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1 School of Physiology and Pharmacology, The University of New South Wales, Sydney, New South Wales 2052; and 2 Department of Obstetrics and Gynaecology and 3 Department of Physiology, The University of Adelaide, Adelaide, South Australia 5005, Australia
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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In the adult, insulin-like growth factor I (IGF-I) increases glomerular filtration rate (GFR) and renal blood flow (RBF) during both acute and chronic treatment. To study its effects on the developing kidney, chronically catheterized fetal sheep (120 ± 1 days gestation) were infused intravenously for up to 10 days with 80 µg/h IGF-I (n = 5) or vehicle (0.1% BSA in saline, n = 6). In contrast to previous acute studies in adult rats and humans, after 4 h of IGF-I fetal GFR and RBF were unchanged. Fractional sodium reabsorption increased (P < 0.05). However, by 4 days, GFR per kilogram had risen by 35 ± 13% (P < 0.05), whereas RBF remained unchanged. Tubular growth and maturation may have occurred, as proximal tubular sodium reabsorption increased by ~35% (P < 0.005). Therefore, despite a marked increase in filtered sodium (~30%, P < 0.05), fractional sodium reabsorption did not change. Although the effects of IGF-I on renal function were delayed, plasma renin activity and concentration were both elevated after 4 h and remained high at 4 days (P < 0.05). Despite this, arterial pressure and heart rate did not change. Kidneys of IGF-I-infused fetuses weighed ~30% more (P = 0.05) and contained ~75% more renin than control fetuses (P < 0.005). Thus, in the fetus, the renal effects of long-term IGF-I infusion are very different from the adult, possibly because IGF-I stimulated kidney growth.
kidney; development; renin-angiotensin system; glomerular filtration
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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IN THE ADULT, INTRAVENOUS INFUSION of insulin-like growth factor I (IGF-I) leads to a rapid rise in renal blood flow (RBF), glomerular filtration rate (GFR), and the glomerular ultrafiltration coefficient (LpA) (22), but it does not change renal perfusion pressure. IGF-I alters renal hemodynamics in this way in anesthetized and conscious rats, and in conscious humans, both acutely (as soon as 20 min after beginning administration) (12, 20) and in a sustained manner over several days of treatment (14, 33). These observations have led others to postulate that during adult life, endogenous IGF-I may contribute to the physiological regulation and maintenance of glomerular filtration (8).
Despite the well-characterized response of the adult kidney to IGF-I, there have been no reports of the effects of prolonged IGF-I treatment on fetal renal function. During fetal life, IGF-I is produced by a wide variety of tissues, including the fetal kidney (5, 18), and the kidney expresses the type 1 IGF receptor (13). In fetal sheep, circulating levels of IGF-I increase over the latter half of gestation, from ~25 to almost 100% of adult levels by term (4). In the developing kidney, IGF-I is a potent stimulus for growth, both in late-gestation fetal sheep (25) and in the neonatal rat (41). This means that IGF-I may affect renal function in the fetus not only via direct actions, as occurs in the adult, but also by stimulating renal growth. Our study was carried out to determine whether the function and/or growth of the fetal kidney were altered during prolonged infusions of IGF-I into late-gestation fetal sheep. We wanted to compare any acute effects with those that occurred after 4 days of IGF-I infusion, by which time any growth-related changes in renal function were likely to be apparent.
The second aim of this study was to determine whether the fetal renin-angiotensin system was affected by prolonged IGF-I infusion. The renin-angiotensin system makes an important contribution to the functional development of the kidney. During fetal life, ANG II maintains fetal GFR through its effects on efferent arteriolar tone, so that in fetal sheep, GFR falls during pharmacological blockade of the fetal renin-angiotensin system and is restored by an infusion of ANG II (28). Not only does the renin-angiotensin system support fetal renal function, but normal renal development depends on the integrity of this system. Disruption of the renin-angiotensin system by a variety of methods during development results in severe, persistent abnormalities in renal morphology and function (15, 19, 34, 43). Recent evidence that the renin-angiotensin system and IGF-I might interact to influence renal development is that in neonatal rats treated with the angiotensin-converting enzyme (ACE) inhibitor enalapril, the presence of renal structural defects such as papillary atrophy was associated with suppressed IGF-I gene expression in the renal medulla (35). Furthermore, cotreatment with IGF-I prevented the development of these histological defects and their associated functional abnormalities (35). Because we postulated that both the function and growth of the fetal kidney might be altered by IGF-I, it was clearly important to determine whether the activity of the fetal renin-angiotensin system was also altered.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Experiments were approved by the Animal Care & Ethics Committee of the University of New South Wales. They were carried out in 11 chronically catheterized pregnant ewes and their fetuses.
Surgical preparation and animal care.
At 109-117 days gestation (term 
150 days), ewes were
anesthetized with an intravenous injection of 1-2 g thiopentone
sodium (Pentothal; Abott, Kurnell, New South Wales, Australia) and
following tracheal intubation were maintained by 2-3%
halothane (Fluothane; Clifford Hallam Pharmaceuticals, Riverwood, New
South Wales, Australia) in oxygen. As described previously
(31), catheters were placed in the fetus into a femoral
artery, both tarsal veins, and the bladder. An amniotic catheter was
sutured to the fetal skin over the flank for measurement of
intra-amniotic pressure and injection of antibiotics. The left renal
artery was located through a paravertebral incision, and the cuff of a
20-MHz directional pulsed Doppler flow probe (1.6- or 1.8-mm internal
diameter cuff; Iowa Doppler Products, Iowa City, IA) was tied around it
for measurement of fetal RBF. A maternal femoral artery and vein were
also catheterized.
