Ear!v Human Development, 9 (1984) 347-361 Elsevier 347 EHD 00572 The renal response to acute asphyxia in spontaneously breathing newborn lambs * Barbara S. Stonestreet, Abbot R. Laptook, Sharon R. Siegel and William Oh Department of Pediotrrcs. Women and Infants Hospital of Rhode Island. and The Program in Medicine. Brown Universrty, Providence, RI. U.S.A. Accepted for publication 2 February 1984 Summary The effects of acute asphyxia on neonatal renal function were examined in spontaneously breathing newborn lambs. 25 min of asphyxia were induced by addition of a respiratory dead space to reduce PaO, to 41 + 3 mm Hg, pH to 7.03 + 0.05 and increase PaCOz to 68 + 5 mm Hg (mean + S.E.M.). Glomerular filtration rates did not change significantly during or after asphyxia. Immediately following asphyxia significant (P < 0.05) increases over the baseline were found in urinary flow rates, fractional sodium excretion, absolute sodium excretion and osmolar clearances. These changes were in part secondary to significant (P < 0.05) increases in plasma glucose concentrations associated with increases (P < 0.05) in circulating arterial catecholamine concentrations. The percentage of tubular reab- sorption of glucose decreased significantly (P < 0.05) and urinary glucose excretion increased significantly. Renal blood flow was unchanged. Therefore, asphyxia in- duced significant hyperglycemia which contributed to the concomitant natriuresis and osmotic diuresis in these newborn lambs. kidney; newborn lamb; asphyxia; osmotic diuresis; hyperglycemia Introduction Perinatal asphyxia is a major cause of neonatal morbidity in newborn infants. Compromised renal function has been reported to result from perinatal asphyxia or * Supported by the Basil O’Connor ‘Starter Research Grant No. 5-256 and in part by the Charles H. Hood Foundation. Address/or reprint requests: Barbara S. Stonestreet. M.D., Women and Infants Hospital of Rhode Island. 50 Maude Street. Providence, Rhode Island 02908-9976. U.S.A. 0378-3782/84/$03.00 0 1984 Elsevier Science Publishers B.V
Transcript
Ear!v Human Development, 9 (1984) 347-361 Elsevier
347
EHD 00572
The renal response to acute asphyxia in spontaneously breathing newborn lambs *
Barbara S. Stonestreet, Abbot R. Laptook, Sharon R. Siegel and William Oh
Department of Pediotrrcs. Women and Infants Hospital of Rhode Island. and The Program in Medicine. Brown Universrty, Providence, RI. U.S.A.
Accepted for publication 2 February 1984
Summary
The effects of acute asphyxia on neonatal renal function were examined in spontaneously breathing newborn lambs. 25 min of asphyxia were induced by addition of a respiratory dead space to reduce PaO, to 41 + 3 mm Hg, pH to 7.03 + 0.05 and increase PaCOz to 68 + 5 mm Hg (mean + S.E.M.). Glomerular filtration rates did not change significantly during or after asphyxia. Immediately following asphyxia significant (P < 0.05) increases over the baseline were found in urinary flow rates, fractional sodium excretion, absolute sodium excretion and osmolar clearances. These changes were in part secondary to significant (P < 0.05) increases in plasma glucose concentrations associated with increases (P < 0.05) in circulating arterial catecholamine concentrations. The percentage of tubular reab- sorption of glucose decreased significantly (P < 0.05) and urinary glucose excretion increased significantly. Renal blood flow was unchanged. Therefore, asphyxia in- duced significant hyperglycemia which contributed to the concomitant natriuresis and osmotic diuresis in these newborn lambs.
