Acute and chronic regulation of atrial natriuretic peptide in human pregnancy: a longitudinal study

To determine whether the increase in extracellular fluid volume (ECFV) that occurs during pregnancy alters: (1) the baseline regulation of atrial natriuretic peptide (ANP); or (2) the ANP response to intravascular volume expansion with either haemaccel or hypertonic saline.

A group of normal pregnant subjects was studied longitudinally on three occasions, commencing before 16 weeks of gestation. They were compared with a group of age-matched non-pregnant women. Dietary sodium intake and posture were carefully controlled.

Plasma volume and total ECFV were determined by tracer dilution methods, using Evans Blue and 20% mannitol, respectively. Plasma ANP and aldosterone concentrations were measured by radioimmunoassay.

Plasma ANP did not increase during pregnancy despite increases in both plasma and total ECFV. The plasma ANP response to acute intravenous volume expansion in later pregnancy appears to be more sensitive than either in early pregnancy or in non-pregnant subjects. The initial ANP response to infusions of haemaccel during pregnancy was greater than the ANP response to saline.

During a normal pregnancy, plasma ANP is maintained in the normal non-pregnant range, despite an increase in plasma volume when the effects of dietary sodium intake and posture are carefully accounted for. The ANP response to intravenous volume expansion is enhanced in late pregnancy. The greater ANP response to haemaccel infusions during pregnancy suggests that an increase in atrial stretch mediates the secretion of ANP following intravenous volume expansion.