Study outline. Five fetuses aged 120 ± 1 days gestation (mean ± SE) were given an intravenous infusion of 80 µg/h recombinant human IGF-I (25). This dose was chosen because when infused for 10 days into fetal sheep of the same gestational age as those used in the present study, it led to an approximate threefold increase in circulating IGF-I levels and stimulated renal growth (25). The IGF-I (GroPep, Adelaide, South Australia, Australia) was dissolved at a concentration of 0.05 mg/ml in 0.15 M saline containing 0.1% BSA (Sigma Chemical, St. Louis, MO). On the first day of the study (day 0) following a 2-h control period, fetuses were given a bolus injection of 80 µg IGF-I, and then IGF-I was infused at a rate of 1.65 ml/h for 4-5 h. The infusion rate was then changed so that fetuses received 0.42 mg/ml IGF-I at a rate of 0.19 ml/h for the next 6-10 days. Six control fetuses aged 120 ± 0.4 days gestation received an infusion of vehicle for 9-10 days at the same rates as the treated group. Infusions were prepared immediately before use and delivered through a filter (0.2-µm pore size, Minisart; Sartorius, Göttingen, Germany).
Experiments were carried out on the first and fourth day of infusion (days 0 and 4, respectively) to compare the acute and chronic effects of IGF-I infusion on fetal renal function and the activity of the circulating renin-angiotensin system. At the end of the study, we also measured renal weights and the levels of renin within the fetal kidney. At the start of each experimental day, the fetal bladder was opened and drained under gravity for at least 45 min. The ewe was given a loading dose of lithium chloride (150 µmol/kg iv). The fetus was given intravenous loading doses of lithium chloride (250 µmol/kg) and 125I-sodium iothalamate (1.8 µCi/kg; Amersham), followed by a continuous intravenous infusion for the duration of the experiment of 10 µmol · kg
1 · h1 and
0.3 µCi · kg1 · h1,
respectively, in 0.15 M saline at 0.95 ml/min.
On day 0, 12 consecutive 30-min urine collections were made;
four control periods followed by eight infusion periods covering the
first 4 h of infusion, during which time the initial infusion rate
of 1.65 ml/min was used. Urine was collected anaerobically under a
layer of mineral oil for acid-base measurements. Blood samples were
taken at the midpoint of the second, fourth, eighth, and final
collection period. On day 4, four 30-min urine collections were made, with blood sampled at the midpoint of the second and final
period. Data were averaged to obtain a single value for each variable
for the 2-h control period on day 0 and for the day
4 experiment. Results from days 0 and 4 are
reported for the control period, the final 30-min period on day
0 (4 h), and the experiment on day 4 (4 days).
Arterial pressure, heart rate, and RBF. Arterial pressure, heart rate, and intra-amniotic pressure were monitored continuously during each experiment using pressure transducers (EasyVent Deadender Cap; Ohmeda, BOC) and a polygraph (model 79D; Grass Instrument, Quincy, MA). RBF was also monitored continuously using the flow probe placed during surgery and a 545C-4 Directional Pulsed Doppler Flowmeter (Bioengineering, University of Iowa) connected to the polygraph. Output from the polygraph was interfaced to an IBM compatible computer using a Metrabyte DAS16 interface card (Keithley, MA). Intra-amniotic pressure was subtracted from the recorded blood pressure to obtain true fetal arterial pressure. Mean values for fetal arterial pressure, heart rate, and RBF were obtained for the control period and for each of the infusion periods on days 0 and 4. As the probe measured relative blood flow only, mean values for RBF during the final infusion period on day 0 and during the day 4 experiment were expressed as a percentage of control RBF.
Blood samples.
Arterial blood samples (5 ml) were withdrawn anaerobically into
syringes containing 50 IU heparin and replaced with equivalent volumes
of heparinized saline. Arterial PO2,
PCO2, and pH were measured at 37°C and
corrected to 39.5°C using a Ciba-Corning Blood Gas System (model 288;
Medfield). Hematocrit was determined in duplicate using a
microhematocrit centrifuge and reader (Hettich, Tuttlingen, Germany).
Plasma was separated by centrifuging for 10 min at 2,500 rpm, 4°C in
tubes containing an additional 50 IU heparin, and stored at 
20°C
for biochemical analysis.
80°C for measurement
of renal renin levels.
Biochemical analysis.
Concentrations of IGF-I in plasma were measured by specific RIAs
(10) calibrated with ovine IGF-I (9) after
removal of IGF-binding proteins by size-exclusion HPLC of plasma at pH
2.5 (36). The intra- and interassay coefficients
of variation were 3 and 2.9%, respectively. Plasma glucose and lactate
were measured using a Glucose/L-Lactate Analyzer 2300 Stat
(J Morris Scientific, Chatswood, New South Wales, Australia).
