Renal and adrenal responses to converting-enzyme inhibition in fetal and newborn life

Ontogeny of renin gene expression in the chicken, Gallus gallus

Renin or a renin-like enzyme evolved in ancestral vertebrates and is conserved along the vertebrate phylogeny. The ontogenic development of renin, however, is not well understood in nonmammalian vertebrates. We aimed to determine the expression patterns and relative abundance of renin mRNA in pre- and postnatal chickens (Gallus gallus, White Leghorn breed). Embryonic day 13 (E13) embryos show renal tubules, undifferentiated mesenchymal structures, and a small number of developing glomeruli. Maturing glomeruli are seen in post-hatch day 4 (D4) and day 30 (D30) kidneys, indicating that nephrogenic activity still exists in kidneys of 4-week-old chickens. In E13 embryos, renin mRNA measured by quantitative polymerase chain reaction in the adrenal glands is equivalent to the expression in the kidneys, whereas in post-hatch D4 and D30 maturing chicks, renal renin expressions increased 2-fold and 11-fold, respectively. In contrast, relative renin expression in the adrenals became lower than in the kidneys. Furthermore, renin expression is clearly visible by in situ hybridization in the juxtaglomerular (JG) area in D4 and D30 chicks, but not in E13 embryos. The results suggest that in chickens, renin evolved in both renal and extrarenal organs at an early stage of ontogeny and, with maturation, became localized to the JG area. Clear JG structures are not morphologically detectable in E13 embryos, but are visible in 30-day-old chicks, supporting this concept.

Physiology and Pathophysiology of Compensatory Adaptations of a Solitary Functioning Kidney

Children born with a solitary functioning kidney (SFK) have an increased risk of hypertension and kidney disease from early in adulthood. In response to a reduction in kidney mass, the remaining kidney undergoes compensatory kidney growth. This is associated with both an increase in size of the kidney tubules and the glomeruli and an increase in single nephron glomerular filtration rate (SNGFR). The compensatory hypertrophy and increase in filtration at the level of the individual nephron results in normalization of total glomerular filtration rate (GFR). However, over time these same compensatory mechanisms may contribute to kidney injury and hypertension. Indeed, approximately 50% of children born with a SFK develop hypertension by the age of 18 and 20–40% require dialysis by the age of 30. The mechanisms that result in kidney injury are only partly understood, and early biomarkers that distinguish those at an elevated risk of kidney injury are needed. This review will outline the compensatory adaptations to a SFK, and outline how these adaptations may contribute to kidney injury and hypertension later in life. These will be based largely on the mechanisms we have identified from our studies in an ovine model of SFK, that implicate the renal nitric oxide system, the renin angiotensin system and the renal nerves to kidney disease and hypertension associated with SFK. This discussion will also evaluate current, and speculate on next generation, prognostic factors that may predict those children at a higher risk of future kidney disease and hypertension.

Chronic captopril treatment reveals the role of ANG II in cardiovascular function of embryonic American alligators (Alligator mississippiensis)

Angiotensin II (ANG II) is a powerful vasoconstrictor of the renin-angiotensin system (RAS) that plays an important role in cardiovascular regulation in adult and developing vertebrates. Knowledge of ANG II's contribution to developmental cardiovascular function comes from studies in fetal mammals and embryonic chickens. This is the first study to examine the role of ANG II in cardiovascular control in an embryonic reptile, the American alligator (Alligator mississippiensis). Using chronic low (~ 5-mg kg embryo^-1), or high doses (~ 450-mg kg embryo^-1) of captopril, an angiotensin-converting enzyme (ACE) inhibitor, we disrupted the RAS and examined the influence of ANG II in cardiovascular function at 90% of embryonic development. Compared to embryos injected with saline, mean arterial pressure (MAP) was significantly reduced by 41 and 72% under low- and high-dose captopril treatments, respectively, a greater decrease in MAP than observed in other developing vertebrates following ACE inhibition. Acute exogenous ANG II injection produced a stronger hypertensive response in low-dose captopril-treated embryos compared to saline injection embryos. However, ACE inhibition with the low dose of captopril did not change adrenergic tone, and the ANG II response did not include an α-adrenergic component. Despite decreased MAP that caused a left shifted baroreflex curve for low-dose captopril embryos, ANG II did not influence baroreflex sensitivity. This study demonstrates that ANG II contributes to cardiovascular function in a developing reptile, and that the RAS contributes to arterial blood pressure maintenance during development across multiple vertebrate groups.

