VASOPRESSIN AND OXYTOCIN LEVELS DURING NORMAL-PREGNANCY - EFFECTS OF CHRONIC DIETARY-SODIUM RESTRICTION

Neurohypophysial hormones are thought to be involved in alterations in fluid balance during pregnancy and delivery. In the course of normal pregnancy intravascular volume is increased whereas sodium restriction is thought to reduce plasma volume and cardiac output. In the present study, we measured the effect of long-term severe sodium restriction on vasopressin (AVP) and oxytocin (OT) levels during normal pregnancy and after delivery. Fifty-nine healthy nulliparous women were randomiz ed either for a low sodium diet (20 mmol sodium daily) or for a normal diet from week 12 of pregnancy onwards. Circulating plasma levels and urinary excretion of AVP and OT, their neurophysins (Np-AVP and Np-OT ) and AVP bound to platelets were determined at regular intervals duri ng pregnancy and after delivery. After completion of the study, women on a sodium-restricted diet were compared with control women on a norm al diet using repeated measurement ANOVA with adjustment for potential ly confounding variables. After randomization, a reduction in urinary sodium excretion of on average, 40-82% was found. In general, no effec t of sodium restriction could be demonstrated on the various parameter s (0.53 < P < 0.98) with the exception of a significantly lower 24-h u rinary AVP excretion by non-smokers with sodium restriction compared w ith non-smokers having a normal diet (P = 0.018). For all parameters, clear changes were found in the course of pregnancy and puerperium (P < 0.0001 to P < 0.005). Platelet-bound AVP decreased and Np-OT increas ed during pregnancy. After birth, free plasma AVP, platelet-bound AVP, OT, osmolality, sodium and potassium increased, while Np-AVP and Np-O T decreased. Although elevated Np-AVP and Np-OT levels during pregnanc y seem to indicate increased release of neurohypophysial hormones, pre gnancy up to 36 weeks of gestation is accompanied by low circulating A VP and OT levels. Long-term severe sodium restriction diminishes urina ry AVP excretion in (non-smoking) pregnant women, without changing cir culating levels of AVP and OT, despite the known reduction in circulat ing volume. The reduced circulating (platelet-bound) AVP levels during pregnancy, whether or not in combination with severe sodium restricti on, support the absence of significant non-osmotic stimulation of AVP during pregnancy.