Osmolality was measured using a Fiske One-Ten osmometer (Needham
Heights, MA). Concentrations of sodium, chloride, potassium, and
phosphate were measured on a Beckman Synchron CX3 Clinical System
(Beckman Instruments, Gladesville, New South Wales, Australia). GFR was
measured as the renal clearance of 125I-sodium iothalamate.
The concentration of 125I-sodium iothalamate in plasma and
urine was determined from the activity of 125I using a
Packard Auto Gamma counter (model 5650; Downers Grove, IL). Lithium
concentrations in plasma and urine were measured using a Perkin-Elmer
272 Atomic Absorption Spectrophotometer (Norwalk, CT). Fractional
reabsorption of sodium by the proximal (FRNaP) and distal
tubules was calculated from the renal clearance of lithium, where the
FRNaP was calculated from the formula
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Measurement of plasma renin activity and concentration, plasma angiotensinogen levels, and renal renin. Plasma renin activity (PRA) was measured by determining the rate of formation of ANG I in nanograms per milliliter per hour when 200 µl of plasma were incubated for 2 h at pH 7.5 and 37°C. Because the ANG I measured is formed by endogenous renin acting on endogenous substrate, PRA gives a measure of the capacity of plasma to generate ANG I. Plasma renin concentration (PRC) was determined as the rate of formation of ANG I in nanograms per milliliter per hour when 100 µl of fetal plasma were incubated with an excess of sheep angiotensinogen [100 µl of nephrectomized sheep plasma (NSP)] at pH 7.5 and 37°C for 2 h. In this assay, the activity of renin is not limited by substrate availability, and so PRC is a measure of the amount of active renin in the circulation. The concentration of angiotensinogen in fetal plasma was measured by determining how much ANG I had been generated in 20 µl of plasma by an excess of human renin (0.5 mU; Calbiochem-Novabiochem, Alexandria, New South Wales, Australia) during a 1-h incubation at pH 7.5 and 37°C. The incubation time and total reaction volume (200 µl) were determined in preliminary experiments. All measurements were made in duplicate and averaged to give a single result for each plasma sample. ANG I levels were measured by RIA using methods previously described (30).
Renal renin was measured following the same principles that were used to measure PRC. Homogenates of renal cortex were incubated with NSP, and the concentration of ANG I formed by the end of the incubation was measured by RIA. Preliminary experiments were carried out to determine how much to dilute the homogenate, so that the generation of ANG I in the presence of 100 µl NSP at 37°C and pH 7.5 occurred in a linear manner depending on the amount of renin present, and so that the maximum rate of ANG I formation was reached by the end of 1 h of incubation. To ensure that the assay measured only the ANG I that was generated by renal renin acting on NSP, a pair of duplicates from each sample was placed on ice, whereas a second pair of duplicates was incubated at 37°C and pH 7.5 for 1 h. Samples were then treated in the same way as plasma samples (30). The protein concentration of each renal homogenate was determined in triplicate following the method of Lowry et al. (26). Renal renin levels were calculated by dividing the ANG I formed during the 1-h incubation by renal protein concentration and are expressed as micrograms ANG I per milligram of protein.Derived variables and statistical methods.
For the purposes of calculating doses, fetal body weight at the time of
each experiment was estimated from gestational age using a formula
derived from the body weights and ages of 84 fetuses in this laboratory
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BWD) was compared
between the two groups using a nonpaired t-test. The mean
deviation of each group was found to be similar (vehicle: 47 ± 206 g; IGF-I: 13 ± 195 g; means ± SE). Also,
when estBWD and BWD were compared within each group using Student's
paired t-tests, no difference was found.
Plasma bicarbonate concentrations were calculated from the measured
arterial PCO2 and pH using a formula derived
from the Henderson-Hasselbach equation (2). Filtration
fraction relative to control was calculated from the formula
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Fetal plasma IGF-I levels. Infusion of recombinant human IGF-I increased fetal plasma IGF-I levels from preinfusion concentrations of 154 ± 14 to 325 ± 16 ng/ml at 4 h (P < 0.001, n = 5), and they remained high at 4 days (277 ± 33 ng/ml, P < 0.001, n = 5). In the vehicle group, plasma IGF-I concentrations did not change from control values of 118 ± 36 ng/ml (n = 4).
Maternal responses. No change was apparent in maternal variables of either treatment group.
Fetal arterial blood gases, pH, and plasma composition. Fetal arterial oxygen tension (PO2) fell from control values of 22.6 ± 0.6 to 19.1 ± 0.9 mmHg after 4 days of IGF-I infusion (P < 0.05, n = 5). Fetal PCO2 and pH remained similar to control values (53.7 ± 0.9 mmHg and 7.4 ± 0.0, respectively, n = 5). PO2, PCO2, and pH remained at control levels during vehicle infusion (21.5 ± 1.1 mmHg, 55.7 ± 2.1 mmHg, and 7.3 ± 0.01, respectively, n = 4). Hematocrit did not change in either group (control values IGF-I: 29 ± 1%, n = 5; vehicle: 32 ± 1%, n = 5).