Renal effects of angiotensin II in the newborn period: role of type 1 and type 2 receptors

BACKGROUND

Evidence suggests a critical role for the renin-angiotensin system in regulating renal function during postnatal development. However, the physiological relevance of a highly elevated renin-angiotensin system early in life is not well understood, nor which angiotensin receptors might be involved. This study was designed to investigate the roles of angiotensin receptors type 1 (AT1R) and type 2 (AT2R) in regulating glomerular and tubular function during postnatal development.

METHODS

The renal effects of the selective antagonist to AT1R, ZD 7155 and to AT2R, PD 1233319 were evaluated in two groups of conscious chronically instrumented lambs aged ~ one week (N = 8) and ~ six weeks (N = 10). Two experiments were carried out in each animal and consisted of the assessment of renal variables including glomerular and tubular function, for 30 min before (Control) and 60 min after infusion of ZD 7155 and PD 123319, respectively. Statistical significance was determined using parametric testing (Student t-test, analysis of variance ANOVA) as appropriate.

RESULTS

ZD 7155 infusion was associated with a significant decrease in glomerular filtration rate and filtration fraction at one but not six weeks; urinary flow rate decreased significantly in older animals, whereas sodium excretion and free water clearance were not altered. There was an age-dependent effect on potassium handling along the nephron, potassium excretion decreasing after ZD 7155 infusion in younger but not in older lambs. PD 123319 had no significant effects on glomerular filtration rate and tubular function in either age group.

CONCLUSIONS

These results provide evidence to support an important role for AT1Rs in mediating the renal effects of angiotensin II during postnatal maturation in conscious developing animals. In contrast to a role for AT2Rs later in life, there appears to be no role for AT2Rs in influencing the renal effects of Angiotensin II in the postnatal period.

Aldo-keto reductase 1b7, a novel marker for renin cells, is regulated by cyclic AMP signaling

We previously identified aldo-keto reductase 1b7 (AKR1B7) as a marker for juxtaglomerular renin cells in the adult mouse kidney. However, the distribution of renin cells varies dynamically, and it was unknown whether AKR1B7 maintains coexpression with renin in response to different developmental, physiological, and pathological situations, and furthermore, whether similar factor(s) simultaneously regulate both proteins. We show here that throughout kidney development, AKR1B7 expression-together with renin-is progressively restricted in the kidney arteries toward the glomerulus. Subsequently, when formerly renin-expressing cells reacquire renin expression, AKR1B7 is reexpressed as well. This pattern of coexpression persists in extreme pathological situations, such as deletion of the genes for aldosterone synthase or Dicer. However, the two proteins do not colocalize within the same organelles: renin is found in the secretory granules, whereas AKR1B7 localizes to the endoplasmic reticulum. Interestingly, upon deletion of the renin gene, AKR1B7 expression is maintained in a pattern mimicking the embryonic expression of renin, while ablation of renin cells resulted in complete abolition of AKR1B7 expression. Finally, we demonstrate that AKR1B7 transcription is controlled by cAMP. Cultured cells of the renin lineage reacquire the ability to express both renin and AKR1B7 upon elevation of intracellular cAMP. In vivo, deleting elements of the cAMP-response pathway (CBP/P300) results in a stark decrease in AKR1B7- and renin-positive cells. In summary, AKR1B7 is expressed within the renin cell throughout development and perturbations to homeostasis, and AKR1B7 is regulated by cAMP levels within the renin cell.

The actions of the renin-angiotensin system on cardiovascular and osmoregulatory function in embryonic chickens (Gallus gallus domesticus)

Using embryonic chickens (Gallus gallus domesticus), we examined the role of the renin-angiotensin system (RAS) in cardiovascular and osmotic homeostasis through chronic captopril, an angiotensin-converting enzyme (ACE) inhibitor. Captopril (5 mg kg⁻¹ embryo wet mass) or saline (control) was delivered via the egg air cell daily from embryonic day 5-18. Mean arterial pressure (MAP), heart rate (ƒ(H)), fluid osmolality and ion concentration, and embryonic and organ masses were measured on day 19. Exogenous angiotensin I (ANG I) injection did not change MAP or ƒ(H) in captopril-treated embryos, confirming ACE inhibition. Captopril-treated embryos were significantly hypotensive, with MAP 15% lower than controls, which we attributed to the loss of vasoconstrictive ANG II action. Exogenous ANG II induced a relatively greater hypertensive response in captopril-treated embryos compared to controls. Changes in response to ANG II following pre-treatment with phentolamine (α-adrenergic antagonist) indicated a portion of the ANG II response was due to circulating catecholamines in captopril-treated embryos. An increase in MAP and ƒ(H) in response to hexamethonium indicated vagal tone was also increased in the absence of ACE activity. Captopril-treated embryos had lower osmolality, lower Na⁺ and higher K⁺ concentration in the blood, indicating osmoregulatory changes. Larger kidney mass in captopril-treated embryos suggests disrupting the RAS may stimulate kidney growth by decreasing resistance at the efferent arteriole and increasing the fraction of cardiac output to the kidneys. This study suggests that the RAS, most likely through ANG II action, influences the development of the cardiovascular and osmoregulatory systems.

