A study of factors affecting blood pressure and angiotensin II in newborn infants

Levels of angiotensin peptides in healthy and cardiovascular/renal-diseased paediatric population-an investigative review

The renin-angiotensin-aldosterone system (RAAS) plays a major role in the regulation of blood pressure and homeostasis. Therefore, it is a commonly used target for pharmacotherapy of cardiovascular diseases in adults. However, the efficacy of this pharmacotherapy can only be limitedly derived into children. Comprehensive knowledge of the humoral parameters acting in the paediatric RAAS (e.g. angiotensin I, angiotensin II, angiotensin 1-7, angiotensin III, and angiotensin IV) might facilitate a more effective and rational pharmacotherapy in children. Therefore, this review aims to provide an overview of the maturing RAAS. Out of 925 identified records, 35 publications were classified as relevant. Physiological and pathophysiological concentrations of angiotensin peptides were compiled and categorised according to European Medicines Agency age groups. Age has a major impact on circulating angiotensin I, angiotensin II, and angiotensin 1-7, which is reflected in an age-dependent decrease during childhood. In contrast to data obtained in adults, no gender-related differences in angiotensin levels were identified. The observed increase in peptide concentrations regarding cardiac- and renal-diseased children is influenced by surgical repair, while evidence for a pharmacological impact is conflicting. A comprehensive set of angiotensin I, angiotensin II, and angiotensin 1-7 values from neonates up to adolescents was compiled. Indicating age as a strong effector. However, evidence about potential promising targets of the RAAS like angiotensin III and angiotensin IV is still lacking in children.

Differential responses to systemic and local angiotensin II infusions in conscious postnatal sheep

Angiotensin II (ANG II) increases blood pressure (MAP) via specific ANG II receptors (AT) and is considered important in regulating MAP after birth. In adult animals, AT(1) receptors predominate in vascular smooth muscle (VSM) and mediate vasoconstriction. In newborn sheep, AT(2) receptors, which do not mediate vasoconstriction, predominate in vascular smooth muscle until 2 wk postnatal when they are replaced by AT(1). Thus, the mechanisms whereby ANG II increases MAP after birth are unclear. We examined the effects of ANG II on femoral vascular resistance (FmVR) and blood flow (FmBF) in serial studies of newborn sheep (n = 7) at 7-14 d, 15-21 d, and 22-35 d. Animals had femoral catheters implanted for systemic ANG II infusions and cardiovascular monitoring, and a flow probe was implanted on the contralateral artery proximal to the superficial saphenous artery, which contained a catheter for intra-arterial ANG II infusions. Studies were performed using a range of systemic and intra-arterial ANG II doses. Systemic ANG II increased MAP dose-dependently at all ages (p < 0.001); however, responses were not age dependent. FmBF rose dose dependently at 7-14 d (p < 0.001) and was unchanged at older ages. FmVR was unaffected at 7-14 d, but values increased dose dependently at 15-21 d and 22-3 5d (p < 0.001), although never exceeded relative increases in MAP. Local ANG II did not alter MAP, FmBF, or FmVR at any age. Although systemic ANG II increases MAP and FmVR dose dependently after birth, ANG II-induced vasoconstriction is attenuated. Furthermore, intra-arterial ANG II does not alter FmVR in the absence of systemic responses, suggesting incomplete vascular smooth muscle AT(1) expression, stimulation of local ANG II antagonists, or ANG II-mediated release of another vasoconstrictor.