Perinatal asphyxia is a major cause of neonatal morbidity in newborn infants. Compromised renal function has been reported to result from perinatal asphyxia or
* Supported by the Basil O’Connor ‘Starter Research Grant No. 5-256 and in part by the Charles H. Hood Foundation. Address/or reprint requests: Barbara S. Stonestreet. M.D., Women and Infants Hospital of Rhode Island. 50 Maude Street. Providence, Rhode Island 02908-9976. U.S.A.
hypoxemia in newborn infants [2,6,9,11,12,17,18,24,25]. Renal complications range from minor fluid and electrolyte abnormalities to overt renal failure. Normal [22] to reduced [6,12] glomerular filtration rates have been reported following these peri- natal insults. Other diverse renal abnormalities, such as oliguria, polyuria, increased and decreased electrolyte excretion have been reported to result from perinatal asphyxia [2,6,9,12]. Therefore, studies of renal function in human infants have yielded conflicting results. The etiology of these heterogeneous renal functional responses is probably multifactoral, including: the gestational and postnatal ages of the newborn infants, degree and duration of asphyxia, therapeutic measures applied, hormonal changes occurring and changes in renal hemodynamics [2,6,9,11,12,17,24,25,27].
The main objective of the current study was to examine the effects of a well demarcated acute period of asphyxia on renal functional changes in awake, sponta- neously breathing newborn lambs. In order to explore some of the mechanisms of change in renal function associated with asphyxia, glomerular and tubular functions along with renal hemodynamic and hormonal changes were studied.
Materials and Methods
37 spontaneously delivered mixed breed newborn lambs were the subjects of this study. All lambs remained with the ewes until the morning of study. At the time of study (4.4 + 0.5 days of age, mean f S.E.M.). the lambs weighed 4.3 + 0.2 kg (mean + S.E.M.).
The lambs were divided into four groups: Groups I (n = 10) and II (n = 15) were subjected to asphyxia, with measurements of renal functions in one group (Group I) and renal blood flow in the other (Group II). The remaining two groups were sham-control: One group (Group III, n = 7) was for renal functions and the other (Group IV, n = 5) was for renal blood flow determinations. The renal blood flow determinations in Groups II and IV were from previously studied newborn lambs
v31. All surgical procedures on lambs were performed under local anesthesia (1%
xylocaine). The urinary bladder was catheterized in female lambs via the urethra (Foley, 8-12 French). In male lambs, the urinary bladder was isolated under sterile conditions and percutaneously catheterized through a small midline incision. The bladder catheter was then tunneled onto the flank of the lamb, wrapped in gauze and protected by a stockinette. These lambs received ampicillin (100 mg/kg) intramuscularly and were returned to the ewes for 48 h to permit healing of the bladder until the morning of study, when all vascular catheters were placed acutely as described below: For Groups I and III (renal function groups) polyethylene catheters (Argyle, 5 French) were placed in the jugular vein for infusion of [i4C]inulin and another in the descending aorta via the femoral artery for arterial sampling and measurement of mean systemic arterial pressure and heart rate, In Groups II and IV (renal blood flow study groups), the left ventricular catheter was
349
placed for microsphere injection, a descending aortic (via the femoral artery) catheter was placed for blood withdrawal, and a femoral vein catheter was inserted for blood replacement. All procedures were performed on an infant radiant warmer (Airshield, Inc., Narco, Hatboro, PA) and the lambs’ body temperature maintained between 38 and 39°C.
Following placement of the vascular catheters all lambs were permitted to recover for one hour. For Groups I and III a priming dose of [‘4C]inulin (0.75 PCi) was infused and followed by a constant infusion of 0.03 PCi in 5% dextrose and 0.2% NaCl at a rate of 0.05 ml/kg per min using a calibrated infusion pump (Holter. Extracorporal Medical Specialities, Inc., King of Prussia, PA). The infusion was continued for 1 h prior to the onset of the studies to allow for equilibration of the plasma inulin concentration. After the period of recovery from surgery and equi- libration of inulin, urine was collected for 20 min along with baseline data. After the baseline period, a cuffed endotracheal tube (3.5 mm, American Hospital Supply. McGraw-Hill Park, IL) was placed. Asphyxia was induced in the lambs by adding a dead space of 38 5 2 ml/kg (mean + S.E.M.) to the endotracheal tubes for 25 min. During the period of the asphyxial insult. urine was collected for 20 min. At the termination of asphyxia, the dead space was removed from the endotracheal tubes and the animals were allowed to recover. During the post asphyxial period, three 20-min urine collections were obtained i.e., immediately, 60 and 120 min following the termination of asphyxia. The lambs in the sham operated control groups (III and IV) were treated in exactly the same manner, however the dead space was not added to the endotracheal tubes. All lambs were breathing spontaneously throughout the studies. At the termination of the study, the lambs were killed with a dose of sodium thiamylal and a solution of potassium chloride. An autopsy was performed for verification of catheter placement and removal of the kidneys.