ANG II and baroreflex control of heart rate in embryonic chickens (Gallus gallus domesticus)

ANG II alters the short-term blood pressure buffering capacity of the baroreflex in many adult animals. In embryonic chickens, high plasma ANG II levels contribute to baseline mean arterial pressure (MAP, kPa) without changing heart rate (ƒH, beats/min). We hypothesized, on the basis of these features, that an ANG II-induced reduction in baroreflex sensitivity is present in embryonic chickens as in adults. We examined baroreflex function in day 19 embryonic chickens ( Gallus gallus domesticus) after chronic depletion of endogenous ANG II via angiotensin-converting enzyme (ACE) inhibition with captopril (5 mg/kg) from days 5–18 of incubation. The correlation between MAP and ƒH was assessed using increasing doses of sodium nitroprusside, a vasodilator, and phenylephrine, a vasoconstrictor. We used two analytical methods to evaluate baroreflex function: a conventional “static” method, in which maximal MAP and ƒH responses were examined, and a “dynamic” method that assessed beat-to-beat changes during the response to pharmacological manipulation. Captopril-treated embryos were hypotensive by 19% with baroreflex slopes ∼40% steeper and normalized gains ∼50% higher than controls, and differences across treatments were similar using either analytical method. Furthermore, reintroduction of ANG II via infusion raised MAP back to control levels and decreased the baroreflex gain in captopril-treated embryos. Therefore, during typical chicken development, ANG II dampens the baroreflex regulatory capacity and chicken embryos can be used as a natural model of elevated ANG II for studying developmental cardiovascular function. This study is the first to demonstrate that reduction of embryonic ANG II alters normal baroreflex function.

Effects of maternally administered drugs on the fetal and neonatal kidney

The number of pregnant women and women of childbearing age who are receiving drugs is increasing. A variety of drugs are prescribed for either complications of pregnancy or maternal diseases that existed prior to the pregnancy. Such drugs cross the placental barrier, enter the fetal circulation and potentially alter fetal development, particularly the development of the kidneys. Increased incidences of intrauterine growth retardation and adverse renal effects have been reported. The fetus and the newborn infant may thus experience renal failure, varying from transient oligohydramnios to severe neonatal renal insufficiency leading to death. Such adverse effects may particularly occur when fetuses are exposed to NSAIDs, ACE inhibitors and specific angiotensin II receptor type 1 antagonists. In addition to functional adverse effects, in utero exposure to drugs may affect renal structure itself and produce renal congenital abnormalities, including cystic dysplasia, tubular dysgenesis, ischaemic damage and a reduced nephron number. Experimental studies raise the question of potential long-term adverse effects, including renal dysfunction and arterial hypertension in adulthood. Although neonatal data for many drugs are reassuring, such findings stress the importance of long-term follow-up of infants exposed in utero to certain drugs that have been administered to the mother.

Thyroid hormone modulates renin and ANG II receptor expression in fetal sheep

Fetal renin-angiotensin system (RAS) activity is developmentally regulated, increasing in late gestation toward term. At the same time, fetal hemodynamic parameters change, with blood pressure increasing and heart rate decreasing. During this period, fetal plasma thyroid hormone concentrations also increase significantly. In this study we utilized the technique of thyroidectomy (TX), which removes thyroid hormone from the circulation, to investigate the importance of thyroid hormone on the developmental changes in the RAS (in plasma, kidney, heart, and lung) and hemodynamic regulation in fetal sheep. TX was performed at 120 days of gestational age (dGA), and control fetuses were sham operated. Immediately before necropsy (∼137 dGA), fetuses were infused with isoproterenol and the hemodynamic responses were noted. TX significantly decreased plasma thyroid hormone concentrations and renal renin mRNA and renal active renin levels but did not change fetal plasma active renin levels. TX decreased both angiotensin II receptor subtype 1 (AT1) mRNA and protein levels in kidney and lung but not in the left ventricle. TX also was associated with increased ANG II receptor subtype 2 (AT2) mRNA and protein at the 44-kDa band in kidney, whereas AT2 protein was decreased at the 78-kDa level in kidney and lung tissue only. TX fetuses had significantly lower basal mean arterial blood pressures (MAP) and heart rates than controls. Isoproterenol infusion decreased MAP in TX fetuses. These findings support the hypothesis that thyroid hormone is important in modulating maturation of RAS and cardiovascular function in the late-gestation fetal sheep.

