Response of the renin-aldosterone system and antidiuretic hormone to oral water loading and hypertonic saline infusion during and after pregnancy.

Amongst the man physiological changes in human pregnancy are the sustained stimulation of the renin-angiotensin system and a decreases in plasma osmolality (Posm). In this study the effect osmolar and water loading on the renin-angiotensin system and arginine vasopressin (AVP) secretion has been tested in seven women during the third trimester and again 8-10 weeks after delivery. Pregnant women had markedly increased plasma renin substrate (PRS), plasma renin activity (PRA) and plasma renin concentration (PRC) values as well as aldosterone levels when compared to their post-partum values. Osmolar loading with intravenous infusion of hypertonic saline resulted in a decrease in PRA and aldosterone levels both during and after pregnancy but even at the end of the infusion the pregnant women still had values greater than the pre-infusion levels obtained post-partum. Surprisingly, oral water loading also significantly decreased PRA and aldosterone levels in pregnancy, possibly related to the redistribution of extracellular fluid centrally when the pregnant women were in left lateral recumbency. Despite the decreased basal Posm of pregnancy, urinary AVP increased and decreased appropriately during osmolar and water loading. Exact characterisation of the resetting of the threshold for AVP secretion in pregnancy awaits the development of a reliable radioimmunoassay for the determination of AVP in human pregnancy plasma.