AVPR1A and hydrops fetalis: However, during hemorrhage, when VP is released in excess in blood, in support of circulation, it acts similarly in normotensive and hypertensive rat strains, and prevents the respiratory related high frequency (HF-BPV) increase, possibly as a consequence of V1aR-mediated vasoconstriction which prevents the unloading of thoracic vessels underlying HF-BPV increase.