The effect of hypothalamo-pituitary disconnection on the renin-angiotensin system in the late-gestation fetal sheep

The activity of the renin-angiotensin system (RAS) increases significantly in the late-gestation fetal sheep. Fetal cortisol is also increased during this time, and it is thought that the increase in cortisol may modulate the RAS changes. Previous studies have examined the effects of cortisol infusion on RAS activity, but the effects of blocking the peripartum increase in cortisol concentrations on the developmental changes in the RAS are not known. Therefore, we utilized the technique of hypothalamic-pituitary disconnection (HPD), which prevents the cortisol surge from occurring, to investigate the importance of the late-gestation increase in cortisol on the ontogenic changes in RAS activity. HPD of fetal sheep was performed at 120 days of gestational age (dGA), and fetuses were delivered between 135 and 139 dGA. Control fetuses were sham operated. HPD blocked the late-gestation cortisol increase but did not alter renal renin mRNA, renal renin or prorenin protein content, nor plasma renin levels compared with sham operated. However, HPD fetuses had increased ANG II receptor subtype 1 (AT1) mRNA and protein expression in the kidney and lungs. ANG II receptor subtype 2 (AT2) expression was not altered in these tissues at either mRNA or protein level. HPD did not change AT1 or AT2 mRNA in the left ventricle but did result in decreased protein levels for both receptors. These studies demonstrate that blockade of the naturally occurring increase in fetal cortisol concentration in late gestation is associated with tissue-specific alterations in expression of AT1 and AT2 receptors. These changes may impact on fetal tissue maturation and hence have consequences in postnatal life.

Periconceptional nutrition programs development of the cardiovascular system in the fetal sheep

It has been proposed that fetal adaptations to intrauterine nutrient deprivation permanently reprogram the cardiovascular system. We investigated the impact of restricted periconceptional nutrition and/or restricted gestational nutrition on fetal arterial blood pressure (BP), heart rate, rate pressure product, and the fetal BP responses to ANG II and the angiotensin-converting enzyme inhibitor captopril during late gestation. Restricted periconceptional nutrition resulted in an increase in fetal mean arterial BP between 115 and 125 days gestation (restricted 41.5 +/- 2.8 mmHg, n = 12; control 38.5 +/- 1.5 mmHg, n = 13) and between 135 and 147 days gestation (restricted 50.5 +/- 2.2 mmHg, n = 8; control 42.5 +/- 1.9 mmHg, n = 10) as well as an increase in the rate pressure product in twin, but not singleton, fetuses between 115 and 147 days gestation. Mean BP and fetal plasma ACTH were also positively correlated in twin, but not singleton, fetuses. This is the first demonstration that maternal undernutrition during the periconceptional period results in an increase in fetal arterial BP. This increase occurs concomitantly with an increase in fetal ACTH but is not dependent on activation of the fetal renin-angiotensin system.

Targeted disruption of the bradykinin B(2) receptor gene in mice alters the ontogeny of the renin-angiotensin system

Angiotensin II type 1 (AT(1)) receptor knockout (KO) mice exhibit an activated kallikrein-kinin system (KKS) that serves to attenuate the severity of the renal vascular phenotype in these mice (Tsuchida S, Miyazaki Y, Matsusaka T, Hunley TE, Inagami T, Fogo A, and Ichikawa I, Kidney Int 56: 509-516, 1999). Conversely, gestational high salt suppresses the fetal renin-angiotensin system (RAS) and provokes aberrant renal development in bradykinin B(2)-KO mice (El-Dahr SS, Harrison-Bernard LM, Dipp S, Yosipiv IV, and Meleg-Smith S, Physiol Genomics 3: 121-131, 2000). Thus the cross talk between the RAS and KKS may be critical for normal renal maturation. To further define the developmental interactions between the KKS and RAS, we examined the consequences of B(2) receptor gene ablation on the expression of RAS components. Renal renin mRNA levels are 50% lower in newborn B(2)-KO than wild-type (WT) mice. Also, the age-related decline in renin mRNA is greater in B(2)-KO than WT mice (3.5- vs. 2-fold, P < 0.05). Although renal angiotensinogen (Ao) protein levels are higher in newborn B(2)-KO than WT mice, Ao mRNA levels are not, suggesting accumulation of Ao as a result of decreased renin-mediated cleavage. Similar age-related increases (8-fold) in angiotensin I-converting enzyme (ACE) activity are observed in B(2)-KO and WT mice. Renal AT(1) protein levels are not different in B(2)-KO and WT mice. Furthermore, the developmental increases in renal kallikrein mRNA and enzymatic activity are more pronounced in B(2)-KO compared with WT mice (mRNA: 8- vs. 3-fold; activity: 13- vs. 6-fold, P < 0.05). We conclude that 1) bradykinin stimulates renin gene expression, 2) renal kallikrein is regulated via a negative feedback loop involving the B(2) receptor, and 3) Ao, ACE, and AT(1) are not bradykinin-target genes.