Arterial blood samples (2 ml) were obtained at the midpoint of each 20-min urine collection for plasma concentrations of [ “C]inulin, sodium, glucose and plasma osmolality. Arterial blood hematocrit, PaO,, PaCO, and pH were also measured. After removal of the plasma, the animal’s own packed cells were resuspended in an equal volume of intravenous fluid (5% dextrose and 0.2% NaCl) and returned to the animal. At the end of the collections, urine was obtained for volume, concentrations of [‘4C]inulin, osmolality, sodium and glucose. Concentrations of sodium were determined by flame photometry (Beckman Instruments. Fullerton, CA) and of glucose by the glucose oxidase method on a YSI Glucose Analyzer 23A (Yellow Springs, OH). Osmolalities of plasma and urine were determined on a vapor pressure osmometer (Wescor, Logan, UT). Concentrations of [‘4C]inulin were determined by liquid scintillation counting on a Searle Analytic Mark III Well Counter (Chicago, IL). Arterial PaO,, PaCO, and pH were measured using appropriate electrodes on a Corning Blood Gas Analyzer No. 165 (Corning Scientific Instruments, Medford, MA). Hematocrits were measured by the microhematocrit method. Mean systemic arterial pressure and heart rate were monitored continuously using a Bentley Trantec pressure transducer and recorded on a Hewlett Packard (7700 series) polygraph.
Plasma renin activity and catecholamine concentrations (epinephrine and norepinephrine) were determined during the baseline, asphyxial and at the end of
350
the post-asphyxial (recovery) periods. Blood samples for plasma renin activity were collected in chilled tubes containing EDTA, kept on ice and centrifuged at 4’C. Plasma renin activity was determined by radioimmunoassay [13]. Blood samples for plasma catecholamine concentrations were collected in chilled tubes, placed on ice and centrifuged at 4O C and the plasma supernatant removed and frozen at - 70 ‘C until assayed. Norepinephrine and epinephrine were determined using a radioen- zymatic assay (Cat-a-kit, Upjohn) as previously described [19].
Inulin, osmolar and free water clearances, sodium, and glucose excretions and filtered glucose and absolute tubular glucose reabsorption were calculated by con- ventional formulae and expressed per kg body weight. Fractional sodium excretion and tubular reabsorption of glucose were expressed as the percentage of the amount filtered.
In Groups II and IV, renal blood flow was determined by the microsphere method at the end of the baseline, asphyxial and post asphyxial study periods as previously described [23]. Renal blood flow was measured with 15 + 5 pm diameter microspheres labelled with one of the following radionuclides: 46Sc 95Nb “Cr “Sr or i13Sn (New England Nuclear, Boston, MA). The isotope sequence was randomly assigned for the blood flow determinations. Approximately 8.5 X lo5 microspheres suspended in 2.5 ml of a solution of 0.01 Tween 80 and 20% Dextran were injected within 45 s into the left ventricle and flushed with 3 ml of 5% dextrose and 0.2% NaCl. A reference sample of aortic blood was collected continuously for 2 min beginning 5 s prior to the injection of microspheres, at a rate of 3.88 ml/min using a constant withdrawal pump (Harvard Apparatus, Millis, MA) [14]. At the end of the study the kidneys were removed, weighed and sectioned into 0.3-1.2 g pieces placed in counting vials containing 1 ml of 10% formalin. The radioactivity from the kidney samples and reference blood samples was measured in a gamma well counter (Packard Autogamma Scintillation Spectrometer, Packard Instrument Company, Downers Grove, IL) [23]. All kidney sections and reference blood samples contained at least 800 microspheres [14].