Plasma osmolality had increased after 4 days of IGF-I infusion, largely due to increases in plasma concentrations of sodium and chloride ions (Table 1). Plasma glucose and lactate concentrations did not change from control values of 1.43 ± 0.01 and 1.24 ± 0.04 mmol/l, respectively. In vehicle-infused fetuses, plasma potassium levels increased from 3.8 ± 0.2 mmol/l during control to 4.3 ± 0.2 mmol/l at 4 days (P < 0.05, n = 5). There were no changes during vehicle infusion in plasma osmolality nor in the plasma levels of sodium, chloride, phosphate, glucose, or lactate (control levels: 284 ± 2 mosmol/kgH2O, 143 ± 1 mmol/l, 107 ± 2 mmol/l, 2.52 ± 0.14 mmol/l, 1.35 ± 0.19 mmol/l, and 1.20 ± 0.07 mmol/l, respectively, n = 5).
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Fetal arterial pressure and heart rate. In IGF-I-infused fetuses, systolic, diastolic, and mean arterial pressures and heart rate remained similar to control values (53.6 ± 1.2 mmHg, 32.1 ± 1.0 mmHg, 41.1 ± 1.0 mmHg, and 189 ± 4 beats/min, respectively, n = 5). In vehicle-infused fetuses, these variables also remained similar to control values of 50.9 ± 1.1 mmHg, 30.9 ± 1.7 mmHg, 39.0 ± 1.6 mmHg (n = 4), and 184 ± 4 beats/min (n = 6), respectively.
Fetal renal function.
Although there was no change in fetal GFR after 4 h of IGF-I
infusion, in absolute terms it had risen 60 ± 16% above control by day 4 (P < 0.005, n = 5)
and 35 ± 13% above control after correction for fetal body
weight (P < 0.05; Fig.
1A). RBF tended to fall during
infusion, although this did not reach significance (Fig. 1B). The increased GFR at 4 days meant that there was a
large increase in filtration fraction relative to control on day
4 (Fig. 1C). GFR and RBF did not change significantly
with vehicle infusion, although there was a small increase in
filtration fraction in the control group by day 4 (Fig. 1).
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1 · kg1
and 5 ± 1, 4 ± 1, 2.6 ± 1.9, and 0.28 ± 0.18 µmol · min1 · kg1,
respectively. These variables did not change in vehicle-infused fetuses. Urinary osmolality did not change from control values in
either treatment group (IGF-I: 128 ± 6 mosmol/kg H2O,
n = 5; vehicle: 129 ± 8 mosmol/kgH2O,
n = 6).
Renal acid-base handling.
The filtered load of bicarbonate was increased after 4 days of IGF-I
infusion (from 39.1 ± 5.1 µmol · min1 · kg1 during
control to 52.0 ± 4.9 µmol · min1 · kg1,
P < 0.05, n = 5). Bicarbonate
reabsorption was higher on day 4 compared with 4 h
(53.8 ± 5.1 and 33.1 ± 5.3 µmol · min1 · kg1,
respectively, P < 0.05, n = 4).
Urinary pH, fractional bicarbonate reabsorption, and excretion rates of
bicarbonate, titratable acid, and ammonium ions did not change (control
values: 6.6 ± 0.1, 96.5 ± 0.7%, 1.5 ± 0.3 µmol · min1 · kg1,
0.2 ± 0.1 µmol · min1 · kg1, and
0.9 ± 0.2 µmol · min1 · kg1,
respectively, n = 4). Renal acid-base handling was
measured in four control animals, and there were no changes with
vehicle infusion.
Fetal renin-angiotensin system.
Over the first 4 h of IGF-I infusion, PRA (Fig.
3) rose almost threefold (to 397 ± 119% of control, P < 0.05), and on day 4 it was still elevated (385 ± 67% of control, P < 0.05). There was a very large, sustained increase in PRC in the
IGF-I-treated fetuses (Fig. 3), to 438 ± 117% of control at
4 h (P < 0.05) and 543 ± 119% of control
at 4 days (P < 0.01). Plasma angiotensinogen levels
tended to fall (not significant; Fig. 3). In fetuses infused with
vehicle, PRA had increased by day 4 to 9.2 ± 2.1 ng
ANG I · ml1 · h1 (control:
5.1 ± 1.9 ng ANG
I · ml1 · h1;
P < 0.05, n = 4). This was an increase
to 247 ± 78% of control (not significant), but neither PRC nor
plasma angiotensinogen concentrations changed significantly (control:
7.3 ± 2.1 ng ANG I · ml1 · h1 and 1.8 ± 0.2 µg ANG I/ml, respectively, n = 4). Renal renin levels in IGF-I fetuses on day 10 were ~75% higher than
those of vehicle-infused animals (Fig.
4).
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Fetal body weight and kidney weight. Fetuses infused with IGF-I had a larger total kidney mass than control animals (27.4 ± 2.6 g, n = 5 compared with 20.9 ± 1.5 g, n = 6; P = 0.05). Kidney weight was also higher when expressed as a percentage of body weight (1.0 ± 0.1 compared with 0.8 ± 0.04%, P = 0.05). Fetal body weights were similar between groups (IGF-I: 2,636 ± 141 g; vehicle: 2,522 ± 227 g).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The present study compares the short- and long-term effects of infusions of IGF-I into fetal sheep on fetal renal function and the activity of the fetal renin-angiotensin system. We report that after 4 h of IGF-I infusion, there was a marked increase in renin secretion by the fetal kidney. There was a small increase in fractional sodium reabsorption (P < 0.05), without an increase in GFR. After 4 days, changes in renal function followed a very different pattern. There was a large increase in GFR, which was accompanied by a compensatory increase in tubular reabsorption that prevented excessive solute loss. Also, even after long-term IGF-I infusion, the renin-angiotensin system remained intensely activated. Renin secretion was still high, and, interestingly, at the end of the study, renal renin levels were increased.