Targeted disruption of the bradykinin B2 receptor gene in mice alters the ontogeny of the renin-angiotensin system

First published July 12, 2001; 10.1152/ajprenal.0020.2001.—Angiotensin II type 1 (AT1) receptor knockout (KO) mice exhibit an activated kallikrein-kinin system (KKS) that serves to attenuate the severity of the renal vascular phenotype in these mice (Tsuchida S, Miyazaki Y, Matsusaka T, Hunley TE, Inagami T, Fogo A, and Ichikawa I, Kidney Int 56: 509–516, 1999). Conversely, gestational high salt suppresses the fetal renin-angiotensin system (RAS) and provokes aberrant renal development in bradykinin B2-KO mice (El-Dahr SS, Harrison-Bernard LM, Dipp S, Yosipiv IV, and Meleg-Smith S, Physiol Genomics 3: 121–131, 2000). Thus the cross talk between the RAS and KKS may be critical for normal renal maturation. To further define the developmental interactions between the KKS and RAS, we examined the consequences of B2 receptor gene ablation on the expression of RAS components. Renal renin mRNA levels are 50% lower in newborn B2-KO than wild-type (WT) mice. Also, the age-related decline in renin mRNA is greater in B2-KO than WT mice (3.5- vs. 2-fold, P < 0.05). Although renal angiotensinogen (Ao) protein levels are higher in newborn B2-KO than WT mice, Ao mRNA levels are not, suggesting accumulation of Ao as a result of decreased renin-mediated cleavage. Similar age-related increases (8-fold) in angiotensin I-converting enzyme (ACE) activity are observed in B2-KO and WT mice. Renal AT1 protein levels are not different in B2-KO and WT mice. Furthermore, the developmental increases in renal kallikrein mRNA and enzymatic activity are more pronounced in B2-KO compared with WT mice (mRNA: 8- vs. 3-fold; activity: 13- vs. 6-fold, P < 0.05). We conclude that 1) bradykinin stimulates renin gene expression, 2) renal kallikrein is regulated via a negative feedback loop involving the B2 receptor, and 3) Ao, ACE, and AT1 are not bradykinin-target genes.

Maternal undernutrition increases arterial blood pressure in the sheep fetus during late gestation

1. We have investigated the effect of a 50 % reduction in maternal nutrient intake during the last 30 days of pregnancy on arterial blood pressure and on arterial blood pressure responses to angiotensin II (AII) and the angiotensin converting enzyme (ACE) inhibitor captopril in the sheep fetus at 115-125 and at 135-145 days gestation (term = 147 +/- 3 days gestation). 2. Fetal plasma glucose concentrations were lower in the undernourished (UN) group compared to the control animals. There was no difference, however, in fetal plasma cortisol or adrenocorticotrophic hormone (ACTH) concentrations between the UN and control groups between 115 and 145 days gestation. 3. During the first 10 days of undernutrition, maternal plasma concentrations of cortisol were increased in the UN group compared to controls. At 115-125 days gestation, fetal arterial blood pressure was also higher in the UN group compared with controls and there was an inverse relationship (r = -0.62, P < 0.05) between mean arterial pressure and the fetal plasma concentrations of ACTH in the UN group. Fetal blood pressure responses to increasing doses of angiotensin II were also higher (P < 0.05) in UN compared to control animals at 115-125 days gestation. 4. Between 135 and 145 days gestation, fetal arterial blood pressure was increased in UN fetal sheep and mean arterial blood pressure was correlated with fetal plasma concentrations of cortisol. 5. Increased arterial blood pressure and responsiveness to AII measured in the fetuses of nutrient-restricted ewes may be related in part to fetal exposure to the actions of cortisol derived from transplacental transfer during the first 10 days after the start of the restricted feeding regime.

Role of cortisol in the ontogenic control of pulmonary and renal angiotensin-converting enzyme in fetal sheep near term

