/g KW
to 0.078 ± 0.023) (p < 0.05). The NM excretion increased significantly with volume expansion from
1.252 ± 0.225 to 1.951 ± 0.352 ng/min/g KW
(p < 0.05).
There was a significant increase in the clearance of
epjnephrine (CE), norepinephrine (CNE), dopamine
(CPA), and NM (CNM) after isotonic volume expansion
(fig. 3). The CE rose from 0.453 ± 0.075 ml/min/g K
to 0.652 ± 0.104 (p < 0.05). The fractional clearance
of E rose insignificantly from 74% ± 6.1% to 93.6% ±
12.7% (p > 0.05). The clearance of NE increased from
0.438 + 0.062 to 0.680 + 0.108 ml/min/g KW (p <
0.025), and the fractional clearance rose insignificantly from 76.1% ± 12.4% to 96.3% ± 8.1% (p
> 0.05). The clearance of DA rose from a control
clearance of 0.711 ± 0.214 to 1.092 ± 0.316 ml/min/g
KW (p < 0.05) with volume expansion. The fractional
clearance rose insignificantly from 126.5% ± 14.2% to
160.8% ± 20.3% (p > 0.05). The clearance of NM increased significantly from 1.335 ± 0.308 to 2.055 ±
0.522 ml/min/g KW (p < 0.05), and fractional
clearance rose significantly from 235.8% ± 44.1% to
295.2% ± 39.2% (p < 0.05). Further, the clearance of
NM was significantly greater than that of Cr in the
control period (p < 0.05), and the clearance of NM
and DA both exceeded the clearance of Cr after
volume expansion (p < 0.05). The clearance of NE
and E was less than that of Cr in the control period (p
< 0.05) but did not differ statistically from Cr
clearance after volume expansion. The net turnover of
free CA in the renal metabolic compartment was
determined by the following formula:
VOL. 2, No 4, JULY-AUGUST 1980
EXPERIMENTAL
•
ARTERIAL
Q
VENOUS
p < 0.001
p<0.01
FIGURE 1. Arterial and venous plasma catecholamine
concentrations in the control and volume-expanded period.
Brackets represent standard error of mean. Abbreviations
are: NE = norepinephrine, E = epinephrine, DA = dopamine, and NM = normetanephrine.
2 . 2 - i
2.0
-
to
UJ
2
1.8 -
5
O
U
1.6
|
Y//\
I CONTROL
EXPERIMENTAL
p<0.05
UJ
£
J.« -I
U
1.2
-
1.0
-
in en
c
O
c
NE
NE
2. Urinary excretion rates during control and
volume-expanded period. Abbreviations as for figure 1.
FIGURE
RENAL CATECHOLAMINE HANDLING IN THE DOG/Boren et al.
387
p<0.05
320
CONTROL
Q
280
EXPERIMENTAL
CONTROL
EXPERIMENTAL
2.4_
200
z
i
o
I
s
1.6-
-I
"V I
1.2-
;120.
p<0.025
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LLJ
z
a.
u
j
£200-
2.0-
LU
p<0.05
E
0.8H
o.a_
NE
I
I
80.
40-
DA
NM
11
NE
DA
NM
FIGURE 3. Clearance of catecholamines in control and volume expanded period.
Abbreviations as for figure 1.
net catecholamine turnover =
RPF X VCA + V X U CA - RPF X ACA
= venous return + urine loss — arterial delivery
where RPF = renal plasma flow; V = urine flow; and
VCA, ACA» and U C A represent renal venous, aortic, and
urinary concentrations of catecholamines respectively.
Positive values indicate a net production of catecholamine, and negative values indicate a net removal
of catecholamine by the kidney. The net return is
denned as the sum of venous return and the urine
excretion.
The net amount of catecholamines and N M produced or removed by the kidney was calculated using
this equation (table 3). In the control period E was extracted by the kidney; after volume expansion, the extraction fell. Delivered E was significantly greater
than the net return of E in both the control and experimental periods (p < 0.05). In the control period,
NE, DA, and N M were produced by the kidney (table
3); after volume expansion, the production of DA increased (p < 0.05) but did not change for NE or N M .
