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A method has been developed which allows estimation of the kinetic parameters of the plasma renin-angiotensinogen reaction from data obtained by autologous renin assays at several plasma dilutions. The quantitative aspects of the renin-angiotensin system were examined in 28 plasma samples from 23 healthy, normotensive pregnant women. They were compared with 20 women who were not pregnant, of whom 12 were taking oral contraceptives and eight were not. In the first 3 months of pregnancy, there was a sharp increase in plasma renin activity and concentration. Plasma angiotensinogen rose steadily throughout pregnancy. The higher concentrations of renin and angiotensinogen would lead to an increase in angiotensin formation with potentially adverse consequences. However, this increase may be reduced by the fall in the affinity between renin and antiotensinogen which is suggested by the present observation that the Michaelis-Menten constant (K_m) attained values five to six times higher than those seen in women not taking oral contraceptives.

The smaller increases in plasma renin and angiotensinogen induced by oral contraceptives were less effectively compensated by lower affinity.

Circumstantial evidence is provided which suggests that the observed high K_m values may be due to oestrogen/pregnancy-induced synthesis of an angiotensinogen with a lower affinity for renin.

J. Endocr. (1987) 112, 465–472