1. This study examined the ontogeny of angiotensin-converting enzyme (ACE) concentration in the lungs and kidneys of fetal, newborn and adult sheep, and investigated the effects of cortisol infusion on tissue and plasma ACE in the chronically catheterised ovine fetus. 2. Pulmonary and renal ACE in utero increased from 113 days of gestation towards term; peak tissue ACE concentrations were observed in fetuses studied at 143 days (term, 145 +/- 2 days). The high level of ACE seen in the fetal lungs close to term was maintained in the lambs and adult ewes whereas renal ACE decreased immediately after birth and rose to a maximal value in the adult ewes. In all groups of animals studied, higher mean concentrations of ACE were observed in the kidneys than in the lungs. Ontogenic increments in pulmonary and renal ACE in utero were coincident with the prepartum cortisol surge. In untreated and saline-infused fetuses, plasma cortisol correlated with both pulmonary (r = 0.83, P < 0.0001) and renal (r = 0.53, P < 0.01) ACE concentrations, irrespective of gestational age. 3. An intravenous infusion of cortisol (2-3 mg kg-1 day-1) at either 113 or 129 days raised plasma cortisol to the level seen near term and caused an increase in pulmonary ACE at both gestational ages. Pulmonary ACE concentration in the cortisol-infused fetuses at 129 days, but not at 113 days, was similar to that observed in the fetuses near term. In contrast, cortisol infusion had no effect on renal ACE concentration at either 113 or 129 days of gestation. Plasma ACE concentration was also increased by exogenous cortisol at 129 days. 4. Therefore, these findings suggest that the ontogenic rise in ACE concentration observed in the lungs of the sheep fetus near term is induced, at least in part, by the prepartum cortisol surge.

Restriction of placental and fetal growth in sheep alters fetal blood pressure responses to angiotensin II and captopril

1. We have measured arterial blood pressure between 115 and 145 days gestation in normally grown fetal sheep (control group; n = 16) and in fetal sheep in which growth was restricted by experimental restriction of placental growth and development (PR group; n = 13). There was no significant difference in the mean gestational arterial blood pressure between the PR (42.7 +/- 2.6 mmHg) and control groups (37.7 +/- 2.3 mmHg). Mean arterial blood pressure and arterial PO2 were significantly correlated in control animals (r = 0.53, P < 0.05, n = 16), but not in the PR group. 2. There were no changes in mean arterial blood pressure in either the PR or control groups in response to captopril (7.5 microg captopril min-1; PR group n = 7, control group n = 6) between 115 and 125 days gestation. After 135 days gestation, there was a significant decrease (P < 0.05) in the fetal arterial blood pressure in the PR group but not in the control group during the captopril infusion (15 microg captopril min-1; PR group n = 7, control group n = 6). 3. There was a significant effect (F = 14.75; P < 0.001) of increasing doses of angiotensin II on fetal diastolic blood pressure in the PR and control groups. The effects of angiotensin II were different (F = 8.67; P < 0.05) in the PR and control groups at both gestational age ranges. 4. These data indicate that arterial blood pressure may be maintained by different mechanisms in growth restricted fetuses and normally grown counterparts and suggests a role for the fetal renin-angiotensin system in the maintenance of blood pressure in growth restricted fetuses.

Angiotensin II and cardiovascular chemoreflex responses to acute hypoxia in late gestation fetal sheep

1. In six intact and nine carotid sinus denervated (CSD) fetal sheep (125-128 days gestation) we measured heart rate (FHR), mean systemic arterial blood pressure (MAP), femoral and carotid blood flows (FBF and CBF), and femoral and carotid vascular resistances (FVR and CVR). Three experiments were conducted on successive days: normoxia followed by acute isocapnic hypoxia (Pa,O2 to ca 12 mmHg) with infusion of vehicle (HV experiment), the same protocol but with infusion of the angiotensin converting enzyme (ACE) inhibitor, captopril (HC experiment), and normoxia alone with captopril infusion (NC experiment). Plasma angiotensin II concentration ([AII]) was measured in these fetuses, and in a separate group of fetuses (n = 5) that were infused with the nitric oxide (NO) synthesis inhibitor N G-nitro-L-arginine methyl ester (L-NAME) or saline vehicle. 2. During normoxia, cardiovascular parameters and plasma [AII] were unaltered by captopril infusion, apart from a fall in MAP (NC experiment only, P < 0.05) and FHR (HC experiment only, P < 0.05) in intact and CSD fetuses, respectively. No differences were observed between intact and CSD groups. 3. At the onset of hypoxia the rapid initial fall in FHR and rise in FVR was attenuated in CSD fetuses. In all fetuses FHR returned towards prehypoxic levels as hypoxia continued. In contrast, during hypoxia with vehicle infusion (HV experiment) plasma [AII] rose to a similar level in intact and CSD fetuses. 4. In both intact and CSD fetuses, the rise in [AII] during hypoxia was blocked by captopril or L-NAME infusion. In CSD, but not intact, fetuses infused with captopril the rise in MAP was absent, and the fall in FBF and rise in FVR did not reach significance during hypoxia. 5. Thus, during normoxia CSD alone, or combined with ACE inhibition, does not consistently alter basal cardiovascular control in the late gestation fetus. The rise in [AII] during hypoxia is not mediated by carotid reflexes but may involve NO-dependent mechanisms. In intact fetuses, AII does not appear to be pivotal in cardiovascular control during hypoxia. It is only when carotid reflex mechanisms are removed that a role for AII in the regulation of MAP and peripheral blood flow during hypoxia becomes apparent. These findings lend weight to the idea of multiple mechanisms of fetal cardiovascular control during hypoxia.

