Autoregulation of the glomerular filtration rate and the single-nephron glomerular filtration rate despite inhibition of tubuloglomerular feedback in rats chronically volume-expanded by deoxycorticosterone acetate


Häberle, D.A.; Königbauer, B.; Davis, J.M.; Kawata, T.; Mast, C.; Metz, C.; Dahlheim, H.

Pflügers Archiv 416(5): 548-553

1990


Tubuloglomerular feedback (TGF) function and autoregulation (renal blood flow RBF; glomerular filtration rate, GFR; single-nephron glomerular filtration rate, SNGFR) were examined in rats chronically treated with deoxycorticosterone acetate (DOCA) and given isotonic saline to drink. DOCA treatment depressed arterial plasma renin activity, expanded plasma volume by 25% and increased arterial blood pressure. Autoregulation of RBF and GFR was maintained in the DOCA animals above 90 mm Hg and 110 mm Hg respectively, whereby both GFR and RBF were lower than in controls. Micropuncture experiments demonstrated the absence of TGF in the DOCA animals. There was no difference between SNGFR values measured in the distal and proximal tubules, nor was there a significant response of SNGFR when loops of Henle were perfused with Ringer's solution at 20 nl/min. Loop perfusion in control rats with tubular fluid collected in DOCA rats elicited a normal TGF response, showing that TGF inhibition in the DOCA animals is due to changes in the function of the juxtaglomerular apparatus. In contrast to control rats, proximal SNGFR was perfectly autoregulated. These results suggest that TGF is not primarily responsible for autoregulation and that the vasodilatation normally resulting from acute TGF interruption is therefore compensated by some other mechanism such that RBF and GFR are lower than in controls.

Pfliigers
Arch
(1990)
416:548
553
Journal
of
Physiology
©
Springer-Verlag
1990
Autoregulation
of
the
glomerular
filtration
rate
and
the
single-nephron
glomerular
filtration
rate
despite
inhibition
of
tubuloglomerular
feedback
in
rats
chronically
volume-expanded
by
deoxycorticosterone
acetate
D. A.
Haberle
l
,
B.
Konigbaner
l
,
J.
M.
Davis',
T.
Kawata
2
,
C.
Mast',
C.
Metz',
and
H.
Dahlheini
l
Physiologisches
Institut
der
Universitat
Munchen,
Pettenkoferstrasse
12,
D-8000
Munich
2,
Federal
Republic
of
Germany
2
Tokyo
Women's
Medical
College,
Kidney
Center,
Kawada-cho
8--1,
Shinjuku-Ku,
Tokyo,
Japan
Received
July
31,
1989
/
Received
after
revision
January
22
/
Accepted
February
8,
1990
Abstract.
Tubuloglomerular
feedback
(TGF)
function
and
autoregulation
(renal
blood
flow
RBF;
glomerular
filtration
rate,
GFR;
single-nephron
glomerular
filtration
rate,
SNGFR)
were
examined
in
rats
chronically
treated
with
deoxycorticosterone
acetate
(DOCA)
and
given
isotonic
saline
to
drink.
DOCA
treatment
depressed
arterial
plasma
renin
activity,
expanded
plasma
volume
by
25
%
and
in-
creased
arterial
blood
pressure.
Autoregulation
of
RBF
and
GFR
was
maintained
in
the
DOCA
animals
above
90
mm
Hg
and
110
mm
Hg
respectively,
whereby
both
GFR
and
RBF
were
lower
than
in
controls.
Micropuncture
experi-
ments
demonstrated
the
absence
of
TGF
in
the
DOCA
animals.
There
was
no
difference
between
SNGFR
values
measured
in
the
distal
and
proximal
tubules,
nor
was
there
a
significant
response
of
SNGFR
when
loops
of
Henle
were
perfused
with
Ringer's
solution
at
20
nl/min.
Loop
perfusion
in
control
rats
with
tubular
fluid
collected
in
DOCA
rats
elicited
a
normal
TGF
response,
showing
that
TGF
inhibi-
tion
in
the
DOCA
animals
is
due
to
changes
in
the
function
of
the
juxtaglomerular
apparatus.
In
contrast
to
control
rats,
proximal
SNGFR
was
perfectly
autoregulated.
These
re-
sults
suggest
that
TGF
is
not
primarily
responsible
for
au-
toregulation
and
that
the
vasodilatation
normally
resulting
from
acute
TGF
interruption
is
therefore
compensated
by
some
other
mechanism
such
that
RBF
and
GFR
are
lower
than
in
controls.
Key
words:
Autoregulation
Tubuloglomerular
feedback
Renal
blood
flow
Glomerular
filtration
rate
Plasma
renin
activity
Deoxycorticosterone
acetate
Plasma
volume
rats
Introduction
Since
the
work
of
Rein
[28]
and
Medes
[20,
21]
more
than
50
years
ago,
it
is
known
that,
within
a
certain
range,
vari-
Offprint
requests
to:
D.
