Atrial natriuretic peptide (ANP) decrease during normal pregnancy as related to hemodynamic changes and volume regulation

Abstract

Volume regulation and hemodynamic functions change during pregnancy, leading to marked increases in blood volume and cardiac output, peripheral vasodilatation and reduced sensitivity to angiotensin. Atrial natriuretic peptide (ANP) is intimately involved in fluid and sodium homeostasis and exerts marked relaxant activity on vascular smooth muscle pre-contracted with angiotensin. This study was performed to clarify the role of ANP as a regulator of maternal physiology. 40 normal primigravidae were examined five times during pregnancy plus 12 weeks after delivery. Each time were measured: ANP, aldosterone, renin, blood volume (carbon monoxide), cardiac output (Doppler), blood pressure and sodium excretion. Interdependence of the changes in ANP and in the other parameters was tested using Spearman's rank correlation test on the delta (delta)-values (the differences between investigations). P-ANP in the 20th week was 11.4 (8.5-18.9) pmol.l-1 (median, 25 and 75 percentiles), the same as 12 weeks after delivery, 11.5 (9.6-15.2) pmol.l-1, and in a non-pregnant control group, 10.4 (9.0-12.5) pmol.l-1 (n = 20). All measurements of P-ANP during the 3rd trimester were lower than in the 20th week and 12 weeks after delivery, p < 0.0001 (Wilcoxon matched-pairs test). There was a negative correlation between changes in P-ANP and changes in: a) blood volume. R = 0.69, p < 0.0001, b) aldosterone, R = 0.58, p < 0.0001, c) renin, R = -0.54, p < 0.001, d) cardiac output, R = 0.61, p < 0.001. There was a positive correlation between changes in P-ANP and changes in: a) fractional excretion of sodium, R = 0.54, p < 0.0001, and b) total peripheral resistance. R = 0.52, p < 0.0001. Decrease in p-ANP is one of the mechanisms whereby blood volume is increased and maintained during pregnancy. The competitive relationship between ANP and the renin aldosterone system in regulating sodium balance and fluid volume is preserved during pregnancy. The results substantiate the physiological importance of ANP as a regulator of blood volume. ANP does not function as a vasodilator during pregnancy.