Platelet angiotensin II binding in non-pregnant women and in normotensive pregnant and hypertensive pregnant women
A method for measuring human platelet Angiotensin II (All) binding was validated, and characterisation studies involving 67 non-pregnant subjects were performed. Platelets were found to possess high affinity binding sites with many of the characteristics of receptors. No correlation was found between platelet All binding and the rise in intracellular free calcium after ex vivo All infusion, thus formal validation of the binding sites as receptors was not achieved. In 25 non-pregnant subjects, there was an inverse correlation between platelet All binding and simultaneously measured plasma All (P<0.02). In 10 ovulatory subjects, platelet All binding diminished in the luteal phase of the menstrual cycle (P<0.02). In a pilot cross-sectional study of platelet All in normotensive pregnancy, incorporating 125 pregnant/postnatal patients, platelet All binding was significantly lower in 1st trimester patients as compared to non-pregnant subjects (P<0.001). Platelet All binding remained low throughout pregnancy. Higher values, approximating to the non-pregnant level, were found 6 weeks postnatally. These findings were confirmed in a longitudinal study .of 30 pregnant women, with a diminution in platelet All binding being suggested by 5-8 weeks gestation (p= 0.02). Inverse correlations in pregnancy between platelet All binding and the components of the renin-angiotensin system were found (P<0.01). There were also significant correlations between platelet All binding and the levels of serum sodium, urea and osmolality (P<0.01). When platelet All binding was measured in 67 patients with established hypertension in pregnancy, binding in patients with pregnancy induced hypertension (PIH) was significantly higher than in normotensive primigravidae (P<0.0001). No differences in binding were found in the puerperium. In a prospective comparison of platelet All binding and the All sensitivity test in predicting the development of PIH, involving 34 subjects, platelet All binding was a more effective discriminant than any of the parameters derived from the All sensitivity test. There was a significant correlation between platelet All binding and the slope of the curve relating the diastolic pressor response to infused All (P<0.01).