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

Mechanisms regulating angiotensin II responsiveness by the uteroplacental circulation

Pregnancy is associated with increases in cardiac output and uterine blood flow (UBF) and a fall in systemic vascular resistance. In ovine pregnancy, UBF rises from approximately 3% of cardiac output to approximately 25% at term gestation, reflecting a >30-fold rise in UBF by term. This increase in UBF supports exponential fetal growth during the last trimester and maintains fetal well-being by providing excess oxygen and nutrient delivery. These hemodynamic changes are associated with numerous hormonal changes, including increases in placental steroid hormones and enhanced activation of the renin-angiotensin and sympathetic nervous systems, all of which are believed to modulate systemic and uterine vascular adaptation and vascular reactivity. Systemic pressor responses to infused ANG II are attenuated in normotensive pregnancies and the uteroplacental vasculature is even less sensitive, suggesting development of mechanisms to maintain basal UBF and permit the rise in UBF necessary for fetal growth and well-being. The effects of ANG II on the uteroplacental vasculature are reviewed, and the mechanisms that may account for attenuated vascular sensitivity are examined, including ANG II metabolism, vascular production of antagonists, ANG II-receptor subtype expression, and the role of indirect mechanisms.

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.

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.

Role of angiotensin II and alpha-adrenergic receptors during estrogen-induced vasodilation in ewes

Estradiol-17 beta (E2 beta) produces uterine and systemic vasodilation in nonpregnant ewes without altering mean arterial pressure (MAP). Mechanisms responsible for maintaining MAP and thus uterine blood flow (UBF) may include activation of the renin-angiotensin and/or adrenergic systems. We therefore investigated the effects of systemic blockade of angiotensin II (ANG II) and/or alpha-adrenergic receptors in nonpregnant, castrated ewes, using saralasin (Sar) and/or phentolamine (Phen) in the presence or absence of intravenous E2 beta (1.0 microgram/kg). In nonestrogenized ewes neither antagonist alone had substantial cardiovascular effects; however, Sar + Phen decreased systemic vascular resistance (SVR) 20 +/- 7.4% (SE) and increased heart rate (HR) 50 +/- 19% (P < 0.01); MAP and UBF were unaffected. Following E2 beta treatment SVR fell 17 +/- 2.4% (P < 0.01), UBF increased more than fourfold, and MAP was unchanged. Compared with E2 beta alone, Phen + E2 beta decreased SVR 42 +/- 4.7%, and MAP fell 11 +/- 1.8% (P < 0.05) despite 40-50% increases in HR and cardiac output (P < 0.05). Responses to Sar + E2 beta were similar to E2 beta alone, except for a fall in MAP, whereas responses to Sar + Phen + E2 beta resembled those of Phen + E2 beta. E2 beta-induced uterine vasodilation was unaltered by Sar and/or Phen. During E2 beta-induced vasodilation, MAP is maintained by enhanced activation of the alpha-adrenergic and renin-angiotensin systems; however, uterine vascular responses to E2 beta are independent of both systems and perfusion pressure.

Local and systemic estradiol-17 beta: effects on uterine and systemic vasodilation

Systemic estradiol-17 beta (E2 beta) administration increases uterine blood flow (UBF), cardiac output (CO), heart rate (HR), and plasma renin activity (PRA). We sought to determine if the E2 beta-induced systemic responses were dependent on the observed uterine responses. Nonpregnant, ovariectomized ewes (n = 5) received 3 micrograms E2 beta into both uterine arteries followed 120 min later by systemic E2 beta, 1 microgram/kg. At 120 min after local E2 beta, UBF increased from 26 +/- 5 to 161 +/- 21 ml/min (P less than 0.05); uterine vascular resistance (UVR) decreased 83 +/- 2.5% (P less than 0.05); and systemic parameters were unchanged. At 120 min after systemic E2 beta, UBF remained elevated and CO had increased gradually from 4.4 +/- 0.2 to 5.5 +/- 0.32 l/min (26 +/- 3.4%, P less than 0.05), reflecting a 37 +/- 3.9% (P less than 0.05) increase in HR; mean arterial pressure (MAP) remained unchanged. The increased CO was associated with a 20 +/- 3.1% (P less than 0.05) fall in systemic vascular resistance (SVR), with % delta SVR less than % delta UVR (P less than 0.05). Base-line PRA and angiotensin II, 1.31 +/- 0.2 ng.ml-1.h-1 and 10.3 +/- 2.1 pg/ml, respectively, were unchanged by local E2 beta; systemic E2 beta caused increases to 3.56 +/- 0.51 ng.ml-1.h-1 (P less than 0.05) and 34.1 +/- 11.3 pg/ml (P less than 0.05), respectively. E2 beta-induced uterine hyperemia occurs independent of its systemic effects and is not responsible for systemic cardiovascular alterations, and the relative uterine vascular responses exceed systemic responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II vascular receptors in fetal and neonatal rats

