Renin-angiotensin and autonomic mechanisms in cardiovascular homeostasis during haemorrhage in fetal and neonatal sheep.

The present study examined the roles of the renin-angiotensin and autonomic nervous systems in cardiovascular homeostasis during slow progressive haemorrhage (20% of measured blood volume over 1h) in fetal (128-132 and 143-148 days gestation) and neonatal (5-9 and 12-20 days post-natal) sheep. Basal plasma renin activity (PRA) was not significantly different in the 4 sheep groups and increased to a similar degree (approximately 2 to 3-fold) during haemorrhage. Mean arterial pressure (MAP) exhibited modest falls in response to haemorrhage in all sheep groups and while heart rate (HR) was well maintained in the fetal groups there was a tendency to bradycardia in neonates. None of these responses was significantly different in age-matched fetal sheep subjected to bilateral vago-sympathectomy, cervical cord transection or bilateral nephrectomy, with the exception of PRA in the latter group which was close to zero throughout. Treatment with the angiotensin II (AII) antagonist, (Sar1-Ala8) AII (Saralasin), significantly increased basal PRA in both fetal and neonatal sheep (approximately 5 to 7-fold). The PRA response to haemorrhage was absent in neonatal sheep treated with Saralasin but significantly increased in fetal sheep. Saralasin significantly reduced resting MAP in both sheep groups and increased the hypotensive and bradycardic effects of haemorrhage in neonatal (approximately 3 to 5-fold) but not fetal sheep. It is concluded that in the perinatal period studied, fetal and neonatal sheep are equally well able to maintain cardiovascular homeostasis in response to moderate haemorrhage.(ABSTRACT TRUNCATED AT 250 WORDS)