Effects of Positional Change and Sodium Balance on the Renin-Angiotensin-Aldosterone System, Big Renin and Prostaglandins in Normal Pregnancy*

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Abstract
Plasma renin activity and concentration, angiotensin II, aldosterone, big renin and prostaglandins[PG]A, E and F were determined in 11 [human] subjects during late normal pregnancy. Samples were obtained in lateral recumbency and 5 min after change to supine position, during a control period and after (10 meq/d[day]) and high (300 meq/d) Na intake. Plasma concentrations of all substances were unaffected by change from lateral to supine position and those of big renin and the PG were unaffected by change from low to high Na intake. Alterations in Na intake were associated with significant changes in plasma renin activity and concentration, angiotensin II and aldosterone in a manner qualitatively similar to those of non-pregnant subjects. Correlation coefficients determined from mean values for the group and from all individual values for the 11 subjects suggested partial dissociation of the components of the renin-angiotensin system. However, correlation coefficients calculated from normalized individual values were similar to those reported for non-pregnant individuals subjected to manipulation of Na intake. The findings suggest that apparent dissociation of the usual relationships of renin-angiotensin-aldosterone previously reported to occur in pregnancy and observed in this study may in part be accounted for by the methodology of renin determinations and in part by between-patient variation in absolute values.