Specific binding sites for angiotensin II in aorta and renal arteries have been studied in rat fetuses (18th day of pregnancy) and 1-day-old newborn rats by binding studies in arterial membranes using [125I] ileu-5-angiotensin II. One type of angiotensin receptor was found both in fetuses and in the newborns; the capacity of this (RT) decreased immediately after birth (from 0.06 +/- 0.01 nM to 0.02 +/- 0.005 nM; +/- SEM) and the affinity (Kd) increased at birth (from 3.5 +/- 0.6 nM to 19.5 +/- 1.2 nM; +/- SEM). Localization of the specific binding sites was studied by autoradiography on arteries from fetal and newborn rats either perfused with iodinated angiotensin II by cannulation of the aorta or in vitro on cryostat sections incubated with the radioactive angiotensin II. Both in fetuses and in the newborn the binding sites were located in the tunica media of the arteries.

The effect of enalapril (MK421), an angiotensin converting enzyme inhibitor, on the conscious pregnant ewe and her foetus

The effects of enalapril, an angiotensin converting enzyme (ACE) inhibitor, on maternal and foetal blood pressure, heart rate and components of the renin-angiotensin-aldosterone system were studied in 9 chronically-cannulated pregnant ewes and their foetuses. Six ewes received 1 mg kg-1 enalapril i.v. while 3 were given 2 mg kg-1. Although the initial fall in blood pressure was slightly greater in the higher dose group, there was substantial overlap of data. The pressor response to angiotensin I, assessing ACE activity, was abolished within 10 min of administration, and did not recover during 3 h of observation. Maternal systolic and diastolic pressures reached a nadir 90 min after administration (P less than 0.001, P less than 0.002 respectively). The maximum tachycardia was seen at 60 min (P less than 0.05). The foetuses of the ewes given 1 mg kg-1 enalapril showed no change in systolic or diastolic blood pressure or heart rate. Those of the ewes given the higher dose showed late-onset hypotension, coincident with the lowest maternal blood pressures. Maternal plasma renin concentration (PRC) had risen significantly by 30 min (P less than 0.02), reaching a maximum at approximately 90 min. Maternal plasma angiotensin II and aldosterone concentrations both fell initially (P less than 0.05) but were almost at basal levels by the end of the experiment. Foetal plasma renin, angiotensin II and aldosterone concentrations were unchanged throughout the experiment. Peak values of enaprilic acid, the active principle, were recorded in maternal plasma 65-90 min after administration of 1 mg kg-1, and 25-30 min after the administration of 2 mg kg-1. A trace amount of the active principle was recorded in the foetal plasma of one lamb, whose mother had been given the higher dose. None was recorded in the plasma from three other lambs. Maternal plasma ACE concentrations fell by an average of 84%; in 4 of the 6 ewes in which concentrations were measured they were undetectable after treatment. Foetal plasma ACE concentrations were unchanged throughout. Enalapril at 1 mg kg-1 thus exerts a depressor effect on the pregnant ewe which is probably related to its blockade of the renin-angiotensin system. Both direct measurement of the drug and foetal observation suggest that it does not cross the sheep placenta at this dose. This is consistent with its failure to cross the blood-brain barrier. Foetal effects were noted at 2 mg kg-1, and there was an unexpected foetal death in this group.

Effect of captopril on bradykinin inactivation by human foetal placental circulation

The inactivation of bradykinin on passage across the human foetal placental circulation was investigated in six full-term human placentas. The placentas were perfused with a modified Krebs-Henseleit solution and placenta perfusion pressure was recorded. Samples collected at the arterial inflow and at the venous effluent were assayed on the isolated guinea-pig ileum as an estimation of bradykinin activity. Bradykinin (100 ng ml-1) was infused through the foetal placental vessels before and during captopril 4 X 10(-7) M. Bradykinin produced a transient increase in placental vascular resistance that was not potentiated by captopril. Bradykinin activity was completely abolished after passage through the foetal placental circulation, and the inactivation was blocked by captopril. These data suggest that angiotensin I converting enzyme (kininase II) might occur in the foetal placental vessels.

