Accuracy of single concentration estimations of platelet angiotensin II receptor number. Its usefulness in screening for pregnancy-induced hypertension

Mechanisms underlying the decrease in circulating angiotensin II concentration after sodium loading

1. Acute sodium loading causes a rapid decrease in the circulating concentration of angiotensin II (AngII), which is apparent from 5 min after sodium administration. This could result from an increase in AngII catabolism and/or a decrease in AngII synthesis/secretion. However, the major determinant of AngII synthesis is thought to be a change in plasma renin activity, which occurs over a longer time frame (15 min). 2. To investigate the mechanisms underlying the rapid decrease in plasma AngII engendered by sodium administration, we performed metabolic clearance studies in male New Zealand white rabbits before and after a hypertonic sodium load of 1.5 mmol/kg as 0.513 mol/L saline i.v. bolus. 3. The metabolic clearance rate of AngII increased significantly from 42.2 +/- 9.0 mL/min per kg before sodium to 110.8 +/- 33.7 mL/min per kg after sodium administration (P < 0.05). The calculated or theoretical secretion rate decreased from 1470.7 +/- 404.2 to 573.5 +/- 139.5 fmol/min per kg (P < 0.025) in response to sodium. 4. We conclude that an increase in AngII metabolism and a decrease in synthesis/secretion contribute to the reduction in circulating AngII, which occurs in the first 60-90 min after sodium loading.

A prospective longitudinal study of platelet angiotensin II receptors for the prediction of preeclampsia

OBJECTIVES

We developed a simplified method using a relatively small volume of blood for the determination of platelet angiotensin II receptors by saturation analysis and we evaluated its performance for the prediction of preeclampsia.

DESIGN AND METHODS

A platelet suspension with minimal contamination by leukocytes and erythrocytes is obtained by centrifugation and washing. The platelet concentrate is incubated in a multi-well plate with increasing concentration of radiolabelled angiotensin II in the presence or absence of an excess of unlabelled angiotensin II. Bound and free fractions are separated using an oil mixture. Maximum binding is determined by Scatchard plot. This method was compared with a previously reported method. Our method was prospectively evaluated in 801 women attending our institution for routine prenatal care. A specimen was obtained at each trimester of pregnancy whenever possible. Diagnosis of preeclampsia was done postnatally by an experienced obstetrician.

RESULTS

The method showed acceptable correlation with a previously published method although a proportional bias of 2.1 was observed between the two methods. No differences in mean maximum binding were observed between normal and affected pregnancies at either trimester. Even when the results were analyzed longitudinally, using the change in maximum binding between two trimesters for each patient, no significant increase could be documented in preeclamptic pregnancies.

CONCLUSIONS

Platelet angiotensin II receptor measurement is not a clinically useful marker for the prediction of preeclampsia.

Changes in angiotensin II metabolism contribute to the increased pressor response to angiotensin after chronic treatment with L-NAME in the spontaneously hypertensive rat

1. Administration of nitric oxide (NO) synthase inhibitors, such as L-NAME, is associated with an increase in blood pressure and an increase in pressor responsiveness to infused angiotensin II (AngII). The present study was designed to investigate the contribution of changes in the metabolism of AngII to the enhanced pressor response to AngII in the spontaneously hypertensive rat (SHR; 14 weeks old) chronically treated with L-NAME. 2. Group I rats received L-NAME for 7 days (5 mg/kg per day) in their drinking water. Group II rats received water only. On day 7, rats were anaesthetized and metabolic clearance studies were performed. AngII concentrations in plasma and infusate were measured by radioimmunoassay. 3. Urinary NO2 was unchanged after L-NAME treatment, while NO3 decreased compared with control. Mean arterial pressure (MAP) was higher in the L-NAME treated rats than in control. After 30 min infusion of AngII, MAP increased significantly in both groups, although the increase was larger in L-NAME-treated than control rats. The metabolic clearance rate of AngII was significantly lower in L-NAME-treated rats than in the control group. 4. We conclude that chronic NO synthase inhibitors, such as L-NAME, cause a decrease in the rate at which AngII is metabolized. This decrease, in combination with the increase in the number of vascular AngII receptors, may account for the reported increase in pressor responsiveness to infused AngII.

A study of angiotensin II receptors after chronic inhibition of nitric oxide synthase in the spontaneously hypertensive rat

1. Nitric oxide (NO) synthase inhibition, induces a sustained increase in blood pressure and amplifies the pressor response to infused angiotensin II (AngII). This study was designed to investigate the contribution of AngII receptors in the elevated blood pressure and enhanced pressor response to AngII in the spontaneously hypertensive rat (SHR) chronically treated with N(G)-nitro-L-arginine-methyl ester (L-NAME). 2. Two groups of 13 week old female SHR were housed four to a box. Group I rats received L-NAME for 7 days (2.5 mg/kg per day) in their drinking water. Group II rats received water only. Blood pressure was monitored daily by tail-cuff plethysmography. Plasma AngII was measured by radioimmunoassay. Aortic and uterine receptor binding was determined by saturation analysis using [125I]-Sar8, Ile1)AngII. Data was analysed using the computer program LIGAND. 3. Mean systolic blood pressure was significantly elevated in rats treated with L-NAME compared with the control group. Plasma AngII concentration was slightly decreased in rats treated L-NAME compared with control. Densities of both aortic and uterine AngII receptors increased significantly following NO synthase inhibition. Receptor affinity in the aorta decreased in the L-NAME group compared with control. However, uterine AngII receptor affinity was unchanged. 4. We conclude that the increased blood pressure and enhanced pressor responsiveness that occurs with chronic inhibition of NO synthesis may result partly from increased vascular AngII receptor expression.