A.
Haberle
ations
of
systemic
blood
pressure
or
renal
perfusion
pressure
are
compensated
by
the
kidney
by
parallel
changes
in
renal
vascular
flow
resistance
so
that
the
renal
blood
flow
(RBF)
and
the
glomerular
filtration
rate
(GFR)
remain
constant
(autoregulation).
Whilst
it
is
generally
agreed
that
regula-
tory
mechanisms
intrinsic
to
the
kidney
are
responsible
for
this
phenomenon
there
is
still
no
general
agreement
on
the
nature
of
the
mechanism.
In
particular,
debate
continues
concerning
the
relative
importance
of
two
regulatory
mech-
anisms:
(a)
a
"myogenic"
mechanism
whereby
increased
transmural
pressure
or
wall
tension
directly
elicits
smooth
muscle
contraction
in
the
preglomerular
renal
vasculature
[2]
and
(b)
a
negative
(metabolic)
feedback
mechanism,
the
so-called
tubuloglomerular
feedback
mechanism
(TGF),
in
which
changes
of
blood
pressure
induce
parallel
changes
in
the
luminal
NaC1
concentration
in
the
macula
densa
segment
of
the
nephron.
These
in
turn
trigger
parallel
changes
in
renal
vascular
flow
resistance
[5,
25].
Immedi-
ately
relevant
to
this
debate
are
findings
from
studies
on
various
states
of
chronic
volume
expansion.
In
these
states
autoregulation
of
GFR
and
RBF
have
been
reported
to
persist
(for
literature
see
below)
whereas
TGF
function
has
been
reported
to
be
attenuated
or
even
abolished
[8,
22,
23,
34].
This
especially
holds
for
the
model
in
which
volume
expansion
is
achieved
by
daily
deoxycorticosterone
acetate
(21-hydroxy-4-pregnene-3,20-dione
acetate;
DOCA)
administration
with
isotonic
saline
to
drink,
for
at
least
2
weeks.
Under
these
conditions
TGF
in
superficial
neph-
rons
is
almost
completely
inhibited
[6]
whereas
RBF
and
GFR
in
both
rats
[1,
17]
and
dogs
[10,
16, 18, 19,
24,
27]
appear
to
remain
normally
autoregulated.
Since
under
these
experimental
conditions
uncontrolled
differences
in
exper-
imental
protocol,
as
well
as
inhomogeneity
of
superficial
and
subcortical
nephron
function,
cannot
be
excluded,
it
was
the
aim
of
the
present
study
to
examine
TGF
function
and
autoregulation
of
GFR
in
superficial
nephrons
in
par-
allel
and
to
relate
these
data
to
the
autoregulation
of
RBF
and
GFR
in
the
whole
kidney.
Experiments
were
performed
in
rats
in
which
the
renal
perfusion
pressure
was
experimentally
lowered
stepwise
be-
low
the
spontaneous
pressure
and
the
effect
of
this
on
TGF,
549
single-nephron
glomerular
filtration
rate
(SNGFR),
GFR
and
RBF
determined.
Materials
and
methods
Dietary
pretreatment
Male
Wistar
rats
(Savo-Invanovas,
Kisslegg,
FRG)
were
divided
into
two
age-
and
weight-matched
groups.
One
group
was
fed
a
control,
low-sodium
diet
(C1324,
Altromin,
Lage,
FRG,
containing
.2
g
kr=
Na)
and
given
tap
water
to
drink;
the
other
received
the
same
diet,
2.5
mg
kg
body
wt.'
DOCA
(Percorten
M,
Ciba-Geigy,
Basel,
Switzer-
land)
s.c.
or
i.m.
daily
on
5
days
per
week
and
9
g
NaC1
to
drink
for
at
least
2
weeks.
Animal
preparation
Rats
weighing
between
180
g
and
230
g
were
anaesthetized
by
Inactin
(Byk-Gulden,
Konstanz,
FRG),
100
mg
kg
body
wt.'
i.p.
Body
tem-
perature
was
maintained
at
approximately
37.5
°C
by
a
heated
oper-
ating
table
controlled
by
feedback
from
a
rectal
thermistor
thermo-
meter.
A
tracheal
cannula
was
inserted
and
the
left
femoral
artery
and
vein
catheterized,
the
artery
to
permit
continuous
recording
of
blood
pressure
by
means
of
a
strain-gauge
transducer
(Statham
model
P
23
Db,
Puerto
Rico)
and
blood
sampling
as
required,
the
vein
to
allow
a
sustaining
infusion
of
a
150
mmol
NaCl
solution
containing
80
gr
i
polyfructosan
(Mutest,
Laevosangesellschaft,
Linz,
Austria)
at
5.0
ml
11
-1
kg
body
wt.'.