Blood losses because of sampling were replaced with equal quantities of blood from the ewes which had been drawn into polyethylene bags containing CPD anticoagulant solution (Travenol Laboratory, Inc., Deerfield, IL), within 24 h prior to study and stored at 4” C. It has been previously shown that exchange transfusions with adult blood increases cardiac output in newborn lambs [16]. Therefore, we compared renal blood flow during the baseline, asphyxial and post-asphyxial periods from 10 lambs that received adult blood and a separate group of 5 lambs in whom blood sampling was limited and adult blood not given; no differences in renal blood flow were found (231.
Variables within each group were compared by analysis of variance for repeated measurements. If a significant difference was found, the Dunnet’s multiple range f-test was used to compare the means to the baseline values [28]. Baseline values between the groups were compared by the unpaired Student l-test. All values were expressed as mean f standard error of the mean.
35 I
Results
Table I summarizes the arterial blood gas changes of Groups I and III. The baseline values were similar between the two groups. After the addition of the dead space, the experimental lambs showed the expected changes in arterial pH, PaCO, and PaO,, and base excess. All blood gas values within the experimental group (Group I) returned to baseline values 130 min after removal of the respiratory dead space. The base excess remained significantly (P < 0.01) below the baseline im- mediately after the termination of asphyxia in the experimental lambs (Group I). The sham control lambs (Group III) showed no changes in the arterial blood gases during the study. The blood gas determinations of the experimental lambs (II) and the sham-operated control lambs (Group IV) in whom renal blood flow was measured have previously been reported [23]. As seen in Table IA, the values were comparable to those shown in Table I for Groups II and IV, respectively.
Mean systemic arterial blood pressure and heart rate did not change in any group during the entire study, and baseline differences among the groups were not noted. During the study periods in Group I, mean arterial blood pressures were as follows: baseline, 81 f 4 mm Hg; asphyxia, 92 + 5 mm Hg; and recovery periods, 85 + 5, 79 + 5, and 78 & 7 mm Hg, respectively; the results were similar for the experimental animals in whom renal blood flow was determined. Baseline values for the sham- control lambs were: Group III: 79 + 4 mm Hg and IV: 91 k 4 mm Hg, no changes from baseline were observed during the studies. During the study periods in Group I, the lambs’ heart rates were as follows: baseline, 230 + 11; asphyxia, 240 + 12; and recovery periods, 227 k 9, 222 1 15 and 222 + 10 beats/mm, respectively; the results were similar for Group II lambs. Baseline values for the sham-control lambs were: Group III: 259 + 4 and IV 246 + 5, no changes from baseline were observed during the studies. The baseline hematocrit values were similar among the four groups (mean 33-36s). No changes were noted in hematocrit values of Groups I, III, and IV; at the end of the study, Group II lambs had lower hematocrit values than baseline (35 _+ 1 vs. 33 k 1, P < 0.05).
The effects of asphyxia on plasma arterial concentrations of sodium, glucose and osmolality are summarized in Table II. Plasma sodium concentrations were unal- tered during and following asphyxia in Group I; however, the sham-operated control lambs demonstrated small significant (P < 0.05) decreases in plasma sodium con- centrations during the two final sham-recovery periods, presumably resulting from intravenous fluid administration (5% dextrose and 0.2% NaCl). Significant (P < 0.05) increments in plasma arterial glucose concentrations were observed during and 10 min following asphyxia, plasma osmolality increased significantly (P < 0.05) during asphyxia.
The effects of acute asphyxia on urinary flow rates, glomerular filtration rates (inulin clearance), urinary osmolality, and osmolar and free water clearances are summarized in Table III for Groups I and III lambs. In Group I, a diuresis, as evidenced by a significant (P < 0.05) increase in the urinary flow rates, was observed for 20 min following asphyxia, 1 h following the removal of the respiratory dead space and the termination of asphyxia, the urine flow rates returned to pre-asphyxial
TAB
LE
I
Arte
rial
bloo
d ga
s va
riabl
es
in
the
new
born
la
mbs
in
whi
ch
rena
l fu
nctio
ns
wer
e de
term
ined
Expe
rimen
tal
grou
p I
(10)
Bas
elin
e A
sphy
xia
min
: 10
35
45
Con
trol
grou
p II
I (7
)
Rec
over
y B
asel
ine
Sham
-asp
hyxi
a Sh
am-re
cove
ry
60
130
190
10
35
45
60
130
190
PH
PaO
,
(mm
H
g)
PaC
O,
(mm
H
g)
Bas
e ex
cess
(m
equi
v./l)
7.37
It
0.01
79
lt2
31
+2
-4
+2
7.10
*
+ 0.