Effect of cortisol on blood pressure and the renin-angiotensin system in fetal sheep during late gestation

The effects of cortisol on blood pressure and the circulating components of the renin-angiotensin system (RAS) were investigated in sheep fetuses during late gestation and after exogenous cortisol infusion. Plasma cortisol concentration was greater in fetuses at 140 +/- 1 days of gestation (term 145 +/- 2 days) compared to those studied earlier in gestation (128 +/- 1 days), although, because of wide inter-animal variation, no differences were observed in blood pressure or plasma angiotensin II (AII), renin or angiotensinogen (Ao) concentrations. At 129 +/- 1 days of gestation, an infusion of cortisol for 5 days (2-3 mg kg(-1) day(-1) i.v.) increased plasma cortisol concentration to a value normally seen close to term. This rise in plasma cortisol was accompanied by increases in blood pressure and plasma concentrations of AII, renin and Ao. When observations from all fetuses were considered, plasma cortisol concentration correlated with plasma AII and renin, and blood pressure correlated with plasma cortisol and AII concentrations. Intravenous administration of an AII type 1 (AT(1))-specific receptor antagonist (3 mg kg(-1) GR138950) caused a reduction in blood pressure in all fetuses; the hypotensive response was greatest in fetuses studied near term and in the cortisol-treated fetuses. Overall, the magnitude of the hypotension induced by GR138950, and the concomitant rise in plasma renin, both correlated with the plasma cortisol concentration before GR138950 treatment. These findings show that, in the sheep fetus during late gestation, the RAS becomes more important in the maintenance of resting blood pressure when plasma cortisol concentration is elevated, whether endogenously or exogenously.

Factors determining prevalence of maternal antibody to measles virus throughout infancy: a review

The effectiveness of vaccination against measles, the leading cause of vaccine-preventable deaths in infants globally, is greatly impacted by the level of maternal antibody to measles virus (or "measles maternal antibody"; MMA) during infancy. Variation in the prevalence of maternal antibody to measles virus between infant populations across countries and sociodemographic strata is poorly understood. We reviewed the literature on the prevalence of MMA, focusing on 3 principal determinants: starting level of maternal antibody, placental transfer of maternal antibody, and rate of decay of maternal antibody after birth. Our review identified placental transfer as an important determinant, with greater efficiency found in studies performed in developed countries. Placental transfer was influenced by gestational age, human immunodeficiency virus infection, and malaria. Antibody levels in mothers varied widely between countries, although predictably according to vaccination status within populations. Rates of antibody decay across studies were similar. Future studies should evaluate the utility of the cord blood level of MMA as a predictor of vaccine efficacy in infancy; inclusion of World Health Organization international reference sera will facilitate comparisons. Greater understanding of the determinants of the prevalence of MMA will help national policy makers determine the appropriate age for measles vaccination.

Failure of the loop diuretic torasemide to improve renal function of hypoxemic vasomotor nephropathy in the newborn rabbit

The use of diuretic therapy in vasomotor nephropathy is a controversial topic. It is generally agreed that diuretics in this situation enhance urinary output, reduce the degree of volume expansion, and improve cardiac and especially the compromised lung function. It is less clear whether diuretic therapy improves kidney function. The present study evaluates the effects of intravenous torasemide on renal function in 25 anesthetized ventilated hypoxemic newborn New Zealand White rabbits with vasomotor nephropathy. This well-established animal model mimics the renal physiology of the human newborn and, in particular, that of the premature infant. Three groups of hypoxemic rabbits were studied. The animals of group 1 (n = 8) received no diuretic therapy (hypoxemic control), and those of group 2 (n = 9) were given a single intravenous dose of torasemide (1 mg/kg) in a curative fashion 2 h after hypoxemia was induced. In the third group of animals (n = 8), torasemide was given preventively; a bolus intravenous dose of torasemide (0.2 mg/kg) was given before the induction of hypoxemia and sustained by the addition of 0.2 mg x kg(-1) x h(-1) of the drug to the continuous intravenous infusion given throughout the entire 3-h hypoxemic period. Hypoxemia alone (group 1) caused acute renal insufficiency with a significant fall in mean arterial blood pressure, GFR, and renal blood flow; the renal vascular resistance increased, and the filtration fraction was unchanged. The curative dose of torasemide (group 2) induced a significant diuresis and natriuresis with minimal augmentation of urinary potassium excretion, converting mildly oliguric to nonoliguric acute renal insufficiency. In this group of animals, torasemide did not improve the glomerular dysfunction. The preventive dose of torasemide (group 3) even somewhat worsened the already impaired renal functions without further increasing the diuretic effect of the drug. We conclude that in hypoxemic newborn animals with renal dysfunction, torasemide is an effective potassium-sparing diuretic that unfortunately does not improve renal blood flow and GFR. The failure of torasemide to attenuate the glomerular dysfunction of hypoxemic vasomotor nephropathy in the newborn rabbit is disturbing. These data certainly caution against the overzealous use of loop diuretics in hypoxemic oliguric neonates.