The
bladder
was
catheterized
via
a
suprapubic
incision
along
the
linea
alba
and
the
urethra
and
left
ureter
were
ligated.
The
left
kidney
was
exposed
by
a
transverse
flank
incision,
and,
taking
care
not
to
damage
the
nerve
plexus
at
the
renal
hilus,
dissected
free
from
perirenal
fat
and
laid
in
a
Plexiglas
cup.
The
ureter
was
catheterized,
the
kidney
embedded
in
agar
(30
g
=
in
150
mmol
NaC1)
and
the
renal
surface
bathed
with
warmed
(38
°C)
mineral
oil.
Clearance
measurement
of
GFR,
renal
plasma
flow
and
RBF
autoregulation
GFR,
renal
plasma
flow
(RPF)
and
RBF
were
measured
in
normal
and
DOCA/saline
rats,
prepared
as
above,
from
the
inulin
clearance,
the
inulin
extraction
and
the
haematocrit.
After
the
kidney
had
been
placed
in
the
Plexiglas
cup
and
the
ureter
cannulated,
the
aorta
was
carefully
dissected
free
from
surrounding
attachments
as
far
above
the
origin
of
the
left
renal
artery
as
possible
and
an
adjustable
screw
clamp
placed
around
it.
Left
renal
arterial
blood
pressure
was
set
at
one
of
five
nominal
pressures
by
adjustment
of
the
aortic
clamp
and
the
inulin
infusion
begun.
The
pressures
employed
were
130,
115, 105,
90
and
75
mm
Hg;
the
sequence
followed
was
varied
randomly.
After
equilibra-
tion
(30
min)
a
timed
urine
collection
was
begun.
Arterial
and
renal
venous
blood
samples
were
collected
as
described
elsewhere
[6].
Blood
pressure
was
changed
by
adjustment
of
the
clamp
and,
after
a
further
equilibration
period,
the
above
procedure
was
repeated.
Measurement
of
plasma
volume
In
a
separate
series
of
experiments,
plasma
volume
was
measured
in
both
groups
by
the
Evan's
blue
dilution
technique
as
described
else-
where
[6].
Measurement
of
renin
activity
in
arterial
plasma
Renin
activity
was
determined
in
arterial
plasma
in
a
separate
series
on
animals
kept
on
the
dietary
treatment
for
1-6
weeks.
After
pre-
paring
the
rats
as
above
and
allowing
an
equilibration
period
of
about
20
min,
200µl
arterial
blood
was
withdrawn
from
the
femoral
artery
into
heparinized
haematocrit
tubes.
After
separation
the
plasma
was
stored
at
—30°C
(1-3
weeks)
until
assay.
Micropuncture
experiments
In
vivo
activity
of
TGF.
Rats
of
both
groups
were
prepared
as
above.
Following
equilibration,
the
proximal
and
distal
loops
of
a
nephron
were
identified
as
described
earlier
[6].
Using
an
oil-filled
micropipette
(outer
diameter,
8
pm)
a
timed,
quantitative
fluid
collection
was
first
made
from
the
distal
tubule.
Under
these
circumstances
flow
at
the
TGF-sensing
site
(macula
densa)
is
quasi-normal
and,
hence,
the
TGF
control
loop
intact.
Subsequently
a
timed,
quantitative
collection
was
made
from
the
proximal
tubule.
Under
these
circumstances
the
TGF
control
loop
is
interrupted.
The
difference
between
"proximal"
and
"distal"
SNGFR
is
thus
an
index
of
the
in
vivo
activity
of
the
TGF
mechanism.
Loop
of
Henle
perfusion
studies.
"Open-loop"
TGF
activity
was
tested
in
rats
of
both
groups
by
perfusing
loops
of
Henle
with
either
Ringer's
solution
stained
with
FD&C
green,
or
homologous,
late
proximal
tubular
fluid
previously
harvested
in
the
experimental
animal
and
simultaneously
measuring
SNGFR
from
the
proximal
tubule.
This
method
is
described
in
detail
elsewhere
[6,
15].
Perfusion
rates
were
0,
20
and
40
nl/min.
In
any
one
nephron
at
least
three
perfusion
rates,
including
0,
were
tested.
Each
subsequent
collection
was
made
further
upstream
to
avoid
the
possibility
of
leakage.
Autoregulation
experiments.
Rats
from
both
groups
were
prepared
as
above.
After
selecting
one
of
the
nominal
blood
pressures by
adjust-
ment
of
the
aortic
clamp,
a
random
proximal
tubule
was
punctured
and
distal
loops
identified
as
above.
Several
distal-proximal
paired,
quantitative
tubular
fluid
collections
were
made
and
the
blood
pressure
was
then
changed.
