Cardiovascular effects and clearance of arginine vasopressin in the fetal lamb

Increased fetal secretion of arginine vasopressin (AVP) occurs in association with complicated pregnancies, asphyxia, and meconium-stained amniotic fluid (AF); yet, the role of AVP in fetal homeostasis remains unclear. Using chronically instrumented, near-term lamb fetuses (n = 8), we have ascertained the cardiovascular responses to doses of AVP ranging from 1.94 to 8.73 mU/min, the clearance of AVP from plasma, and the occurrence of meconium release into AF during AVP infusions. AVP administration resulted in a dose-related rise in arterial pressure; although heart rate fell, the decline was not dose-related and occurred before the pressor response. Clearance of plasma AVP was 60 ± 8.7 ml · kg^-1 · min^-1, and the half-time in plasma was 2.8 min. AVP was not cleared across the fetal placenta nor by fetal-maternal transport. However, the AF concentration of AVP rose fourfold after the infusion of AVP was stopped. Meconium release into AF occurred in fetuses infused with AVP at rates ≥7.76 mU/min. AVP has hemodynamic effects that mimic fetal responses to intrauterine 'stress' and may be causally associated with the fetal release of meconium into AF. Furthermore, plasma AVP is cleared in part by the fetal kidney.