Comparison of angiotensin II type 1 receptor blockade and angiotensin-converting enzyme inhibition in pregnant sheep during late gestation

1. The effects of antagonism of the maternal renin-angiotensin system (RAS) with either an angiotensin II type 1-(AT1) specific receptor blocker (GR138950) or an angiotensin-converting enzyme (ACE) inhibitor (captopril) were compared in chronically-catheterised ewes and their foetuses during late gestation. 2. Daily from 127 +/- 1 days of gestation until parturition at 145 +/- 2 days, each ewe received i.v. either GR138950 (3 mg kg-1; n = 10), captopril (3 mg kg-1; n = 6) or an equivalent volume of vehicle solution (0.9% w/v saline; n = 10). 3. Within 2 h of drug administration, GR138950 abolished the maternal, but not the foetal, pressor responses to angiotensin II (AII; 100-188 ng kg-1, i.v.; P < 0.05), whereas captopril abolished both the maternal and foetal pressor responses to angiotensin I (AI; 400-750 ng kg-1, i.v.; P < 0.05). 4. On the first day of treatment, maternal blood pressure decreased in all GR138950-treated (-21 +/- 4 mmHg; P < 0.05) and captopril-treated (-13 +/- 5 mmHg; P > 0.05) ewes at 2 h after drug administration. Captopril also significantly decreased foetal blood pressure by 5 +/- 1 mmHg (P < 0.05). However, foetal blood pressure in the GR138950-treated animals remained unchanged. Maternal and foetal heart rates were unaffected by any treatment. Uterine blood flow was significantly reduced within 2 h of both GR138950 (-130 +/- 20 ml min-1; P < 0.05) and captopril (-72 +/- 16 ml min-1; P < 0.05) administration. 5. On the first day of treatment, maternal arterial haemoglobin (Hb) concentration and oxygen (O2) content increased at 2 h in all GR138950-treated and captopril-treated ewes. Foetal arterial pH and oxygenation (O2 content, O2 saturation and Pao2) were reduced by a similar extent in both groups of drug-treated ewes. 6. After one week of daily GR138950 administration, maternal blood pressure decreased from a pretreatment value of 96 +/- 5 mmHg on day 1 to 79 +/- 2 mmHg by day 7 (P < 0.05). Captopril treatment had no long-term effect on maternal blood pressure. Although foetal blood pressure increased by 3 +/- 1 mmHg over a week of vehicle treatment (P < 0.05), no significant differences were observed between the long-term changes in foetal blood pressure in all three groups of animals. 7. There were no long-term effects of drug administration on maternal Hb concentration or oxygenation, or on the foetal haematological parameters. However, changes in maternal PaCo2 observed in the GR138950-treated (+1.4 +/- 0.5 mmHg; P < 0.05) and captopril-treated (+3.3 +/- 1.1 mmHg; P > 0.05) ewes were significantly different from those seen in the vehicle-treated animals (P < 0.05). 8. There were no apparent adverse effects of maternal GR138950 or captopril treatment on foetal viability. 9. The present study demonstrated that administration of either GR138950 or captopril to pregnant ewes effectively blocked the maternal RAS, and caused hypotension and a decrease in uterine blood flow. However, only captopril appeared to cross the placenta to influence directly the RAS of the sheep foetus. This suggests that the fall in foetal oxygenation observed after AT1-specific receptor blockade and ACE inhibition originates primarily from changes in the maternal and/or placental vasculature. Despite these changes, neither GR138950 nor captopril were detrimental to the outcome of pregnancy when foetal blood loss was kept to a minimum.

Prevention by blockade of angiotensin subtype1-receptors of the development of genetic hypertension but not its heritability

