Changes in steady state plasma arginine vasotocin levels affect ovine fetal renal and cardiovascular function

The neuropeptides arginine vasopressin (AVP) and arginine vasotocin (AVT) are present in ovine fetal plasma in similar concentrations. Although AVP appears to be an important factor in the regulation of fetal water metabolism, the significance of AVT in the fetus is not clear. In the present study we assessed the effects of changes in steady plasma AVT levels on renal and cardiovascular function of fetal lambs (mean gestation, 128 days) maintained with vascular and bladder catheters. Successive 1-h infusions of synthetic AVT (n = 8) at rates of 1.0 and 2.0 ng/min X kg resulted in steady state plasma AVT levels of 30 +/- 7 and 59 +/- 12 pg/ml, respectively. The AVT infusions increased both urinary AVT concentrations and urinary AVT excretion. Plasma and urinary AVT clearance rates (45 +/- 8.4 ml/min X kg and 0.38 +/- 0.13 ml/min, respectively) remained constant over the observed range of plasma AVT levels. Urinary AVT clearance accounted for less than 1% of the total plasma AVT clearance and was nonsaturable. Increases in plasma AVT augmented urinary osmolar excretion and osmolar clearance, and evoked a significant natriuresis. AVT at either 1 or 2 ng/min X kg decreased fetal heart rate and tended to increase blood pressure. These results suggest that AVT, like AVP, may be important in modulating ovine fetal electrolyte homeostasis.