The net return of DA was significantly greater than
the DA delivered in both control and volume expansion periods (p < 0.05).
Discussion
We have previously reported that in standing
humans a significant portion of urinary NE is derived
from processes other than glomerular filtration.2 In
addition, we have observed in humans that chronic
oral salt-loading paradoxically suppresses the urinary
NE secretion rate to a lesser degree than forearm
plasma venous NE concentration." These observations, as well as a vast literature related to the
urinary excretion of catecholamines and their
metabolites, mandate a clearer understanding of how
the kidney handles these substances. In our present
T A B L E 3.
Calculated
Catecholamiae
Epinephrine (E)
Control
Experimental
Renal
n
Turnover
Net production
or addition
(ng/min/g K W )
7
7
of
Catecholamines
N e t extraction
(ng/min/g KW)
0.237 * 0.060
0.039 ± 0.013*
Norepinephrine ( N E )
Control
Experimental
7
7
0.158 =t 0.228
0.346 -= 0.149
—
—
Dopamine (DA)
Control
Experimental
5
5
0.101 ± 0.034
0.159 ± 0.036*
—
—
Normetanephrine
(NM)
Control
Experimental
•p < 0.05.
7
7
- 1.031
1.019 =>
1.409 ** 0.693
—
—
388
HYPERTENSION
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study we examined the role of the kidney and the effect
of acute volume expansion in determining the urinary
excretion rate of not only NE, but also the structurally
related catecholamines E and DA, and NM, the omethylated metabolite of NE, by quantifying renal
clearances of these substances in the anesthetized dog
before and after isotonic volume expansion with Na.
By measuring arterial delivery of the catecholamines
to the kidney and comparing this to urinary excretion
and venous return from the kidney, we were able to
calculate net renal production rates for these amines.
Studies show that NE is synthesized by sympathetic
nerve terminals within the kidney parenchyma and in
blood vessels;18 DA may be synthesized within specific renal dopaminergic neurons,1' liberated from
dopamine conjugates/ or synthesized from circulating dopa by renal aromatic amino acid dccarboxylase.1' The kidney may produce NM either from
NE delivered to the kidney by arterial plasma or from
NE contained within renal sympathetic nerves. Since
the kidney does not contain the enzyme PNMT
necessary for the synthesis of E, this catecholamine
cannot be produced there. The renal handling of E,
therefore, can be taken as a model for the clearance
and catabolism of the catecholamine class of compounds uncomplicated by the possibility of intrarenal
synthesis. Renal arterial E levels were significantly
higher than venous, which would be anticipated in the
absence of intrarenal synthesis. The clearance of E
was less than that of Cr during the control period.
After volume expansion, the fractional clearance of E
increased from 74% to 93.6%. Despite the increase in
renal clearance, volume expansion decreased net intrarenal extraction and metabolism of E since the AV
gradient fell. These observations are compatible with
the postulate that E is passively filtered at the
glomerulus and partially reabsorbed or metabolized
during passage through the remainder of the nephron
by a tubular flow rate-dependent process; E, which is
not filtered at the glomerulus, is metabolized at other
sites within the kidney.
No AV difference was noted for NE in either of the
periods, but net production of NE by the kidney was
observed before and after volume expansion. If NE
delivered to the kidney is handled in a fashion similar
to that of E, it would appear that the majority of renal
venous NE is of renal origin. The arterial concentrations of NE did not change with volume expansion nor
did the urinary excretion rate. The clearance of NE increased with volume expansion but there was no
change in net turnover of NE in the renal metabolic
compartment. The plasma concentrations of DA did
not change with volume expansion, and no AV
difference across the kidney was observed in cither
period. Net renal production of DA occurred in the
control period; production increased after volume expansion, as the apparent clearance of DA increased to
a level greater than Cr clearance. These observations
are compatible with the concept that during normal
Na balance, DA and E are handled by the kidney in a
similar fashion. Volume expansion, by increasing
renal production of DA, distorts these relationships.