05
7.03
*
+ 0.
05
42 *
f4
41 *
76
+3
42
62 *
68
*
f5
+5
-11*
+2
-1
3 *
f2
7.28
?I
0.0
3
32
+2
-1o*
12
7.34
+
0.02
75
+_3
31
51
-7
rt2
7.36
f
0.02
75
+3
31
+I
-7
*1
7.41
*
0.03
81
*5
29
-t3
-5
-t2
7.41
+
0.03
76
+6
25
+_3
-8
-8
_+2
+-2
7.41
+
0.04
77
+6
25
+3
7.40
1.
42
0.03
?I
0.0
3
73
+_5
75
k4
26
28
+1
kl
-1
_+2
-5
+1
7.43
kO
.04
74
+_5
29
t2
-4
+3
Val
ues
are
the
mea
n &
S.E
.M.
Num
ber
of a
nim
als
in p
aren
thes
es.
* P
< 0.
05
com
pare
d to
bas
elin
e.
iom I +I
mm I +I
mm I +I
a rr I +I
mm I +I
r-m I +I
mcr I +I
mm I +I
* 0 FN I +I
*
2N I +I
*
2!, I +I
PN I +I
5
354
TAB
LE
III
The
effe
ct
of a
sphy
xia
on
urin
ary
flow
ra
tes,
glom
erul
ar
filtra
tion
rate
s, ur
inar
y os
mol
ality
. os
mol
ar
and
free
w
ater
cl
eara
nces
in
the
ne
wbo
rn
lam
bs
Expe
rimen
tal
grou
p I
(10)
C
ontro
l gr
oup
111
(7)
Bas
elin
e A
sphy
xia
Rec
over
y B
asel
ine
Sham
- Sh
am-re
cove
ry
asph
yxia
min
: 10
35
60
13
0 19
0 10
35
60
13
0 19
0
Urin
ary
flow
ra
te
(mI/m
in
per
kg
body
w
eigh
t)
0.01
8 0.
046
0.06
8 *
0.02
4 0.
017
f 0.
003
f 0.
011
i 0.
023
+ 0.
003
* 0.
003
0.02
6 +
0.00
6 0.
028
0.02
4 +
0.00
6 k
0.00
6 0.
021
f 0.
003
0.02
1 t
0.00
4
Glo
mer
ular
fil
tratio
n ra
te
(ml/m
in
per
kg
body
w
eigh
t)
1.70
2.
46
1.63
1.
42
1.45
1.
54
1.45
1.
57
1.36
1.
45
k 0.
32
f 0.
32
+ 0.
28
* 0.
24
+ 0.
40
kO.1
5 i_
0.24
*
0.22
+
0.22
+0
.20
Osm
olar
cl
eara
nce
(ml/m
in
per
kg
body
w
eigh
t)
0.02
9 0.
064
0.08
5 *
0.03
5 0.
025
* 0.
004
+_0.
015
f 0.
026
+_0.
005
f0.0
07
0.03
9 fO
.O1
0.04
2 0.
038
10.0
1 io
.01
0.03
2 f
0.00
7 0.
028
+_ 0.
006
Urin
ary
osm
olal
ity
(mos
m/k
g H
,O)
418
411
382
404
399
454
+38
+16
k26
123
i31
i 58
42
5 45
6 *4
7 *4
8 42
7 +5
3 41
9 +5
6
Free
w
ater
cl
eara
nce
(ml/m
in
per
kg
body
w
eigh
t)
- 0.
011
-0.0
19
-0.0
18
-0.0
11
- 0.
009
+ 0.
003
+ 0.
004
f d.
006
* 0.
002
i 0.
003
- 0.
014
k 0.
006
- 0.
