Sequential changes in the human renin-angiotensin system following delivery

Sodium balance in pregnancy

Sodium is the major cation in the extracellular fluid volume (ECFV) and as such, is the most important determinant of osmolality and of the volume of this fluid compartment. Hence any alteration in the control of body sodium will be reflected by changes in the ECFV, including the maternal plasma volume. There is no doubt that expansion of the plasma volume is a necessary and desirable event during pregnancy, influencing positively both maternal and fetal outcome. Therefore, studies of sodium balance in pregnancy provide important information relevant to both mother and fetus.

Nitric oxide-mediated changes in vascular reactivity in pregnancy in spontaneously hypertensive rats

1. To examine the mechanisms which may account for pregnancy-induced vasodilatation in spontaneously hypertensive rats (SHR), we have investigated the changes in vascular reactivity and the effects of endothelial nitric oxide (NO) inhibition in the in situ blood-perfused, mesenteric resistance vessels of 18-20 day pregnant SHR. The effects of NG-nitro-L-arginine (L-NOARG) were compared in pregnant and nonpregnant SHR and gestation matched normotensive Wistar-Kyoto (WKY) rats. 2. Intra-arterial mean blood pressures (MBP) were similar in pregnant and nonpregnant SHR. Basal perfusion pressures (BPP) were decreased in pregnant compared with nonpregnant SHR. Pregnant WKY had lower MBP and BPP than either pregnant or nonpregnant SHR. 3. Vasoconstrictor responses to electrical stimulation (ES) and intra-arterial noradrenaline (NA) were decreased in pregnant compared with nonpregnant SHR. These responses were still greater in pregnant SHR when compared with pregnant WKY. Vascular reactivity to angiotensin II (AII) in pregnant SHR was reduced to a similar level to that in pregnant WKY. 4. L-NOARG (5 mg kg-1, i.v.), an inhibitor of nitric oxide synthase, increased MBP and BPP in all groups. After L-NOARG, BPP were equalized between pregnant and nonpregnant SHR. Pregnant WKY still showed lower MBP and BPP than SHR groups. 5. L-NOARG potentiated vascular responses to ES, NA and AII in all groups. The blunted vascular responses to NA and ES were normalized and the reactivity to AII was only partially reversed in pregnant SHR compared with nonpregnant SHR. Pregnant WKY still had much lower vascular responses to ES and NA than either pregnant or nonpregnant SHR. L-NOARG enhanced vascular responses to All to a greater extent in pregnant SHR than in pregnant WKY.6. These results demonstrate that blunted responses to NA and ES were NO-dependent, while diminished reactivity to AII was only partially dependent on NO in the in situ blood perfused mesenteric resistance vessels of pregnant SHR.7. The present results in pregnant SHR differ from our previous finding with pregnant normotensive WKY, in which blunted responses to NA, but not to ES, were equalized by L-NOARG. Pregnancy induced vasodilatation in hypertensive rats appears to be more dependent on endothelial NO than in normotensive WKY. A defect of the endothelial NO generating pathway which promotes vasodilatation in pregnancy may contribute to the predisposition of women with essential hypertension to develop pre-eclampsia.

Labour-related eclampsia

Twenty-four cases of eclampsia managed in a hospital in Hong Kong over a five-year period were reviewed. Four patients had antepartum eclampsia and one had postpartum eclampsia following elective caesarean section. Four of these five patients had severe pregnancy-induced hypertension (PIH) (greater than 160/100 mmHg). The other 19 patients had eclampsia occurring during labour or within six hours of delivery. This group included 10 patients with labour-onset PIH, none of whom developed severe hypertension before their convulsions. In the remaining nine patients, only five had severe hypertension before convulsion. The results suggest that labour-related eclampsia has become more common than antepartum eclampsia, and severe hypertension before convulsion is an infrequent finding and therefore an unreliable sign in these patients.

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)

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.

The rabbit: a suitable model for investigation of vascular responsiveness during pregnancy

The decreased vascular response to angiotensin II that characterizes normal pregnancy is lost in pregnancy induced hypertension and associated with an increase in response to norepinephrine. These alterations in vascular responsiveness have not been thoroughly investigated and are poorly understood. Suitable models manifesting physiologic conditions known to exist in human pregnancy have not been satisfactorily demonstrated. We investigated the chronically catheterized and unmedicated rabbit for similarities of four conditions known to occur during human pregnancy. We compared mean arterial pressure and vascular response to angiotensin II and to norepinephrine in 19 non-pregnant and 16 pregnant animals. Plasma levels of angiotensin II, norepinephrine, epinephrine, and dopamine levels were measured and compared. We found striking similarities for conditions investigated in our rabbit model when compared with data reported in the literature for human pregnancy. We conclude the rabbit is a suitable model for investigating alterations of vascular response during pregnancy.

Labour-onset pre-eclampsia

In a consecutive series of 1,201 singleton pregnancies with pre-eclampsia, the onset occurred during labour in 290 (24.1%). There was no difference between the primiparous and parous patient in this respect (25.9% v 20.7%; P less than 0.10). The tendency for pre-eclampsia to develop during labour increased with advancing maturity of the pregnancy and seldom occurred before 38 weeks of gestation; this was again equally true of the primiparous and parous patient, as was the incidence of severe hypertension (diastolic pressure greater than 110mm Hg) (36.1% v 34.1%). The high incidences of severe hypertension (35.5%), proteinuria (41.7%), and eclampsia (2.1%), and the 1 maternal death testified to the severity of the disease process and the need for aggressive management. After delivery, the clinical signs tended to subside rapidly, but the early third stage of labour was a time of maternal risk, irrespective of whether ergometrine or Syntocinon was the oxytocic agent administered. Analysis of perinatal results showed that the risk to the fetus was minimal.

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.