In the adult, both short- and long-term administration of IGF-I causes marked increases in both GFR and RBF (12, 14, 20, 33). These effects are very well described, become apparent within as soon as 20 min of beginning IGF-I treatment, and have been attributed to decreased renal vascular resistance and an increase in the LpA (22). IGF-I causes renal vasodilatation; however, there have been conflicting reports as to whether the reduction in resistance was pre- or postglomerular or occurred at both sites (21, 22, 42). Generally, increases in GFR and RBF are not accompanied by changes in filtration fraction (12, 20).
The actions of IGF-I on the fetal kidney in the present study, however, were very different from those reported in the adult. RBF, if anything, fell, and the increase in GFR was delayed for 4 days. The increased filtration fraction on day 4 suggests that preferential vasoconstriction of the efferent arteriole may have occurred, an event that would increase GFR and reduce RBF. This efferent vasoconstriction is unlikely to have been a direct action of IGF-I, because it causes vasodilatation, but it might be expected as a consequence of the intense activation of the renin-angiotensin system that occurred. The fetal renin-angiotensin system is essential for maintenance of fetal GFR through an effect on efferent arteriolar tone; ACE inhibition causes RBF to increase, and GFR falls to such an extent that anuria results. These effects are reversed by infusion of ANG II (28). Increased circulating levels of ANG II in the present study may therefore have contributed to the rise in GFR by day 4. However, it is puzzling that GFR was not increased at 4 h, when the renin-angiotensin system was already intensely activated. Also, a greater reduction in RBF might be expected if efferent arteriolar constriction was alone responsible for such a large increase in GFR.
The rise in GFR may also have been due to an increase in the LpA. In addition to changes in arteriolar resistance, much of the increase in GFR in adult rats receiving IGF-I either as an acute intravenous infusion or for 6-7 days as subcutaneous injections has been attributed to large increases in the LpA (21, 22). If IGF-I acted similarly in the fetus, an increased LpA may have contributed to the rise in GFR at 4 days, and this would be consistent with the absence of a rise in RBF.
Although increases in the LpA and arteriolar resistance may partly
underlie the rise in GFR that occurred by day 4, they do not
explain why the acute renal responses to IGF-I were so different to
those following chronic treatment and why GFR only increased after
several days of high circulating IGF-I levels. The stimulation of renal
growth by IGF-I strongly suggests that the reason for this delay was
that the rise in GFR was at least partly growth related. New nephrons
are formed in the sheep fetus at the gestation age we studied and until
~130 days gestation (38), and an increased number of
glomeruli would result in an increase in total GFR in addition to any
effects of IGF-I on glomerular hemodynamics or the LpA. As well, by
stimulating the growth of existing glomeruli, IGF-I could increase
glomerular surface area and therefore increase LpA over and above any
direct actions. We did not measure the effects of IGF-I on glomerular
size, number, nor its effects on renal tubular length, so we cannot
determine the specific components of the kidney that were affected.
However, there is evidence that IGF-I affects glomerular growth. It is
a cell-cycle progression factor in adult human mesangial cells in
culture (6), and mice transgenic for overexpression of
human IGF-I have an increased glomerular volume (7),
whereas IGF-I(/) mice have small kidneys with a reduced nephron
number and glomerular volume (40). IGF-I has also
been reported to increase renal mass in the neonatal rat
(41) and in fetal sheep at the same gestational age as the present study (25).
The increase in tubular reabsorption on day 4 meant that fractional sodium reabsorption remained constant despite the increase in GFR, so that glomerulotubular balance was maintained. More importantly, from a developmental perspective, fractional reabsorption by the proximal tubule was maintained. This is an adult-like response. It is not unusual in the fetus for fractional sodium reabsorption to remain relatively constant when the filtered sodium load increases. However, the proximal tubule is immature, so that, in general, the compensatory increases in reabsorption occur in both the proximal and distal regions of the nephron (29). This is in contrast to the adult animal, in which glomerulotubular balance resides in the proximal tubule and there is no evidence of a relationship between distal tubular sodium reabsorption and GFR (29).
We have found therefore that IGF-I increases the reabsorptive capacity of the fetal proximal tubule. Again, this could be related to growth of the kidney, and the proximal tubule and loops of Henle are still undergoing a great deal of growth and elongation at this stage in gestation (1). The synthesis of more transport proteins may also have increased tubular reabsorptive capacity. Regardless of the mechanism, maintenance of glomerulotubular balance by the proximal tubule in the face of a 60% increase in GFR, together with the increase in GFR itself, indicates that IGF-I treatment accelerated the functional maturation of the kidney. This raises another possible explanation for the delayed increase in GFR. It may be that the relative immaturity of the fetal kidney at the start of the treatment meant that IGF-I could not increase GFR acutely but that 4 days of high circulating IGF-I levels matured renal function to the extent that actions such as an increased LpA were possible.