1. We determined whether early inhibition of angiotensin II subtype1 (AT1) receptors by the newly synthesized nonpeptidic antagonist, A-81988, can attenuate the development of hypertension in spontaneously hypertensive rats (SHR) and if the altered blood pressure phenotype can be passed on to the subsequent generation, not exposed to the antagonist. 2. Pairs of SHR were mated while drinking tap water or A-81988 in tap water, and the progeny was maintained on the parental regimen until 14 weeks of age. At this stage, A-81988-treated rats showed lower systolic blood pressure and body weight values (136 +/- 5 versus 185 +/- 4 mmHg and 247 +/- 4 versus 283 +/- 4 g in controls, P < 0.01); while heart rate was similar. In addition, mean blood pressure was reduced (101 +/- 7 versus 170 +/- 7 mmHg in controls, P < 0.01), and the pressor responses to intravenous or intracerebroventricular angiotensin II were inhibited by 27 and 59%, respectively. Heart/body weight ratio was smaller in A-81988-treated rats (3.2 +/- 0.1 versus 3.8 +/- 0.1 in controls, P < 0.01). 3. The antihypertensive and antihypertrophic effect of A-81988 persisted in rats removed from therapy for 7 weeks (systolic blood pressure: 173 +/- 4 versus 220 +/- 4 mmHg, heart/body weight ratio: 3.4 +/- 0.1 versus 4.1 +/- 0.1 in controls at 21 weeks of age, P < 0.01 for both comparisons), whereas the cardiovascular hypertensive phenotype was fully expressed in the subsequent generation that was maintained without treatment. 4. These results indicate that chronic blockade of angiotensin AT1-receptors attenuates the development of hypertension in SHR but it does not prevent the transmission of hypertension to the following generation. Thus, heritability of the SHR's hypertensive trait is not affected by pharmacological manipulation of the cardiovascular phenotype.

The effects of a converting enzyme inhibitor (captopril) and angiotensin II on fetal renal function

1. Renal function was studied in chronically catheterized fetal sheep (119-128 days gestation), before and during treatment of the ewe with the angiotensin converting enzyme (ACE) inhibitor, captopril, which crosses the placenta and blocks the fetal renin angiotensin system. 2. An i.v. dose of 15 mg (about 319 micrograms kg-1) of captopril to salt-replete ewes followed by an infusion to the ewe of 6 mg h-1 (about 128 micrograms kg-1 h-1) caused a fall in fetal arterial pressure (P < 0.01), and a rise in fetal renal blood flow (RBF) from 67.9 +/- 5.6 to 84.9 +/- 8.3 ml min-1 (mean +/- s.e. mean) (P < 0.05). Renal vascular resistance and glomerular filtration rate (GFR) fell (P < 0.01); fetal urine flow (P < 0.01); fetal urine flow (P < 0.01) and sodium excretion declined (P < 0.05). 3. Ewes were treated for the next 2 days with 15 mg captopril twice daily. On the 4th day, 15 mg was given to the ewe and fetal renal function studied for 2 h during the infusion of captopril (6 mg h-1) to the ewe. Of the 9 surviving fetuses, 3 were anuric and 3 had low urine flow rates. When 6 micrograms kg-1 h-1 of angiotensin II was infused directly into the fetus RBF fell from 69 +/- 10.1 ml min-1 to 31 +/- 13.9 ml min-1, GFR rose (P < 0.05) and urine flow (P < 0.01) and sodium excretion increased in all fetuses. 4. It is concluded that the small fall in fetal arterial pressure partly contributed to the fall in fetal GFR but in addition, efferent arteriolar tone fell so that the filtration pressure fell further. Thus maintenance of fetal renal function depends on the integrity of the fetal renin angiotensin system. These findings explain why use of ACE inhibitors in human pregnancy is associated with neonatal anuria.

Renin and angiotensin converting enzyme concentrations in the fetal and neonatal guinea-pig

1. Plasma renin concentration (PRC) and plasma and pulmonary angiotensin converting enzyme (ACE) concentration were measured in fetal and neonatal guinea-pigs from 45 days gestational age (GA) until 21 days post-partum. 2. Fetal PRC increased towards term to reach values greater than those measured in normal adult males. Pentobarbitone anaesthesia of the mother resulted in significant elevation of fetal PRC after 66 days GA but not before this time. 3. PRC were very high in the newborn guinea-pig, decreased rapidly during the first 24 h after birth and then more gradually, to reach approximately adult values by day 21. 4. Fetal plasma ACE concentration increased towards term to reach values greater than those measured in adult males and decreased subsequently. 5. Pulmonary ACE concentrations were very low throughout gestation but increased considerably between days 3 and 14 post-partum. Low concentrations of ACE were measured in other fetal tissues but placental concentrations were relatively high. 6. Propranolol (0.1 mg I.P.) or saline was administered (under halothane-nitrous oxide anaesthesia) to fetuses of litters of various GA from 55 days to term. Fetal PRC were measured 3 h later. Propranolol treatment resulted in significantly lower fetal PRC than saline treatment in litters aged 63 days to term but not in younger litters. 7. These data indicate that the renin-angiotensin system is functional in the fetal guinea-pig during the last third of gestation. Fetal plasma renin concentrations near term are greater than those measured in normal adult males. This may, in part, reflect an increased influence of the fetal sympathetic nervous system.