ATRIAL-NATRIURETIC-PEPTIDE AND PLASMA-VOLUME IN PREGNANCY-INDUCED HYPERTENSION

Pregnancy-induced hypertension (PIH) is characterized by a relative decrease in plasma volume and renin and aldosterone concentrations as well as increased capillary permeability compared with normal pregnancy. As many of these features could be explained by the actions of atrial natriuretic peptide (ANP), we examined the relationship between plasma volume and plasma ANP in women with PIH and in normal third trimester pregnant women, and whether ANP responses to alterations in posture were intact in women with PIH. Basal plasma ANP measured after 20 min lateral recumbency in women with PIH was 24.0 (13.9, 33.1) pmol/L (median [25th, 75th percentile]), which was significantly greater than in normal pregnant women (9.9 [6.3, 16.0]), (P < .05). Plasma ANP did not differ between those with and without protein-uria in the PIH group. Plasma volume was decreased in women with PIH (20.1 [19.0, 23.2]) mL/cm) v 23.5 [21.4, 25.3], P < .05). Plasma renin concentration but not plasma aldosterone concentration was also decreased significantly in women with PIH compared with normal pregnant women (P < .001) and both were correlated negatively with plasma ANP. Following prolonged lateral recumbency, plasma ANP rose to 26.9 [19.1, 44.1] pmol/L in women with PIH (P < .05), which was still significantly greater than in normal pregnant women (15.5 [6.7, 21.9] pmol/L) (P < .05). In a subgroup of these subjects, 30 min head-up tilt decreased plasma ANP by 5.2 [0.9, 22.3] pmol/L in women with PIH and by 6.1 [2.2, 10.3] pmol/L in normal pregnant women, a nonsignificant difference. These studies demonstrate that plasma ANP is elevated in women with PIH despite decreased plasma volume. However, the ANP response to acute increases and decreases in atrial stretch induced by posture is preserved. Thus, factors other than plasma volume and atrial stretch are the dominant regulators of chronic ANP release in women with PIH.