There are considerable amounts of endogenous IGF-I in fetal plasma at the stage of gestation in the present study, and levels become increasingly elevated toward term (4). It has been suggested by others that because these plasma levels correlate with fetal body weight (24) and because infusions of IGF-I into the fetus in late gestation are associated with increased renal mass (25), IGF-I plays an important part in regulating both overall body growth and that of the fetal kidney. The functional maturation of the kidney is intimately associated with renal growth (27), and perhaps endogenous IGF-I, in addition to or by virtue of its influence on renal growth, participates in the normal growth-related maturation of renal function that occurs over this time.
The fetal kidney continued to secrete high levels of renin even
following 4 days of stimulation by IGF-I. The resultant increase in
circulating renin levels meant that the activity of renin in fetal
plasma also remained high. Interestingly, by the end of the study, the
renal stores of renin showed no sign of depletion, but, on the
contrary, renal renin levels were increased. The sustained high levels
of renin secretion were therefore supported by increased renin
synthesis. Direct stimulation of renin secretion by IGF-I has been
demonstrated in vitro in rat renal cortical slices (23), and it is likely that this also occurred in vivo in the present study
throughout IGF-I infusion, because previously reported stimuli for
renin secretion in the fetus do not appear to be responsible. Fetuses
did not become hypotensive (32) nor was there any increase in heart rate or arterial pressure indicative of increased
-adrenergic stimulation (30). PRA was increased in the
control animals on day 4, suggesting that blood sampling may
have contributed to the activation of the renin-angiotensin system with
chronic IGF-I infusion (3); however, the increase was
small compared with that during IGF-I and was not accompanied by
changes in PRC. The fall in PO2 by day
4 was probably too small to be a stimulus for renin release
(39). Also, the delivery of sodium past the macula densa
to the distal tubule was not reduced on day 4, and so it is
unlikely that the macula densa was involved in the stimulation of renin release.
In summary, long-term infusions of IGF-I into late-gestation fetal sheep stimulated renal growth and function and resulted in an appreciable, sustained activation of the fetal renin-angiotensin system characterized by an increase in the synthesis and release of renin by the fetal kidney. The renal functional responses to this long-term infusion were very different to those observed both during short-term infusion into the fetus and in adult life. They indicate that IGF-I accelerated the maturation of renal function, possibly through its effects on renal growth. The degree of influence that the increased activity of the renin-angiotensin system had on the renal growth and these functional changes remains to be determined.
Perspectives
The finding that the fetal renin-angiotensin system underwent a sustained activation of such magnitude during long-term IGF-I treatment has exciting implications. The fetal renin-angiotensin system is highly active during late gestation (3, 45), and there is a large body of evidence that shows that an intact renin-angiotensin system is required for normal renal development. Disruption of the renin-angiotensin system during development by targeted gene disruption (19, 34), ACE inhibition (15), or angiotensin-receptor blockade (43) results in severe abnormalities in renal morphology and function. ANG II may act directly as a renal growth factor, as it causes proliferation of cultured human fetal mesangial cells (37) and hypertrophy of cultured murine proximal tubule cells (44). Also, because ANG II contributes to the maintenance of fetal glomerular filtration, it may facilitate renal development by supporting renal function (28). Because IGF-I not only stimulates renal growth in the fetus but also increases the activity of a system important for normal kidney growth and development, we speculate that the renin-angiotensin system contributes to the renotrophic effects of IGF-I during fetal life. This is supported by the finding that the gross renal structural and functional deficits caused by ACE inhibition in neonatal rats were not only associated with reduced renal IGF-I gene expression but were also prevented by coadministration of IGF-I (35), which suggests that IGF-I and the renin-angiotensin system might act in concert to promote normal renal development. Further experiments are required to see whether, conversely, blockade of the fetal renin-angiotensin system prevents the growth-stimulating effects of IGF-I on the kidney.| |
ACKNOWLEDGEMENTS |
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We thank Drs. A. D. Stevens and J. H. Burrell as well as P. Bode for technical assistance. We also thank the Department of Clinical Chemistry, Prince of Wales Hospital, Sydney, Australia for measurement of electrolyte levels.
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FOOTNOTES |
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This work was supported by the National Health and Medical Research Council of Australia. A. Marsh is supported by a Postgraduate Science Research Scholarship from the National Heart Foundation of Australia.
Address for reprint requests and other correspondence: A. Marsh, School of Physiology and Pharmacology, The Univ. of New South Wales, Sydney, New South Wales 2052, Australia.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 18 October 2000; accepted in final form 28 February 2001.
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REFERENCES |
|---|
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
1.
Aperia, A,
and
Elinder G.
Distal sodium reabsorption in the developing rat kidney.
Am J Physiol Renal Fluid Electrolyte Physiol
240:
F487-F491,
1981.
2.
Armentrout, T,
Katz S,
Thornburg KL,
and
Faber JJ.
Osmotic flow through the placental barrier of chronically prepared sheep.
Am J Physiol Heart Circ Physiol (Lond)
233:
H466-H474,
1977.
3.
Broughton Pipkin, F,
Lumbers ER,
and
Mott JC.
Factors influencing plasma renin and angiotensin II in the conscious pregnant ewe and its foetus.
J Physiol (Lond)
243:
619-636,
1974
4.