After
a
further
equilibration
period,
further
sample
pairs
were
collected.
Analyses
Clearance
experiments.
Urine
volume
was
determined
by
weight,
as-
suming
a
relative
density
of
1.
Inulin
concentration
in
plasma
and
urine
was
measured
by
the
anthrone
method
[13].
Haematocrit
was
meas-
ured
in
all
blood
samples.
Plasma
protein
concentration
was
measured
using
a
commercial
kit
(Total
Protein,
Boehringer
Mannheim,
FRG).
Plasma
Evan's
blue
was
measured
spectrophotometrically
by
measur-
ing
the
difference
in
plasma
absorbances
at
610
nm
and
670
nm
against
a
plasma
blank
and
comparison
with
a
standard
curve
also
prepared
in
homologous
plasma.
Plasma
renin
activity.
Arterial
plasma
renin
activity
(PRA)
was
meas-
ured
enzyme-kinetically
using
the
angiotensin
I
radioimmunoassay
technique.
Plasma
samples
(20-60
/Al)
were
diluted
with
50
,u1
150
mmol
1'
phosphate
buffer
(pH
6.5)
containing
peptidase
blockers.
The
mixture
was
incubated
at
37
°C
for
up
to
42
min,
whereafter
10-
20
pl
of
this
incubation
mixture
was
incubated
with
angiotensin
I
antibody
and
radiolabelled
angiotensin
I
in
phosphate
buffer
at
pH
7.4
for
20
h.
The
final
antibody
dilution
was
1:50000.
The
unbound
octa-
peptide
was
separated
from
the
antigen-antibody
complex
on
dextran-
coated
charcoal
in
phosphate
buffer
and
the
concentration
of
the
un-
bound
tracer
determined
in
a
gamma
counter.
The
initial
slopes
of
the
resulting
curves
were
taken
as
measures
of
renin
activity.
With
this
method
samples
could
be
stored
at
—30°C
for
up
to
3
weeks
with
no
loss
of
activity
(for
details
of
this
method
see
[30]).
Micropuncture
experiments.
The
volume
of
the
tubular
fluid
samples
was
measured
by
injecting
the
sample
into
an
oil-filled,
constant-bore
glass
microcapillary
(Minicap,
0.5
pl,
Hirschmann,
Heilbronn-Eber-
stadt,
FRG)
and
measuring
the
length
of
the
column
by
means
of
an
ocular
micrometer.
Inulin
was
determined
in
plasma
and
urine
by
perchloric
acid
hydrolysis
to
fructose,
which
was
assayed
by
the
hex-
okinase/glucose-6-phosphate
dehydrogenase
method
[4,
29].
Reagents
Re
na
l
bloo
d
f
low
l
m
l/
m
in
ig
K
WI
550
were
obtained
in
kit
form
(Boehringer
Mannheim,
Glucose/Fructose
Kit
139
106)
and
made
up
and
employed
according
to
the
accompanying
instructions,
Inulin
in
tubular
fluid
samples
was
measured
by
a
mi-
croadaptation
of
the
same
method.
Calculations.
All
flows
and
variables
calculated
therefrom
are
expressed
per
gram
kidney
weight.
GFR
is
calculated
from
the
standard
clearance
expres-
sion.
In
the
autoregulation
experiments
RPF
is
calculated
from
the
inulin
extraction
(arteriovenous
concentration
difference).
Given
the
risk
of
retrograde
contamination
of
the
renal
venous
blood
samples
from
the
vena
cava,
the
filtration
fraction
was
calculated
for
each
clearance
period
and
those
periods
with
a
filtration
fraction
less
than
20%
discarded.
Renal
blood
flow
is
calculated
from
RPF
and
arterial
haematocrit.
Plasma
volume
was
calculated
from
the
dilution
of
Evan's
blue.
Nephron
filtration
rates
were
calculated
as
the
product
of
tubular
fluid
flow
rate
and
the
tubular
fluid
to
plasma
inulin
concentration
ratio.
Statistics
Means
are
expressed
±
1
SD
unless
otherwise
indicated.
Data
were
subjected
to
analysis
of
variance
using
a
commercial
computer
software
package
(Statgraph).
The
significance
of
differences
between
means
was
assessed
by
Student's
t-test,
for
paired
or
unpaired
data
as
appro-
priate.
A
probability
level
of
less
than
0.05
was
regarded
as
significant.
Results
Systemic
effects
of
dietary
treatment
The
effects
of
chronic
DOCA/saline
treatment
were
re-
flected
by
a
significant
decrease
in
arterial
PRA
from
114.5
±
82.6
(n=
10)
to
26.8
±
19.3
(n
=
11)
ng
angiotensin
I
ml"
h"
and
significant
rises
in
plasma
volume
from
43.7
±
5.6
nil
kg
body
wt.'
