Platelet Angiotensin II Binding and Plasma Renin Concentration, Plasma Renin Substrate and Plasma Angiotensin II in Human Pregnancy

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Abstract
1. The results are presented of a cross-sectional study of 25 non-pregnant and 125 pregnant/postnatal women in whom platelet angiotensin II binding and plasma angiotensin II, plasma renin concentration and plasma renin substrate were measured. 2. Platelet angiotensin II binding was significantly lower in the first-trimester patients as compared with the non-pregnant women (P < 0.001). Specific binding remained low in the second and third trimesters, and in those patients studied 24 h after delivery. However, higher values, approximating to the non-pregnant level, were found 6 weeks postnatally (n = 25 for each group). 3. Plasma angiotensin II, plasma renin concentration and plasma renin substrate increased in pregnancy, with the increase becoming statistically significant as compared with the non-pregnant women in the second trimester. Maximal median values of plasma angiotensin II and plasma renin substrate were found in the third trimester, but maximal median values of plasma renin concentration were found in the second trimester. The concentrations of all three hormones fell after delivery. 4. There was an inverse correlation between platelet angiotensin II binding and simultaneously measured endogenous levels of plasma angiotensin II (P < 0.02) and plasma renin substrate (P < 0.05) in the 25 non-pregnant subjects. These findings support the concept of the angiotensin II receptor concentration being regulated by the plasma angiotensin II level. There was no correlation between platelet angiotensin II binding and plasma renin concentration. 5. In pregnancy, no correlation between platelet angiotensin II binding and plasma angiotensin II, plasma renin concentration or plasma renin substrate was found. This suggests that any regulation by plasma angiotensin II may not operate in pregnancy, when reduced binding concentrations are present. No statistically significant correlation was found at either 24 h or 6 weeks after delivery.