Carr, JM,
Owens JA,
Grant PA,
Walton PE,
Owens PC,
and
Wallace JC.
Circulating insulin-like growth factors (IGFs), IGF-binding proteins (IGFBPs) and tissue mRNA levels of IGFBP-2 and IGFBP-4 in the ovine fetus.
J Endocrinol
145:
545-557,
1995
5.
Dickson, MC,
Saunders JC,
and
Gilmour RS.
The ovine insulin-like growth factor-I gene: characterization, expression and identification of a putative promoter.
J Mol Endocrinol
6:
17-31,
1991
6.
Doi, T,
Striker LJ,
Elliot SJ,
Conti FG,
and
Striker GE.
Insulinlike growth factor-1 is a progression factor for human mesangial cells.
Am J Pathol
134:
395-404,
1989[Abstract].
7.
Doi, T,
Striker LJ,
Gibson CC,
Agodoa LY,
Brinster RL,
and
Striker GE.
Glomerular lesions in mice transgenic for growth hormone and insulinlike growth factor-I. I. Relationship between increased glomerular size and mesangial sclerosis.
Am J Pathol
137:
541-552,
1990[Abstract].
8.
Feld, S,
and
Hirschberg R.
Growth hormone, the insulin-like growth factor system, and the kidney.
Endocr Rev
17:
423-480,
1996
9.
Francis, GL,
McNeil KA,
Wallace JC,
Ballard FJ,
and
Owens PC.
Sheep insulin-like growth factors I and II: sequences, activities and assays.
Endocrinology
124:
1173-1183,
1989
10.
Gatford, KL,
Quinn KJ,
Walton PE,
Grant PA,
Hosking BJ,
Egan AR,
and
Owens PC.
Ontogenic and nutritional changes in circulating insulin-like growth factor (IGF)-I, IGF-II and IGF-binding proteins in growing ewe and ram lambs.
J Endocrinol
155:
47-54,
1997
11.
Gibson, KJ,
and
Lumbers ER.
Extracellular volume and blood volume in chronically catheterized fetal sheep.
J Physiol (Lond)
485:
835-844,
1995
12.
Giordano, M,
and
DeFronzo RA.
Acute effect of human recombinant insulin-like growth factor I on renal function in humans.
Nephron
71:
10-15,
1995[Web of Science][Medline].
13.
Gröne, HJ,
Neumann P,
and
Fuchs E.
Localization and characterization of IGF-I receptors in fetal and adult human kidneys.
Miner Electrolyte Metab
18:
256-263,
1992[Web of Science][Medline].
14.
Guler, HP,
Eckardt KU,
Zapf J,
Bauer C,
and
Froesch ER.
Insulin-like growth factor I increases glomerular filtration rate and renal plasma flow in man.
Acta Endocrinol (Copenh)
121:
101-106,
1989
15.
Guron, G,
Adams MA,
Sundelin B,
and
Friberg P.
Neonatal angiotensin-converting enzyme inhibition in the rat induces persistent abnormalities in renal function and histology.
Hypertension
29:
91-97,
1997
16.
Györy, AZ,
and
Edwards KD.
Simultaneous titrimetric determination of bicarbonate and titratable acid of urine.
Aust J Exp Biol Med Sci
45:
141-147,
1967[Web of Science][Medline].
17.
Györy, AZ,
Edwards KD,
Stewart JH,
and
Whyte HM.
Comprehensive one-day renal function testing in man.
J Clin Pathol
27:
382-391,
1974
18.
Han, VKM,
Lund PK,
Lee DC,
and
D'Ercole AJ.
Expression of somatomedin/insulin-like growth factor messenger ribonucleic acids in the human fetus: identification, characterization, and tissue distribution.
J Clin Endocrinol Metab
66:
422-429,
1988
19.
Hilgers, KF,
Reddi V,
Krege JH,
Smithies O,
and
Gomez RA.
Aberrant renal vascular morphology and renin expression in mutant mice lacking angiotensin-converting enzyme.
Hypertension
29:
216-221,
1997
20.
Hirschberg, R,
and
Kopple JD.
Evidence that insulin-like growth factor I increases renal plasma flow and glomerular filtration rate in fasted rats.
J Clin Invest
83:
326-330,
1989.
21.
Hirschberg, R,
and
Kopple JD.
The growth hormone-insulin-like growth factor I axis and renal glomerular function.
J Am Soc Nephrol
2:
1417-1422,
1992[Abstract].
22.
Hirschberg, R,
Kopple JD,
Blantz RC,
and
Tucker BJ.
Effects of recombinant human insulin-like growth factor I on glomerular dynamics in the rat.
J Clin Invest
87:
1200-1206,
1991.
23.
Jost-Vu, E,
Horton R,
and
Antonipillai I.
Altered regulation of renin secretion by insulinlike growth factors and angiotensin II in diabetic rats.
Diabetes
41:
1100-1105,
1992[Abstract].
24.
Kind, KL,
Owens JA,
Robinson JS,
Quinn KJ,
Grant PA,
Walton PE,
Gilmour RS,
and
Owens PC.
Effect of restriction of placental growth on expression of IGFs in fetal sheep: relationship to fetal growth, circulating IGFs and binding proteins.
J Endocrinol
146:
23-34,
1995
25.