(n
=
5)
to
57.8
±
11.2
ml
kg
body
wt.'
(n=10)
and
in
mean
arterial
blood
pressure
from
109
±
15
mm
Hg
(n
=
12)
to
131
±
14
mm
Hg
(n
=
15).
The
plasma
protein
concentration
was
similar
in
both
groups
(58.2
3.3
g
1
1
,
in
controls
and
61.7
±
3.1
g
in
the
DOCA
group).
Autoregulation
of
GFR,
RBF
and
SNGFR
Figure
1
demonstrates
the
response
of
GFR
and
RBF
to
experimental
reduction
of
mean
renal
arterial
blood
pres-
sure.
Multiple
analysis
of
variance
showed
that
RBF
above
90
mm
Hg
was
neither
related
to
blood
pressure
nor
to
drug
treatment
(F
=
0.85
and
0.01;
P
=
0.48
and
0.93
respec-
tively),
that
is,
that
RBF
in
both
groups
is
autoregulated
and
that
the
DOCA
RBF
values
are
not
significantly
differ-
ent
from
the
control.
Similarly,
GFR
above
90
mm
Hg
was
not
significantly
related
to
blood
pressure
(F
=
0.34,
P
=
0.8),
but
was
significantly
reduced
by
the
DOCA
treatment
(F
=
17.2,
P
=
0.0003).
Autoregulation
of
SNGFR
measured
in
the
proximal
tubule
is
shown
in
Fig.
2.
SNGFR
values
(nl
min'
g
kidney
wt.')
in
control
animals
were
20.9
±
1.1
(n=
18),
29.1
±
1.3
(n
=
46)
,
33.9
±
1.4
(n
=
41),
32.5
±
1.4
(n
=34)
and
40.9
±
1.7
(n=16)
at
blood
pressures
of
75,
90,
105,
115
n.7
n.7
9.0
8,0
n.5
n.5
n.6
7,0
...
n.
B
6.0
5,0
n.5
n=7
4,0
3.0
80
90
100
110
120
130
140
Mean
arterial
blood
pressure
1
ma)
Hg
1
n=7
n=6
1,2
1,0
n=7
0,8
re
U-
0,6
0,4
0,2
80
100
120
140
Mean
arterial
blood
pressure
mm
Hg
1
Fig.
1.
Autoregulation
of
renal
blood
flow
(upper
panel)
and
glomer-
ular
filtration
rate
(GFR;
lower
panel)
in
left
kidneys
of
control
rats
(III)
and
rats
chronically
treated
with
deoxycorticosterone
acetate
(DOCA)
and
given
isotonic
saline
(0)
for
drinking.
Data
are
given
as
means
±
1
SEM
and
120
mm
Hg
respectively.
In
the
DOCA
animals
SNGFR
values
(nl
min'
g
kidney
wt.")
were
17.5
±
1.2
(n=13),
20.1
±
1.3
(n
=
13),
22.0
(n
1)
and
21.0
±
1.4
(n
=
13)
at
blood
pressures
of
90,
105,
110
and
115
mm
Hg
respectively.
Multiple
analysis
of
variance
showed
SNGFR
to
be
depend-
ent
on
both
blood
pressure
and
drug
treatment
(F
=
8.39
and
76.54,
P
<0.001,
P
<0.001
respectively).
Separating
the
drug
treatment
groups
showed
that
in
the
DOCA
ani-
mals
SNGFR
was
not
significantly
related
to
blood
pressure
above
90
mm
Hg
(F
=
1.28,
P
=
0.3);
that
is,
SNGFR
is
autoregulated.
In
contrast,
proximal
SNGFR
in
the
control
rats
showed
a
significant
relationship
to
blood
pressure
above
90
mm
Hg
(F
=
4.11,
P
=
0.0004);
that
is,
control
SNGFR
is
not
autoregulated.
SNGFR
values
in
the
DOCA
group
were
significantly
lower
at
any
blood
pressure
than
in
the
control
rats.
70
1,6
1,4
n=3
n=6
n=7
n=7
n=7
551
SNG
ER
(%
S
NGFRo
)
50
20-
IL
o
10
-
Z
cf)
0
75
90
105
120
Blood
Pressure
[mmHg]
Fig.
2.
Effect
of
experimental
blood
pressure
reductions
on
single-
nephron
glomerular
filtration
rate
(SNGFR)
measured
in
the
proximal
tubule
in
the
absence
of
loop
of
Henle
perfusion
in
control
rats
(0)
and
in
rats
treated
with
DOCA
(•)
and
given
isotonic
saline
for
drinking.
Data
are
given
as
means
±
1
SEM
Analysis
of
TGF
function
Closed-loop
studies.