Protective effect of perindoprilat in the hypoxemia-induced renal dysfunction in the newborn rabbit

The renal effects of acute hypoxemia and the ability of perindoprilat, a potent angiotensin-converting enzyme inhibitor, to prevent these effects were assessed in 31 anesthetized and mechanically ventilated newborn (5 to 8 d of age) rabbits. Renal blood flow (RBF) and GFR were determined by the clearances of para-aminohippuric acid and inulin, respectively. Each animal acted as its own control. In eight normoxemic rabbits (group 1), the i.v. infusion of saline did not change renal and hemodynamic functions. In eight additional rabbits, acute hypoxemia (PaO2= 40 mm Hg) induced a significant decrease in mean blood pressure (-8+/-2%), RBF (-36+/-3%), and GFR (-31+/-3%) and an increase in renal vascular resistance (+50+/-12%). A third group of newborn animals (n=7) was used to determine the renal effects of perindoprilat administration (20 microg/kg) under normoxemic conditions. RBF significantly increased (+15+/-2%) and renal vascular resistance significantly decreased (-15+/-3%), whereas GFR, mean blood pressure, and filtration fraction did not change significantly. In group 4 (n=7), perindoprilat infusion completely prevented the hypoxemia-induced alterations in GFR and renal vascular resistance and partially prevented the fall in RBF. These results demonstrate that angiotensin II modulates the renal immature microcirculation and that inhibition of its formation effectively prevents the hypoxemia-induced decrease in GFR.

Does positive pressure ventilation increase arginine vasopressin in preterm neonates?

AIM

To examine the effect of intermittent positive pressure ventilation (IPPV) on plasma arginine vasopressin concentration (pAVP) in preterm neonates.

METHODS

Thirty five neonates were classified, at the time of blood sampling, into three groups: unstable ventilated; stable ventilated; and stable non-ventilated. A modification of an extraction method for pAVP was developed for use in studies on very small babies, and sampling methods were compared.

RESULTS

The pAVP (median, range) was similar in the ventilated (1.85 pmol/l, 0.5 to 3.4) and non-ventilated (2.0, 0.5 to 2.6) stable babies, but was significantly higher (5.7, 1.1 to 25) in the unstable group. There was an inverse correlation between systolic blood pressure and pAVP concentration.

CONCLUSIONS

This study shows that in preterm neonates pAVP concentration is affected by the clinical condition and blood pressure, but not by treatment with IPPV.

Prevention of hypoxemia-induced renal dysfunction by perindoprilat in the rabbit

The role of angiotensin II, a potent postglomerular vasoconstrictor, in the hypoxemia-induced renal changes is still controversial. The ability of perindoprilat, an angiotensin converting-enzyme inhibitor, to prevent the acute renal effects of hypoxemia was assessed in 22 anesthetized-ventilated rabbits. In 8 untreated rabbits, hypoxemia induced a significant drop in mean blood pressure (MBP) (-12 +/- 2%), glomerular filtration rate (GFR) (-16 +/- 3%) and renal blood flow (RBF) (-12 +/- 3%) with a concomittant increase in renal vascular resistance (RVR) (+18 +/- 5%) and urine flow rate (+33 +/- 14%), and without any changes in filtration fraction (FF) (-4 +/- 2%). This suggests the occurrence of glomerular vasoconstriction during the hypoxemic stress. In 7 normoxemic rabbits, intravenous perindoprilat (20 microg/kg) induced an increase in urine flow rate (+17 +/- 4%) and RBF (+17 +/- 4%), and a decrease in MBP (-6 +/- 1%), RVR (-14 +/- 3%) and FF (-11 +/- 2%) without a significant change in GFR. The drop in FF and the increase in RBF suggests preferential postglomerular vasodilatation. In 7 rabbits, perindoprilat prevented the occurence of the hypoxemia-induced changes in RBF and RVR without improving MBP. FF decreased significantly (-18 +/- 2%), while the drop in GFR (-7 +/- 2%) was partially blunted and the increase in urine flow rate (+25 +/- 9%) was confirmed. These results could be explained by the inhibition of the angiotensin-mediated efferent vasoconstriction and by the inhibition of bradykinin degradation by perindoprilat. These data confirm the ability of converting-enzyme inhibitors to prevent the renal hypoperfusion induced by acute hypoxemia.