Lok, F,
Owens JA,
Mundy L,
Robinson JS,
and
Owens PC.
Insulin-like growth factor I promotes growth selectively in fetal sheep in late gestation.
Am J Physiol Regulatory Integrative Comp Physiol
270:
R1148-R1155,
1996
26.
Lowry, OH,
Rosenbrough NJ,
Farr AL,
and
Randall JR.
Protein measurement with the Folin reagent.
J Biol Chem
193:
265-275,
1951
27.
Lumbers, ER.
A brief review of fetal renal function.
J Dev Physiol
6:
1-10,
1983.
28.
Lumbers, ER,
Burrell JH,
Menzies RI,
and
Stevens AD.
The effects of a converting enzyme inhibitor (captopril) and angiotensin II on fetal renal function.
Br J Pharmacol
110:
821-827,
1993[Web of Science][Medline].
29.
Lumbers, ER,
Hill KJ,
and
Bennett VJ.
Proximal and distal tubular activity in chronically catheterized fetal sheep compared with the adult.
Can J Physiol Pharmacol
66:
697-702,
1988[Web of Science][Medline].
30.
Lumbers, ER,
and
Lee Lewes J.
The actions of vasoactive drugs on fetal and maternal plasma renin activity.
Biol Neonate
35:
23-32,
1979[Web of Science][Medline].
31.
Lumbers, ER,
and
Stevens AD.
Changes in fetal renal function in response to infusions of a hyperosmotic solution of mannitol to the ewe.
J Physiol
343:
439-446,
1983
32.
Lumbers, ER,
and
Stevens AD.
The effects of frusemide, saralasin and hypotension on fetal plasma renin activity and on fetal renal function.
J Physiol (Lond)
393:
479-490,
1987
33.
Miller, SB,
Hansen VA,
and
Hammerman MR.
Effects of growth hormone and IGF-I on renal function in rats with normal and reduced renal mass.
Am J Physiol Renal Fluid Electrolyte Physiol
259:
F747-F751,
1990
34.
Niimura, F,
Labosky PA,
Kakuchi J,
Okubo S,
Yoshida H,
Oikawa T,
Ichiki T,
Naftilan AJ,
Fogo A,
Inagami T,
Hogan BLM,
and
Ichikawa I.
Gene targeting in mice reveals a requirement for angiotensin in the development and maintenance of kidney morphology and growth factor regulation.
J Clin Invest
96:
2947-2954,
1995.
35.
Nilsson, ABM,
Nitescu N,
Chen Y,
Guron GS,
Marcussen N,
Matejka GL,
and
Friberg P.
IGF-I treatment attenuates renal abnormalities induced by neonatal ACE inhibition.
Am J Physiol Regulatory Integrative Comp Physiol
279:
R1050-R1060,
2000
36.
Owens, PC,
Johnson RJ,
Campbell RG,
and
Ballard FJ.
Growth hormone increases insulin-like growth factor-I (IGF-I) and decreases IGF-II in plasma of growing pigs.
J Endocrinol
124:
269-275,
1990
37.
Ray, PE,
Bruggeman LA,
Horikoshi S,
Aguilera G,
and
Klotman PE.
Angiotensin II stimulates human fetal mesangial cell proliferation and fibronectin biosynthesis by binding to AT1 receptors.
Kidney Int
45:
177-184,
1994[Web of Science][Medline].
38.
Robillard, JE,
Weismann DN,
and
Herin P.
Ontogeny of single glomerular perfusion rate in fetal and newborn lambs.
Pediatr Res
15:
1248-1255,
1981[Web of Science][Medline].
39.
Robillard, JE,
Weitzman RE,
Burmeister L,
and
Smith FG, Jr.
Developmental aspects of the renal response to hypoxemia in the lamb fetus.
Circ Res
48:
128-138,
1981
40.
Rogers, SA,
Powell-Braxton L,
and
Hammerman MR.
Insulin-like growth factor I regulates renal development in rodents.
Dev Genet
24:
293-298,
1999[Web of Science][Medline].
41.
Steeb, CB,
Shoubridge CA,
Tivey DR,
and
Read LC.
Systemic infusion of IGF-I or LR3IGF-I stimulates visceral organ growth and proliferation of gut tissues in suckling rats.
Am J Physiol Gastrointest Liver Physiol
272:
G522-G533,
1997
42.
Tönshoff, B,
Kaskel FJ,
and
Moore LC.
Effects of insulin-like growth factor I on the renal juxtamedullary microvasculature.
Am J Physiol Renal Physiol
274:
F120-F128,
1998
43.
Tufro-McReddie, A,
Romano LM,
Harris JM,
Ferder L,
and
Gomez RA.
Angiotensin II regulates nephrogenesis and renal vascular development.
Am J Physiol Renal Fluid Electrolyte Physiol
269:
F110-F115,
1995
44.
Wolf, G,
and
Neilson EG.
Angiotensin II induces cellular hypertrophy in cultured murine proximal tubular cells.
Am J Physiol Renal Fluid Electrolyte Physiol
259:
F768-F777,
1990
45.
Zhang, DY,
Lumbers ER,
and
Simonetta G.
Changes in renal renin gene expression in fetal sheep.
Clin Exp Pharmacol Physiol
23:
682-684,
1996[Web of Science][Medline].
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