Figure
3
(right
panel)
shows
that
there
was
no
proximal/distal
SNGFR
difference
in
DOCA
rats.
SNGFR
measured
distally
was
24.8
±
3.6
nl
min'
g
kidney
wt.'
(n=12),
that
measured
proximally
in
the
same
neph-
rons
was
not
significantly
different
(22.8
±
5.4
nl
g
kidney
wt.
-1
).
In
contrast,
in
control
rats
(left
panel)
distal
SNGFR
was
27.6
±
3.2
nl
g
kidney
wt.'
;
(n=15),
a
value
significantly
lower
than
SNGFR
measured
proximally
in
the
same
nephrons
(37.8
±
6.1
nl
g
kidney
wt.
-1
).
There
is,
thus,
no
significant
reduction
of
SNGFR
by
TGF
in
vivo
in
DOCA
rats.
Open-loop
analysis.
The
absence
of
TGF
control
of
SNGFR
in
DOCA
rats
is
further
confirmed
by
loop
of
Henle
per-
fusion
(Fig.
4,
right
panel).
SNGFR
in
the
absence
of
loop
of
Henle
flow
was
34.0
±
12.1
nl
min
-1
g
kidney
wt.
-1
(n
=
19).
Loop
perfusion
with
Ringer's
solution
at
20
nl/min
decreased
SNGFR
insignificantly
by
3.6
±
5.7
nl
min
-1
g
kidney
wt.
-1
(n=9,
91
±
16
%
of
the
value
in
the
absence
of
perfusion).
At
a
perfusion
rate
of
40
nl
min
1
,
SNGFR
decreased
slightly
but
significantly
by
7.4
±
5.7
nl
min
-1
g
kidney
wt.'
to
79
±
12
%
(n
=
19)
of
the
unperfused
value.
In
contrast,
loop
perfusion
with
Ringer's
at
20
and
40
nl
min
-1
in
control
rats
(Fig.
4,
left
panel)
reduced
SNGFR
from
41.3
±
6.2
nl
min
-1
g
kidney
wt.
-1
in
the
absence
of
loop
perfusion
by
15.1
±
5.8
nl
min'
g
kidney
wt.
-1
to
63
±
13
%
(n=20)
and
by
19.5
±
5.9
nl
min'
g
kidney
wt.
-1
to
53
±
12
%
(n
=
20),
respectively.
In
addition,
loop
per-
fusion
in
control
rats
at
40
nl
min
-1
with
proximal
tubular
fluid
harvested
in
DOCA
rats
also
reduced
SNGFR
signif-
icantly
from
35.8
±
3.8
nl
min
1
g
kidney
wt.
-1
by
13.3
±
4.3
nl
min
-1
g
kidney
wt.
-1
to
63
±
12%
(n
=
9).
This
value
does
not
differ
significantly
from
the
corresponding
value
with
Ringer's
perfusion
but
both
values
are
significantly
different
from
those
obtained
in
the
DOCA
rats.
50-
Control
Rats
50-
DOCA
Rats
y
40
-
Y
40-
cn
cn
30-
30-
20
2 .
20-
0
z
u)
10-
z
0
10
U)
0
0
proximal
distal
proximal
distal
Fig.
3.
Effect
of
the
interruption
of
free
flow
loop
of
Henle
perfusion
in
single
nephrons
on
SNGFR
in
control
rats
(left
panel)
and
rats
treated
with
DOCA
and
given
isotonic
saline
for
drinking.
Data
are
shown
as
means
±
1
SEM
100
90
Control
Rats
100-
9
cc
DOCA
-
Saline
Rats
LL
80
z
80
u")
70
ce
70
loop
perfusion
with
60
0
2
60
(f)
Ringer's
TF
from
DOCA
Rats
A
50
50
0
20
40
loop
of
Henle
perfusion
rate
Inl/min)
Fig.
4.
Dependence
of
SNGFR
on
loop
of
Henle
perfusion
rate
in
control
rats
(left
panel)
and
rats
treated
with
DOCA
and
given
saline
for
drinking
during
perfusion
with
Ringer's
or
harvested
tubular
fluid.
Data
are
shown
as
means
±
1
SEM.
TF
,
tubular
fluid;
SNGFR,
single-
nephron
glomerular
filtration
rate
Discussion
In
this
study
three
relevant
effects
of
DOCA/saline
treat-
ment
were
observed.
First,
DOCA/saline
increases
plasma
volume
and
depresses
arterial
PRA
compared
with
controls.
Secondly,
it
practically
eliminates
the
TGF
control
of
SNGFR
at
physiological
flow
rates
and,
thirdly,
it
has
no
significant
effect
on
the
autoregulation
of
GFR
and
RBF
during
experimental
blood
pressure
reduction.