Effects of captopril on the human foetal placental circulation: an interaction with bradykinin and angiotensin I

1. The mechanism underlying the foetal toxicity induced by captopril is not well understood. Since bradykinin and angiotensin II appear to be important in the regulation of the placental circulation, experiments were performed to assess the effects of captopril on the vascular actions of these peptides on the human foetal placental circulation. 2. Full-term human placentas, obtained from normal pregnancy, were perfused with a modified Tyrode solution bubbled with O2 using a pulsatile pump. The placental perfusion pressure was measured with a Statham pressure transducer and recorded continuously on a Hewlett-Packard polygraph. 3. Bradykinin (0.1, 0.3 and 1.0 nmol) injected into the placental arterial circulation produced an increase in placental perfusion pressure in all experiments. This effect of bradykinin was significantly inhibited by indomethacin (3 x 10(-7) M). 4. Captopril (10(-7) M) significantly potentiated the pressor effect of bradykinin on the human placental circulation (n = 6). This effect of captopril was reversed by indomethacin (3 x 10(-7) M). 5. Angiotensin I (n = 6) and angiotensin II (n = 6), injected into the placental arterial circulation, both produced dose-dependent increases in placental perfusion pressure. The dose-response curves to angiotensin I (n = 6) were significantly displaced to the right by captopril in a concentration-dependent manner. 6. We suggest that the toxic effects of captopril on the foetus, rather than reflecting an inhibition of angiotensin II formation, may instead be related to a potentiation of the vasoconstrictor effect of bradykinin on the foetal placental circulation, thereby reducing blood flow and causing foetal damage. The reasons for this are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of extracellular fluid volume in neonates

This review summarises mechanisms of control of extracellular fluid volume in the neonatal period. 'Normal' body fluid distribution and methods of its measurement are discussed as well as regulatory mechanisms with particular emphasis on hormonal and renal aspects.

Dynamics of vasomotor thermoregulation of the skin in term and preterm neonates

Eighteen fullterm infants and 17 preterm infants were studied on their 3rd day of life to investigate the reactivity of skin blood flow to thermal stimulation. The infants were studied during quiet sleep. After a 10-min control period a constant air current was used to synchronise the external cutaneous stimulus to the distal lower extremity of each infant: the heating element of an air blower was automatically switched on and off to generate successive warm and cool periods of equal duration (5 cycles/min). Heart rate (HR), skin blood flow (SBF) and respiratory waveform signals were recorded and their variability was analysed using the fast Fourier transform and spectral analysis. Fullterm infants showed a clear response to external thermal stimulation: both HR and SBF were synchronised to the stimulation frequency. A response of preterm infants was present but it was markedly attenuated in comparison to term infants. The effect of stimulation did not seem to be dependent on postnatal age. The results suggest that the vasomotor control is immature in preterm infants.

Factors influencing plasma renin and renin substrate in premature infants

Concentrations of plasma renin (PRC) and plasma renin substrate (PRS) were measured during the first week of life in 52 infants born at less than 37 weeks' gestation (mean (SEM) gestation 30 (0.4) weeks, mean (SEM) birth weight 1.35 (0.08) kg). Both PRC (median 35, interquartiles 16.3, 94.5 ng/ml/hour) and PRS (median 2.3, interquartiles 1.3, 5.0 micrograms/ml) were raised compared with adults. The proportional rise in PRC was much greater than that in PRS, suggesting that PRS may be rate limiting in the generation of angiotensin I. Log10 PRC was inversely proportional to gestational age and a high urinary sodium loss was associated with a significantly raised log10 PRC. In hypoxaemic infants, there was a strong correlation between log10 PRS and haemoglobin concentration; this is a new observation in human infants but consistent with data available from other species.