VOL 2, No 4, JULY-AUGUST
1980
The plasma concentration of NM exceeded that of
NE, E, and DA. No AV difference was observed
across the kidney. Plasma concentrations of NM were
not influenced by volume expansion. The renal
clearance of NM exceeded Cr clearance in both the
control and volume expansion period. Net renal
production of NM occurred in both periods and was
not affected by volume expansion. We conclude that
the intrarenal metabolism of NE and NM are closely
linked and that the kidney is the site of synthesis of a
significant portion of urinary NM.
In this study, the hemodynamic and renal function
data in the control period and following volume expansion agree with previous reports.4-" In addition,
Faucheux ct al." and Carrier? et al.4 observed a
prompt fall in the sum of the NE and E levels in
plasma measured as total catecholamines after
volume expansion. We have found E levels to be 41%
of the NE levels in arterial blood during the control
period, and therefore a fall in E levels alone could
produce a significant change if total catecholamines
were quantitated. Similarly, we observed a decrease in
the excretion of E but no change in NE excretion with
volume expansion. Faucheux et al.' reported a fall in
urinary excretion when both catecholamines were
measured together. The extraction of E by the kidney
seen in both periods of the present experiment confirms the observations of Unger et al.7 In addition,
previous studies in the dog have not demonstrated a
significant difference in arterial and renal venous NE
concentrations with acute stress;' our observations are
in agreement with these studies.
Overy et al.,M using infused tritiated NE, reported
that the fractional clearance of NE was 59.1% in
anesthetized dogs. This compares favorably to the
fractional clearance for NE of 76.1% that we found
during the control period. These investigators did not
examine the effects of isotonic saline infusion. We
have previously shown a rise in NE clearance despite a
fall in Cr clearance with upright posture in man,1 and
others have noted that the clearance of NE and DA
can exceed GFR." Using fluorometric methodology
for E determinations, Jones and Blake13 were unable
to detect E in the plasma of anesthetized dogs. After
infusions of E to increase plasma E concentration,
they observed that the fractional clearance of E was
greater than 100%.
Our observations arc supported by those of
Faucheux et al.,6 who reported that, in the dog,
volume expansion with isotonic saline but not with
albumin increased both urinary DA excretion and the
fractional clearance of DA. Similarly, Alexander et
al.10 found that isotonic saline infusion increased
urinary DA excretion in man. Since we found that the
fractional clearance of DA exceeded unity at the same
time that net renal production of DA was observed, it
is likely that the increase in urinary DA seen after
saline infusion was not due simply to increased renal
tubular secretion. Since DA is natriuretic,24- " it is attractive to hypothesize that the DA synthetic system
activated by saline is of physiologic importance in
modulating the fractional clearance of Na.
RENAL CATECHOLAMINE HANDLING IN THE DOG/Boren et al.
In conclusion, despite the chemical similarity of
NE, E, DA, and NM, the apparent renal clearances of
these substances are distinctly different. Control
clearance of NE and E is less than Cr clearance, while
that of DA approximates, and that of NM exceeds, Cr
clearance. Saline volume expansion increases either
the absolute or fractional clearance of all of these substances. Net production was observed for NE, DA,
and NM in the renal metabolic compartment, suggesting that a portion of these compounds excreted in the
urine may result from intrarenal synthesis or
metabolism of these materials. Saline infusion increases intrarenal production of DA. The urinary excretion rate of NE, E, DA, and NM is modulated by
the arterial delivery of these substances to the kidney
and by variations in apparent renal clearance.
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Acknowledgments
We gratefully acknowledge the surgical assistance of Mary Ann
Neel and the analytical assistance of Carol Robideau and Ronald
Bosher. Dr. Naomi Fineberg provided statistical assistance, and
Nina Kingery provided secretarial help.
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D R Boren, D P Henry, E E Selkurt and M H Weinberger
Hypertension. 1980;2:383-389
doi: 10.1161/01.HYP.2.4.383
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