Fetal responses to maternal infusions of angiotensin II

It is unclear whether the fetus is affected by maternal infusions of angiotensin II; therefore we studied maternal and fetal responses (n = 9) to angiotensin II (1.15, 2.29, 11.5 micrograms/min) infused 5 minutes into the vena cava of chronically instrumented sheep (129 to 137 days of gestation) while monitoring PO2, PCO2, pH, heart rate, uterine blood flow, and arterial and umbilical venous pressures. Pregnant sheep demonstrated expected dose-related increases in mean arterial pressure and decreases in uterine blood flow (p less than 0.05). Increases in fetal mean arterial pressure also correlated with the maternal dose of angiotensin II (r = 0.77, p less than 0.001). Fetal heart rate appeared to increase with 2.29 micrograms/min; however, bradycardia was observed with 11.5 micrograms/min (p less than 0.05) and was associated with decreased PaO2, 19.0 +/- 1.0 to 14.3 +/- 1.4 mm Hg (p less than 0.05), increased PaO2 (p less than 0.05), and decreased umbilical venous PO2, 31.4 +/- 2.3 to 27.0 +/- 1.9 mm Hg. The decreases in PO2 correlated with decreases in uterine blood flow (r = 0.60, p less than 0.002, and r = 0.75, p less than 0.005, respectively). Nevertheless, changes in fetal mean arterial pressure also occurred in the absence of altered fetal oxygenation; thus decreased uterine blood flow and fetal oxygenation alone cannot explain the fetal cardiovascular responses. It is suggested that angiotensin II or an active metabolite may cross the ovine placenta.

Hemodynamic effects of indomethacin in chronically instrumented pregnant sheep

Indomethacin administration has produced decreases in uteroplacental blood flow in several animal studies; therefore, it has been suggested that the maintenance of uterine blood flow is critically dependent on the continued synthesis of vasodilating prostaglandins. However, vasoconstriction following indomethacin administration may be due to mechanisms other than reduced prostaglandin synthesis. We administered indomethacin (2, 5, or 10 mg/kg) intravenously to seven unanesthetized sheep in late pregnancy and determined the time courses of the uteroplacental and systemic hemodynamic responses, comparing these to the concurrent changes in circulating prostaglandins. Indomethacin administration resulted in rapid increases in systemic and uteroplacental vascular resistance (80% to 100%) and mean arterial pressure (approximately 30%) and in decreases in systemic (approximately 30%) and uteroplacental (0% to 30%) blood flows within 5 minutes. Vasoconstriction was transient, however, and after 60 minutes there was no evidence of uterine or systemic vasoconstriction, although systemic and uterine plasma prostaglandin levels remained reduced for 180 minutes. Thus substantial inhibition of prostaglandin synthesis existed without evidence of concurrent systemic or uteroplacental vasoconstriction, suggesting that uterine blood flow is not directly dependent on maintained prostaglandin synthesis in unstressed pregnant sheep. Furthermore, the transient indomethacin-induced vasoconstriction may not be due to inhibition of prostaglandin synthesis.

The effect of saralasin in the chronically cannulated ewe in the early puerperium

The effects of angiotensin II receptor blockade with saralasin ((sar1) (ala8) angiotensin II) have been studied 1-3 days post partum in seven ewes with indwelling vascular cannulae. Control experiments were performed 48 h later in five of the same ewes. The infusion of saralasin at 1, 2, 4 and 8 micrograms kg-1 min-1 resulted in an initial small pressor response, followed by a depressor effect. A significant inverse correlation was demonstrated between log10 dose saralasin and the evoked change in diastolic blood pressure (r = -0.5891, P less than 0.005). A small and inconsistent depressor effect was found in the control experiments; there was no evidence for an association between dose and response (r = +0.0411, P greater than 0.85). These data were compared with those from a previously published study in which an identical infusion protocol for saralasin was used in ten chronically cannulated pregnant ewes. The slopes of the two dose-response curves were very similar (-14.9 +/- 4.3 post partum compared with -14.2 +/- 4.2) in the two groups. However, the average response was greater in the pregnant group, by 7.6 +/- 1.2 mmHg (P less than 0.005). Plasma renin concentration rose significantly during saralasin infusion (P less than 0.05) but was unchanged in control experiments, indicating blockade of the renin-angiotensin system by the removal of negative feed-back control by angiotensin II. Saralasin is known to exert an agonist effect when angiotensin II itself is not directly concerned in the regulation of arterial blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the effect of mode of delivery on the renin-angiotensin system in mother and fetus at term