Water deprivation: effects on fluid and electrolyte handling and plasma biochemistry in Long-Evans and Brattleboro rats

1. Intakes and urine outputs of fluid and electrolytes were measured before, during and after water deprivation in normal rats (Long-Evans strain) and in vasopressin-deficient rats (Brattleboro strain). 2. In a parallel experiment it was confirmed that the water-deprivation schedule used (Long-Evans rats 53 h, Brattleboro rats 14 h), and previously shown to cause similar percentage reductions in plasma volumes in the two strains, did produce more marked hypernatraemia and hyperosmolality and a greater percentage reduction in body weight in Brattleboro than in Long-Evans rats. 3. In Long-Evans rats, water deprivation caused a gradual reduction in urine output, a reduction in food intake and, during the first 24 h, increases in Na+ and K+ output. In Brattleboro rats, the reduction in urine output was more pronounced, but despite this total water losses were greater than from Long-Evans rats. Brattleboro rats showed a greater reduction in food intake. Their urinary Na+ and K+ losses were elevated during the first 9 h of water deprivation; thereafter these variables fell but remained above the level of intake. 4. The cumulative Na+ losses during water deprivation were similar in the two strains but the cumulative K+ losses in the Brattleboro rats were greater than in the Long-Evans rats. Thus the relative hypernatraemia and hyperkalaemia in water-deprived Brattleboro rats compared to water-deprived Long-Evans rats cannot be explained simply on the basis of differences in renal fluid and electrolyte handling. 5. There were significant increases in plasma angiotensin II and aldosterone levels at the end of the water deprivation periods in both strains of rat, and after the drinking water was returned there was a marked anti-natriuresis consistent with an expression of one of the renal actions of aldosterone.

Circulating 5-hydroxytryptamine concentrations in preterm newborns

Alterations in circulating 5-hydroxytryptamine (5-HT) concentrations play a role in the pathophysiology of respiratory failure in adults. We undertook a study to develop a micromethod and measure circulating free 5-HT concentrations in preterm newborns with and without respiratory distress. Forty-six samples of platelet-poor plasma were obtained from 29 preterm newborns with varying degrees of respiratory distress. Samples were taken on days 2-3 and 6-7 of life. For measuring 5-HT concentrations we used a precolumn sample enhancement technique followed by ion exchange HPLC with electrochemical detection. The assay allowed detection of extremely small (50 pg) amounts of 5-HT from small (0.2 ml) amounts of blood. The mean 5-HT concentration on days 2-3 was 1.77 +/-0.74 ng/ml (mean +/- 95% confidence limits) and on days 6-7 was 0.69 +/- 0.23 ng/ml. This represented a significant fall in 5-HT concentrations (P = 0.01). All of 16 paired serial samples fell with time (P = 0.006). We conclude that platelet-poor plasma 5-HT concentrations in premature newborns are low, that there is a significant decline in these values over the first week of life, and that, in contrast with adults, the presence of respiratory failure is not associated with increased free 5-HT concentrations. The low 5-HT concentrations seen in newborns may reflect the ability to increase pulmonary uptake.