014
- 0.
014
* 0.
005
i 0.
005
-0.0
11
i 0.
007
- 0.
006
f0.0
04
All
valu
es
liste
d at
the
m
idpo
int
of t
he
urin
ary
colle
ctio
n pe
riods
. V
alue
s ar
e m
ean
+ S.
E.M
. N
umbe
r of
ani
mal
s in
par
enth
eses
. *
P <
0.05
co
mpa
red
to b
asel
ine.
TABL
E IV
The
effe
ct o
f as
phyx
ia o
n re
nal
tubu
lar
func
tions
in
new
born
lam
bs
Expe
rimen
tal
grou
p I
Base
line
Asp
hyxi
a Re
cove
ry
mm
: 10
35
60
13
0 19
0
z Q\
Cont
rol
grou
p III
Base
line
Sham
- Sh
am-re
cove
ry
asph
yxia
-
- 10
35
60
13
0 19
0
Sodi
um
0.80
3.
24
5.88
*
+0.2
5 +
1.22
+
2.53
(1
0)
(10)
(1
0)
1.35
1.
11
+ 0.
29
f 0.
29
(10)
(1
0)
3.04
*
+1.1
0 (1
0)
0.82
+0
.16
(10)
0.66
*0
.13
(10)
4.42
2.
75
f 0.
76
f 0.
42
(9)
(9)
2.26
f
0.87
(9
)
3.63
*
2.56
+
0.60
kO
.39
(9)
(9)
2.45
2.
36
f 0.
92
* 0.
39
(8)
(7)
97.7
97
.0
85.5
*
*1.7
+1
.4
k4.5
(8
) (9
) (9
)
93.3
f
3.0
(9)
0.20
+
0.09
(9
)
96.1
f
2.7
(8)
0.04
0.
23
0.79
*
* 0.
03
kO.1
0 +
0.32
(8
) (9
) (9
)
0.07
*
0.04
(8
)
0.96
1.
1 0.
91
f 0.
20
(7)
0.72
f0
.14
(7)
0.55
f0
.13
(7)
0.51
fO
.ll
(7)
2.74
2.
05
f 0.
36
kO.5
1 (5
) (6
)
2.71
2.
02
f 0.
36
+0.5
0 (5
) (6
)
98.6
99
.0
+0.4
f0
.3
(5)
(6)
0.06
0.
04
f 0.
03
f 0.
02
(7)
(7)
0.57
f
0.09
(7
)
0.39
*
0.05
(7
)
1.93
kO
.52
(7)
2.09
f
0.58
(6
)
99.2
+0
.1
(6)
0.04
*
0.02
(6
)
excr
etio
n (n
EQ/m
Wkg
bo
dy w
eigh
t)
Frac
tiona
l so
dium
ex
cret
ion
(S o
f fil
tere
d)
Filte
red
gluc
ose
(mg/
min
per
kg
body
wei
ght)
Abs
olut
e tu
bula
r re
abso
rptio
n of
gl
ucos
e (m
g/m
in p
er
kg b
ody
wei
ght)
Tubu
lar
reab
sorp
tion
of g
luco
se
(W o
f fil
tere
d)
Urin
ary
gluc
ose
excr
etio
n (m
g/m
in p
er k
g bo
dy w
eigh
t)
kO.1
8 (7
) f
0.3
(7)
0.85
0.
96
* 0.
54
+0.3
0 (1
0)
(10)
0.45
0.
78
* 0.
09
f 0.
26
(7)
(7)
3.08
kO
.81
(9)
6.16
*
+1.1
1 (9
)
2.44
2.
32
f 0.
40
f 0.
40
(7)
(7)
2.40
5.
93 *
f
0.47
+
1.06
(8
) (9
)
2.25
*
0.53
(7
)
96.0
97
.2
*1.3
f0
.9
(7)
(7)
0.08
f
0.03
(7
)
0.07
f
0.03
(7
)
All
valu
es f
isted
at
the
mid
poin
t of
the
urin
ary
colle
ctio
n pe
riods
. Va
lues
are
the
mea
n +
S.E.
M.