Plasma renin substrate (PRS) and renin (PRC) concentrations were measured in the paired umbilical artery and vein blood of 54 babies and in the venous blood of 23 of their mothers at vaginal delivery. Similar samples were collected from 31 babies and their mothers at elective caesarean section. Fetal renin concentrations in both umbilical artery and vein blood were markedly increased at vaginal delivery compared with values at operative delivery. Renin substrate concentration was correlated with the length of second stage of labour. Umbilical vein PRC and PRS were greater than corresponding umbilical artery values at vaginal, but not at operative delivery, suggesting that some factor associated with delivery might stimulate the placenta to produce renin and renin substrate during vaginal delivery. Maternal and umbilical concentrations of both renin and renin substrate were unrelated, supporting the view that the maternal and fetal compartments are independent. It is suggested that hyperactivity of the renin-angiotensin system may be an integral part of the marked changes in cardiovascular homeostasis which occur at birth.

The effect of captopril (SQ14,225) upon mother and fetus in the chronically cannulated ewe and in the pregnant rabbit

1. An inhibitor of angiotensin-I-converting enzyme activity (D-3-mercapto-2-methylpropanoyl-L-proline, Captopril) was given to five chronically cannulated pregnant ewes and eleven rabbits in late pregnancy. 2. Within 2 min of administration to the sheep, Captopril had blocked the maternal conversion of angiotensin I to II, as assessed by the pressor response evoked by the i.v. administration of angiotensin I. Maternal and fetal basal systemic blood pressures had fallen within 10 min of administration. Although maternal systemic blood pressure returned to basal levels within 2 hr, fetal pressures remained low for up to 2 days. 3. All ewes went into spontaneous labour at or near term. One lamb was live-born but very weak and failed to establish suckling. The remaining seven lambs were fresh still-births. 4. Gestation length was significantly prolonged in the treated rabbits by comparison with ten controls. The still-birth rate was 37% in the treated animals and 6% in the controls (P less than 0.001). 5. It is concluded that the administration of Captopril to the two species studied is harmful to the fetus. The observations suggest that the drug rapidly crosses the placenta, and may cross the blood-brain barrier to exert a central effect. It may also interfere with the normal initiation of parturition.

Angiotensin and the cardiac baroreflex response to phenylephrine

1. In adult conscious sheep the pressor actions of infused angiotensin II were prevented by the concomitant intravenous infusions of sodium nitroprusside. The effect of such intravenous infusions of angiotensin II on the cardiac baroreflex response to transient rises in arterial pressure caused by intravenous phenylephrine was studied. 2. Intravenous infusion of angiotensin II caused a reduction in pulse interval in the absence of any change in arterial pressure. It also caused a reduction in baroreflex sensitivity measured by determining the relation between pulse interval and systolic pressure during the rise in pressure caused by injection of phenylephrine. 3. After administration of the converting enzyme inhibitor (captopril) the mean baroreflex sensitivity of 3 of 4 pregnant ewes increased. 4. It is concluded that high levels of angiotensin II can modify the cardiac baroreceptor reflex response so that the heart rate is inappropriately high for a given systolic pressure and that it reduces the sensitivity of the cardiac baroreflex response to transient changes in arterial pressure.

The effect of SAR1ALA8 AII on ovine uterine and umbilical blood flow

The effects of Sar1Ala8 AII (Saralasin), an angiotensin II (AII) receptor blocker, were examined in the late gestation ewe and fetus. Maternal administration of Saralasin resulted in a reduction in both mean maternal systemic arterial pressure and uterine blood flow. There were no changes in the measured fetal indices during maternal drug administration. Infusion of Saralasin to the fetus caused variable reductions in mean fetal arterial blood pressure and umbilical flow. There were no changes in maternal parameters measured. Therefore, in pregnant sheep in the third trimester, a component of normal uterine blood flow is maintained by endogenous AII, primarily through maintenance of adequate perfusion pressure.

Sequential changes in the human renin-angiotensin system following therapeutic termination of pregnancy

Plasma renin and angiotensin II levels were measured in nine patients immediately before and at half-hourly intervals in the four hours following therapeutic termination of pregnancy. There was a small fall in renin and angiotensin II levels over the first 1 to 2 hours, followed by a slight increase. The magnitude of these effects was much smaller than those previously seen following normal delivery. It is concluded that in early pregnancy maternal, rather than feto-placental, factors are controlling the renin-angiotensin system.