These
three
findings
will
be
discussed
in
the
following.
a)
Consistent
with
numerous
earlier
reports,
DOCA
administration
expanded
plasma
volume
significantly
and,
consequently,
depressed
arterial
PRA.
This
expansion
can
be
related
both
to
an
increased
NaCl
uptake
[36]
due
to
the
stimulation
of
sodium
appetite
[11,
12]
and
to
increased
NaC1
retention
[11]
due
to
the
well-known
stimulation
of
Na,K-ATPase
activity.
b)
Inhibition
of
th
TGF
mechanism
is
revealed
by
two
observations.
First,
under
DOCA
treatment
the
proximal-
distal
SNGFR
difference
disappears.
In
control
rats
SNGFR
determined
by
proximal
fluid
collection
(i.e.,
in
the
absence
of
flow
at
the
macula
densa
and,
hence,
with
the
TGF
loop
interrupted)
is
greater
than
SNGFR
determined
in
the
distal
tubule
(with
normal
flow
at
the
macula
densa
and,
hence,
20
40
552
with
the
TGF
control
intact
and
functioning).
This
differ-
ence
disappears
in
rats
treated
with
DOCA,
implying
that
TGF
control
of
the
SNGFR
in
vivo
has
been
abolished.
Secondly,
DOCA
treatment
markedly
attenuates
the
SNGFR
response
to
experimental
variations
in
flow
rate
in
the
loop
of
Henle.
In
control
rats
loop
of
Henle
perfusion
with
Ringer's
solution
at
rates
of
20
nl
min'
and
40
nl
min
-1
reduces
SNGFR
to
63
%
and
53
%
of
the
value
in
the
un-
perfused
loop
of
Henle.
In
contrast,
in
DOCA
rats
loop
perfusion
with
Ringer's
solution
at
20
nl
min
-1
has
no
sig-
nificant
effect
upon
SNGFR.
Increasing
the
perfusion
rate
to
40
nl
min'
reduces
SNGFR
only
slightly
(to
79
%
of
its
value
in
the
absence
of
loop
perfusion).
Thus,
at
flow
rates
in
the
range
of
the
natural
late
proximal
fluid
delivery
into
the
loop
of
Henle
(below
20
nl
min
-1
),
TGF
cannot
be
stimulated.
This
inhibition
apparently
results
from
changes
within
the
juxtaglomerular
apparatus;
this
conclusion
is
sug-
gested
by
the
observation
that
tubular
fluid
harvested
in
DOCA
rats
stimulates
TGF
in
control
rats
in
a
similar
man-
ner
to
Ringer's
solution.
This
excludes
the
possibility
that
some
inhibitory
substance
is
added
to
the
perfusion
fluid
during
perfusion
of
loop
of
Henle
segments
proximal
to
the
macula
densa.
The
finding
of
almost
complete
TGF
inhibi-
tion
by
DOCA
is
entirely
consistent
with
earlier
studies
of
Dev
et
al.
[8]
and
Schnermann
et
al.
[34],
but
only
partially
consistent
with
observations
of
Moore
and
Mason
[22].
In
this
latter
study,
however,
a
smaller
dose
of
DOCA
was
used
and
the
animals
were
given
hypotonic
saline
(115
mmoi
NaCl)
1
-1
for
drinking.
The
mechanism
of
TGF
resetting
in
the
DOCA/saline
model
of
chronic
volume
expansion
is
not
entirely
clear.
It
may
be
speculated
that
this
results
from
the
combination
of
some
unspecific
effects
of
the
treatment
rather
than
from
specific
effects
of
DOCA
or
volume
expansion.
First,
this
protocol
leads
to
the
expansion
of
extracellular
volume,
the
depression
of
systemic
arterial
PRA
and
to
a
diminution
of
angiotensin-II-receptor
density
in
the
glomeruli
[9]
and
ar-
terioles
[31].
Such
changes
are
known
to
attenuate
the
TGF
responsiveness
[26].
Secondly,
since
very
high
DOCA
doses
were
used
(see
[34])
it
is
likely
that
a
fraction
of
the
DOCA
is
converted
by
the
mitochondrial
114-hydroxylase
in
cor-
ticosterone,
the
principal
glucocorticoid
in
the
rat.
If
this
is
so,
and
blood
glucocorticoid
concentration
were
to
increase,
prostaglandin
synthesis
would
be
inhibited,
a
state
in
which
glomerular
arterioles
are
dilated
[3]
and
the
TGF
response
attenuated
[33,
35],
but
in
which
autoregulation
is
main-
tained
[17].
c)
The
most
intriguing
observation
in
this
study
is
the
maintenance
of
autoregulation
of
SNGFR
during
experi-
mental
blood
pressure
reduction
despite
inhibition
of
TGF.