Hypertension in the neonate

Blood pressure (BP) is low at birth. It increases with age, by about 1 mm Hg per day within the period of 3 to 8 days. It rises by about 1 mm Hg per week between ages 5 to 6 weeks. At a latter age, systolic BP is close to 95 +/- 10 mm Hg. Hypertension is a rare condition in the neonate, where it carries a high risk of cardiorespiratory failure and cerebral distress. Causes of neonatal hypertension are either secondary to congenital malformations or to acquired disease states: Congenital malformations: renal artery stenosis, renal artery hypoplasia, abdominal aortic atresia, coarctation of the aorta, kidney cystic disease, reflux nephropathies. Acquired diseases: thrombo-embolic renal artery complications secondary to umbilical artery catheterization or to thrombosis of ductus arteriosus, closure of abdominal wall defects, adrenal hematoma with renal artery compression, seizures in preterm infants, central nervous system disorders, drug-induced hypertension, infants of drug-dependent mothers. The morbidity and the mortality of neonatal hypertension are elevated. Death may be caused by severe uncontrollable hypertension or by concomitant problems. Morbidity may be related to drug-resistant hypertension, or to the side-effects of hypotensive drugs. Surgery in these severely ill infants also carries a significant risk.

The effect of dopamine administration on the activity of the renin-angiotensin-aldosterone system in sick preterm infants

Nine premature infants with birth weight of 1150 to 2500 g and gestational age of 28 to 35 weeks were given dopamine in a dose of 2-4 micrograms/kg/min to treat cardiopulmonary distress. In addition to monitoring of blood gases, blood pressure, acid-base balance, urine flow and urinary sodium excretion, plasma renin activity (PRA) and plasma aldosterone concentration (PA) was also determined prior to and during dopamine therapy. The dopamine-induced increase in urine flow and urinary sodium excretion was associated with a significant increase of PRA from 18.2 +/- 5.1 ng/ml/h to 33.0 +/- 5.6 ng/ml/h (P less than 0.025), while PA and blood pressure remained unaltered by dopamine administration. It is suggested that the angiotensin II-stimulated aldosterone production is overridden by the inhibitory effect of dopamine.

Role of renin and aldosterone in establishment of electrolyte balance in very low birthweight neonates

Urine sodium, potassium and chloride excretion, plasma renin activity (PRA) and urine aldosterone excretion (UAE) were measured in seven very low birthweight (VLBW) infants during the first 6 weeks after birth. Hyponatraemia was most common, and major changes in urine electrolyte excretion occurred, during the first 2 weeks. These changes in urine electrolyte excretion appeared to relate to improvement in distal tubular function. PRA did not correlate with urine excretion of either aldosterone or electrolytes. However, UAE correlated significantly with fractional sodium-potassium exchange in the distal tubule in a non-linear fashion (P less than 0.001) which suggested a threshold of aldosterone responsiveness between 70 and 100 nmol/24 h per 1.73 m2 UAE. We conclude that in VLBW infants the distal tubule can respond to aldosterone during the first 2-3 weeks, but that the threshold for responsiveness appears to be higher than it is in fullterm infants.

Studies on equine prematurity 4: Effect of salt and water loss on the renin-angiotensin-aldosterone system in the newborn foal

Plasma renin substrate concentration was measured in 18, four-day-old pony foals after the administration of the natriuretic agent frusemide. Thirteen foals had been delivered spontaneously; labour had been induced in the remaining five mares. Plasma aldosterone concentration was measured in 12 of the spontaneously delivered foals. Renin substrate concentration had risen sharply within 15 mins (P less than 0.005) and peaked at 1 h. The response was consistently greater in the induced foals. Serum sodium concentration fell rapidly in the induced foals (P less than 0.002 by 60 mins) but was better maintained in the spontaneous group. Individual serum potassium concentrations varied widely and were higher in both the induced group and smaller (30 kg or less) spontaneous foals. Plasma renin substrate concentration was positively correlated with plasma potassium in both groups (P less than 0.05, P less than 0.001 respectively). Plasma aldosterone rose significantly within 30 mins (P less than 0.025) and did not stabilise during the experiment. Plasma aldosterone was significantly correlated with renin substrate concentration in both lighter (P less than 0.005) and heavier (P less than 0.05) spontaneously delivered foals. It is suggested that maturation of renal sodium and potassium handling occurs late in gestation in the foal and is in part related to body mass. 'Premature' delivery following induced labour is associated with a more rapid loss of sodium when challenged with frusemide, even though the response of the renin-angiotensin system may be exaggerated.