Num
ber
of d
eter
min
atio
ns
in p
aren
thes
es.
* P
< 0.
05 c
ompa
red
to b
asel
ine.
357
levels. Glomerular filtration rates (inulin clearance) did not change significantly during and after asphyxia. A significant (P < 0.05) increase in osmolar clearances also occurred during the 20 min following asphyxia. However, urinary osmolality did not change during or after asphyxia. Free water clearances were also not altered during the asphyxial or post asphyxial periods. No changes in urinary flow rates, glomerular filtration rates, osmolar clearances, urinary osmolality and free water clearances were observed during the equivalent study periods in the sham controls,
The effect of acute asphyxia on renal tubular functions is summarized in Table IV. Absolute and fractional sodium excretions increased significantly (P -C 0.05) during the 20 min following the asphyxia insult. Filtered glucose increased signifi- cantly (P -Z 0.05) during asphyxia and absolute tubular reabsorption of glucose increased (P < 0.05) during and following asphyxia. Whereas, the percentage of glucose reabsorbed by the tubule decreased and excreted glucose increased signifi- cantly (P < 0.05) following asphyxia. These changes were not observed in the sham-control lambs.
Table V summarizes the effect of asphyxia on plasma renin activity and catecholamine concentrations in Groups I and III. Asphyxia did not result in alterations in plasma renin activity. However, significant (P < 0.05) increments were seen in plasma arterial catecholamine (epinephrine and norepinephrine) concentra- tions during asphyxia. During the recovery period, these values returned to baseline. There were no changes in plasma renin activity or catecholamine concentrations in the sham operated controls during the studies.
As seen in Table VI, asphyxia did not result in significant changes in renal blood flow during or after asphyxia in the experimental lambs (Group II). There were no changes in renal blood flow in Group IV during the studies.
TABLE V
The effect of asphyxia on plasma renin activity and epinephrine and norepinephrine concentrations in the newborn Iambs
Values are the mean + S.E.M. Number of animals in parentheses.
Discussion
The primary goal of this study was to investigate the effect of asphyxia on renal function in awake, spontaneously breathing newborn lambs and to examine possible mechanisms responsible for the changes observed. During asphyxia the newborn lambs showed the expected arterial blood gas changes. The changes in renal function demonstrated in these lambs were a result of the combined effect of metabolic and respiratory acidemia, hypoxemia and hypercarbia. Because hypoxemia, hypercarbia and acidemia occurred simultaneously, the relative contributions of each arterial blood gas change cannot be assessed in these lambs. However, this type of insult is similar to that found in human infants experiencing perinatal asphyxia. Further- more, the acute changes in renal function in the study lambs occur as a result of normotensive asphyxia.
The increases in arterial plasma glucose concentrations and osmolality during asphyxia in these newborn lambs are consistent with previous findings during hypoxia [4,27]. Fluid shifts from the extracellular-intravascular into the intracellular compartments because of increases in intracellular osmotic pressure [4]. The mod- erate increases in plasma arterial glucose concentrations were associated with increased plasma arterial catecholamine concentrations. Elevations in catecholamine concentrations result in accelerated glucose delivery from the liver and reductions in systemic glucose clearance [7,20]. Increases in plasma osmolality due to hyper- glycemia have been previously reported in well fed animals [4]. Moreover, our lambs were receiving glucose administration (approximately 2.5 mg/kg per min) during the studies.