Besides
the
fact
that
this
finding
is
consistent
with
the
main-
tenance
of
autoregulation
of
RBF
and
GFR
under
compa-
rable
conditions,
this
finding
allows
three
conclusions
to
be
drawn.
First,
autoregulation
is
due
to
mechanisms
other
than
TGF.
Second,
SNGFR
in
normal
rats
is
increased,
and
its
autoregulation
attenuated,
when
loop
of
Henle
flow
is
interrupted.
In
DOCA
rats,
however,
proximal
SNGFR
is
autoregulated
and
similar
to
that
measured
in
the
distal
tubule
(and
to
distal
SNGFR
in
normal
rats).
It
thus
appears
as
though
the
increase
in
SNGFR
in
normal
rats
on
blockade
of
loop
flow
causes
the
attenuation
of
autoregulation
in
those
animals
rather
than
that
DOCA
creates
a
new
capacity
for
autoregulation
of
SNGFR
in
the
DOCA
animals.
This
is
consistent
with
previous
results
from
our
laboratory
[7]
and
Schnermann
and
Briggs
[32].
In
normal
rats
SNGFR
or
stop-flow
pressure
remains
autoregulated
when
the
loops
of
Henle
are
microperfused
at
40
n1
min
-1
,
whereas
in
the
absence
of
loop
flow
autoregulation
is
attenuated.
Since
any
regulatory
influence
of
TGF
on
GFR
is
eliminated
under
both
conditions
(i.e.
in
the
absence
of
loop
flow
and
during
perfusion
at
the
supramaximal
rate
of
40
nl
min"),
it
follows
that
the
attenuation
of
autoregulation
results
simply
from
preglomerular
vasodilatation
rather
than
from
elimination
of
a
specific
autoregulatory
mechanism
(see
[14]).
Finally,
this
view
is
further
substantiated
by
the
persistence
of
"nor-
mal"
GFR,
RBF
and
SNGFR
values
in
the
absence
of
TGF
control
in
the
COCA
animals.
Acute
elimination
of
TGF
control
in
normal
rats
is
accompanied
by
increases
in
GFR
and
glomerular
blood
flow
and,
hence,
a
decrease
in
vas-
cular
flow
resistance.
In
DOCA
rats
this
decrease
seems
to
be
compensated
by
another,
non-TGF
mechanism.
This
conclusion
is
consistent
with
previous
observations
on
rats
kept
on
a
high-salt
diet
[6].
As
in
the
present
case,
TGF
control
was
eliminated
and
intrarenal
vascular
resistance
redistributed
to
other
preglomerular
vascular
segments.
In-
terestingly,
in
these
rats
'proximal'
SNGFR
was
also
auto-
regulated,
as
in
the
present
study.
With
this
concept,
some
of
the
earlier
findings
of
Moore
et
al.
[23]
might
be
explained.
Although
these
authors
used
the
same
protocol
as
in
the
present
study,
they
did
not
detect
autoregulation
of
proximally
or
distally
measured
SNGFR.
However,
if
the
reported
SNGFR
values
are
referred
to
the
(estimated)
kidney
weight,
the
resultant
SNGFR
values
are
substantially
higher
than
those
in
the
present
study,
sug-
gesting
that
the
kidneys
in
that
study
may
have
been
con-
siderably
more
vasodilated
than
in
the
present
study.
In
addition,
these
authors
measured
SNGFR
at
only
two
pres-
sures
("high"
and
"low"),
the
high
values
being
not
only
arbitrarily
chosen
but
also
different.
For
DOCA
the
level
was
150
mm
Hg;
for
the
controls
135
mm
Hg.
Without
prior
knowledge
of
the
whole
kidney
flow
(GFR
or
RBF)/pres-
sure
relationship,
there
is
considerable
risk
of
missing
the
inflection
points
in
the
autoregulation
curve
and
thus
(er-
roneously)
concluding
that
autoregulation
is
absent.
Finally,
the
possibility
cannot
be
excluded
that
differences
in
the
time
course
of
the
development
of
vasoconstriction
[37]
or
unknown
differences
in
preparation
or
protocol
may
have
contributed
to
the
discrepancies
between
the
two
studies.
Acknowledgements.
The
authors
gratefully
acknowledge
the
support
of
the
Deutsche
Forschungsgemeinschaft
through
grants
Ha
775/9,
Da
210/1-1
and
Da
93/8-6.
Dr.
Kawata
was
the
recipient
of
a
Research
Fellowship
from
the
Deutscher
Akademischer
Austauschdienst;
Ms
B.
Konigbauer
is
a
scholar
of
the
Deutsche
Studienstiftung
and
is
sup-
ported
by
a
doctoral
scholarship
from
the
University
of
Munich.
The
authors
also
gratefully
acknowledge
the
expert
technical
assistance
of
Mrs.
E.
Eyl
and
Ms
M.
Stachl.
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