Levels of renin, angiotensin I and II, angiotensin-converting enzyme and aldosterone in infancy and childhood

Basal plasma renin activity (PRA), angiotensin I and II (AI, AII), angiotensin-converting enzyme (ACE) activity and plasma aldosterone (PA) and sodium and potassium concentration were simultaneously measured in 55 healthy recumbent children aged between 1 week and 13 years. A significant (P less than 0.001) age-related decrease for PRA (r = -0.73), AI (r = -0.72), AII (r = -0.51) and PA (r = -0.71) was observed but not for ACE (r = 0.26, P = 0.06). After correction for age the correlation between PRA or PA and AI or AII was still significant (P less than 0.005). The strong correlation between AI and AII in the group as a whole (r = 0.82, P less than 0.001) and also in separate age groups, and an AI to AII ratio which was not different between the various age groups suggest that ACE activity in this age range is not rate-limiting for AII generation.

Central control of vasopressin release and thirst

The effects of changes in the composition and/or the volume of the extracellular fluid on central receptors involved in the regulation of water balance are discussed. A review of results obtained in adults is given, followed by a discussion of possible roles for vasopressin during the neonatal period.

Changes in the renin-angiotensin-aldosterone system in normotensive primigravidae in the four days after normal spontaneous delivery

Plasma renin activity, plasma renin concentration, plasma renin substrate, plasma angiotensin II and plasma aldosterone were measured in 10 true primiparae, normotensive throughout pregnancy, for 4 days after a spontaneous vaginal delivery. The concentration of all the measured variables, except plasma renin substrate, fell steeply over the first 3 days post partum. In four of the 10 patients, all components of the renin-angiotensin system increased in concentration somewhat thereafter, although not to the levels of the first day post partum. There are multiple potential sources of renin in the fetoplacental unit and it is possible that this 'pregnancy' renin may be capable of the partial suppression of the maternal renin production during pregnancy. The slight rebound observed in some patients may represent a hunting for re-establishment of non-pregnant values.

Angiotensin-converting enzyme and the renin-angiotensin system in normotensive primigravid pregnancy

In a prospective study the interactions between ACE and the renin-angiotensin system in 18 primigravid normotensive women were investigated throughout pregnancy. ACE activity was found to be depressed throughout pregnancy, rising in the last trimester and returning to non-pregnant values by 6 weeks post partum. No significant differences were found between cord arterial or venous and maternal venous ACE levels at delivery. ACE was inversely related to systolic and mean arterial blood pressures, inversely to serum sodium and urinary potassium and directly to serum potassium levels. Within the system, ACE was correlated only to aldosterone levels. In company with other components of the system ACE activity appears to be altered in pregnancy and to be influenced by similar control mechanisms. It is possible that ACE plays a modulating role on aldosterone secretion via the (des-asp1), Angiotensin I pathway.

A prospective study of plasma angiotensin-converting enzyme in normotensive primigravidae and their infants

Plasma angiotensin-converting enzyme (ACE) has been measured prospectively throughout pregnancy, at delivery and in the puerperium in 18 normotensive primigravidae and their infants. Plasma ACE was consistently lower during pregnancy than in comparable, non-pregnant controls, but rose progressively from about 30 weeks to term. At vaginal delivery maternal and fetal ACE levels did not differ significantly. There was a steady increase in maternal ACE activity up to 6 weeks post partum, when the levels were not significantly different from non-pregnant controls. No correlation could be found between plasma ACE and plasma renin activity or concentration, or plasma AII. Plasma aldosterone increased in parallel with ACE during the last ten weeks of pregnancy.