In’the fetal lamb partial umbilical cord occlusion is associated with a decrease in glomerular filtration rates (GFR), whereas intermittent umbilical cord occlusion resulted in no change in GFR [8,10]. Recovery from umbilical cord occlusion and fetal hypoxemia in late gestation fetal lambs results in an increase in GFR [10,21]. Glomerular filtration rates remain unaltered in newborn piglets during asphyxia [l]. Thus, our results in newborn lambs are similar to those in piglets and differ from findings in fetal lambs following partial umbilical cord occlusion and hypoxemia. Renal blood flow and plasma renin activity did not change significantly during
asphyxia. Because of the slight non-significant decrease in renal blood flow (Table
VI) and increases in GFR (Table III) and renin activity (Table V) during asphyxia, vasoconstriction of the efferent arterioles cannot be excluded. Moreover, it has been previously demonstrated that newborn puppies respond to renal artery infusions of epinephrine with renal vasoconstriction [15]. The renal vasculature is sensitive to sympathetic activation and elevated norepinephrine levels affect both the afferent and efferent arterioles. The renal response to elevated circulating catecholamine concentrations resulting from asphyxia probably differs considerably from the direct effects produced by epinephrine administration into the renal circulation. The reason for the lack of a significant reduction in renal blood flow associated with a marked elevation in plasma catecholamine concentrations in these lambs remains unclear. In newborn piglets, renal blood flow decreased following asphyxia of a longer duration [l]. The lack of reduction in renal blood flow may relate to the shorter duration of exposure to asphyxia and/or species differences.
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The increase in urinary flow rates in the experimental lambs is consistent with previous findings in newborn piglets [l] and fetal sheep [&lo]. Fractional and absolute sodium excretions and osmolar clearances increased in our lambs. These findings are also consistent with results in human neonates [6], newborn piglets [l], fetal sheep [8,10,21], and adult dogs [26]. These changes occurred concomitantly with a significant increase in arterial plasma catecholamine concentrations in the new- born lambs. Increases in catecholamines are known to induce hyperglycemia as a result of accelerated glucose delivery from the liver and reduced glucose clearance from the circulation [7,20]. Thus, in our newborn lambs, a catecholamine-induced hyperglycemia resulted in increases in urinary glucose excretion and decreases in the percentage of glucose reabsorbed by the renal tubule. However, absolute tubular glucose reabsorption also increased. These changes in renal glucose homeostasis contributed to the natriuresis, increased solute excretion and consequent osmotic diuresis following asphyxia in our newborn lambs. In young animals, it has been previously shown that there is an increased splay in the glucose titration curve as a result of functional heterogeneity in the proximal tubules resulting in a lower threshold for glucose [3]. Hyperglycemia during and after asphyxia exceeded the ability of the proximal tubule to reabsorb glucose, resulting in increased glucose and sodium excretion and contributing to the increased osmolar clearances and conse- quent diuresis.
The diuresis associated with acute moderate hyperglycemia has recently been demonstrated to result from increases in filtration rates, as a result of reductions in systemic oncotic pressure and reductions in proximal tubular reabsorption of glucose. The reductions in tubular glucose reabsorption resulted from hyperglycemic increases in renal interstitial hydrostatic pressure or glucose induced alterations in osmotic pressure gradients across the proximal tubule, rather than because of the effect of unreabsorbed glucose on sodium concentration gradients [5]. The impor- tance of these mechanisms in the diuresis resulting from asphyxial induced hyper- glycemia remains speculative.
Other potential factors such as prostaglandins, vasopressin, direct damage to sodium transport systems [21] and increased capillary permeability resulting in decreased colloid oncotic pressure, which we did not measure, probably contributed
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to the observed osmotic diuresis. In adult dogs, systemic hypoxia results in a diuresis because of increased renal perfusion pressure and glomerular filtration resulting from increases in mean arterial blood pressure [27]. Our renal blood flow data show that renal circulatory changes appear to be of lesser importance during asphyxia in these lambs. The lack of significant changes in heart rate, mean arterial blood pressure, renal blood flow and plasma renin activity tends to support this conten- tion.
In summary, newborn lambs exposed to asphyxia for 25 min exhibit a diuresis immediately following asphyxia. This response is associated with increased sodium and glucose excretions resulting in part from increased circulating catecholamine concentrations and altered glucose homeostasis.
Acknowledgments
Presented in part at the American Pediatric Society, and the Society for Pediatric Research, San Antonio, Texas, May, 1980. Supported by The March of Dimes Birth Defects Foundation, Basil O’Connor Research Starter Award 5-256, and in part by the Charles H. Hood Foundation. The authors wish to thank Mr. Raymond Petit for his skillful technical assistance and Albert S. Most, M.D., FACC Chief of Cardiol- ogy, and the staff of the Cardiac Research Laboratory of Rhode Island Hospital for their help and use of the facilities.
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