Serum Vasopressinase and Platelet Responses to Arginine Vasopressin in Normal Pregnancy, Pregnancy-induced Hypertension and Pre-eclampsia.

Pregnancy is marked by the placental production of vasopressinase, an enzyme which accelerates the metabolic clearance of arginine vasopressin (AVP), and serum vasopressinase is reported to be abnormally elevated in hypertensive pregnancy. Since platelet AVP binding capacity is influenced by the plasma AVP concentration, we sought to determine whether alterations in vasopressinase and AVP concentrations affect platelet responsiveness to AVP in normotensive and hypertensive pregnancy. Four groups of 10 women were studied: non-pregnant subjects, normotensive pregnancy, hypertensive pregnancy, and pre-eclampsia (PE). AVP-induced platelet aggregation was measured turbidometrically, serum vasopressinase by chromogenic assay and plasma AVP by radioimmunoassay. Platelet responses to AVP were similar in all groups, as were plasma concentrations of AVP. Vasopressinase was raised in all pregnant patients compared with non-pregnant subjects, and levels were significantly higher in pregnancy-induced hypertension than in either the normotensive or PE groups (p<0.05). No significant correlation was observed between platelet responsiveness to AVP and circulating concentrations of either AVP or vasopressinase. Thus circulating vasopressinase is increased in pregnancy and abnormally so in hypertensive pregnancy. This does not, however, appear to influence ex vivo platelet responsiveness to AVP.