Importance of plasma angiotensin concentrations in a comparative study of responses to angiotensin in the maturing newborn lamb

Plasma angiotensin concentrations were measured in a longitudinal study of the vascular, renal, and adrenal responses to infusions of angiotensin II (AII) in the maturing newborn lamb. Basal plasma concentrations of angiotensin increased with age and correlated with the rising arterial pressure that occurred with maturation. However, age was a stronger determinant of arterial pressure than was plasma angiotensin concentration. For any given dose of AII per kilogram of body weight, the actual plasma angiotensin concentration achieved increased as the lambs matured and gained weight. Therefore, a comparative study of biologic responses to AII in maturing animals must be based on actual plasma angiotensin concentrations achieved rather than on dose of AII infused per kilogram of body weight. When analyzed on the basis of actual plasma angiotensin concentration, the increase in arterial pressure and the suppression of plasma renin activity in response to increasing plasma angiotensin concentrations did not differ significantly as the lambs matured. However, the increment in plasma aldosterone concentrations in response to increasing plasma angiotensin concentrations was diminished in immature lambs (less than 18 days) when compared to the aldosterone responses in the same lambs at older ages.

Ontogeny of blood pressure and the renin-angiotensin-aldosterone system. Sequential studies in the newborn lamb

To test the hypothesis that the rise in arterial pressure occurring with maturation is due to an increase in the vascular responsiveness to angiotensin II, sequential cumulative doses of angiotensin II were infused intravenously into chronically catheterized newborn lambs maintained on a constant, weight-adjusted sodium intake from birth to 8 weeks of age. Basal mean arterial pressure correlated with both age and weight, but age was a stronger determinant of mean arterial pressure than was weight. No change in the mean arterial pressure response to angiotensin II occurred with maturation. Basal plasma renin activity and plasma aldosterone concentrations were low and did not change significantly with age. Therefore, in the newborn lamb in the sodium replete state, age is a better determinant of arterial pressure than is weight. However, an age-related change in vasoconstrictor responsiveness to angiotensin II does not occur and, therefore, cannot account for the rise in arterial pressure that is observed with maturation.

Renin and angiotensin levels in children

Plasma renin activity, plasma renin concentration, and angiotensin II levels were measured in 63 normal children aged between 2 months and 12 years. The results showed that the high levels of renin and angiotensin II present in infancy remained above adult levels throughout the first decade of life but that there was a decline with age. Boys less than 8 years old had lower plasma renin activity and angiotensin II levels than girls of a similar age; this may be due to a relative substrate deficiency. Serum urea levels were inversely correlated with plasma renin activity in both sexes. A significant inverse relationship was found between both plasma renin activity and angiotensin II, and serum sodium in the girls; a similar, although not statistically significant, relationship was seen with plasma renin activity in the boys. An inverse correlation was found between plasma renin concentration and diastolic blood pressure for the group as a whole.

Comparison of the alpha and beta blocking drug, labetalol, and methyl dopa in the treatment of moderate and severe pregnancy-induced hypertension

Twentysix women with pregnancy-induced hypertension have been randomly treated with either labetalol or Aldomet. A more satisfactory control of blood pressure was obtained with labetalol with minimal side-effects. After two weeks of treatment with labetalol renal function had significantly improved with a markedly lower incidence of proteinuria. More patients went into spontaneous labour following labetalol than following Aldomet; the Bishop score was also higher in this group. No adverse effects attributable to labetalol were noted in the baby either ante- or post-natally.

Effect of the specific angiotensin antagonist (Sar1) (Ala8) angiotensin II on blood pressure and the renin-angiotensin system in the conscious pregnant ewe and fetus

A direct relationship was found between maternal diastolic blood pressure and simultaneously measured angiotensin II (All) levels (P less than 0.001) in chronically cannulated pregnant ewes. The infusion of Saralasin to the ewe resulted in a dose-dependent fall in blood pressure (P less than 0.005), the magnitude of which was proportional to the initial All levels (P less than 0.025). Plasma renin and All levels rose significantly during the infusion. No consistent fetal effects were seen. The infusion of normal saline had no effect on blood pressure or hormone levels. Thus it seems likely that the renin-angiotensin system is involved with the maintenance of normal blood pressure in the pregnant sheep. Fetal blood pressure either fell significantly or was unchanged following direct infusion of Saralasin. This may be related to development of the beta-adrenergic nervous system. The renin-angiotensin system may be more important in cardiovascular homeostasis in the immature than in the adult animal.