Comparison of hormonal responses to hypotension in mature and immature fetal lambs.

We studied norepinephrine (NE) and plasma renin activity (PRA) responses to sodium nitroprusside (NP)-induced hypotension in seven chronically catheterized fetal lambs 0.79-0.94 gestation (mature) and in seven fetuses 0.64-0.72 gestation (immature) 4 or 5 days after surgery. We infused intravenously 5% dextrose in water (DW) or NP in DW to reduce arterial pressure 30% in fetuses for 10 min. Initial infusion choice was random, and the two infusions were separated by 24-48 h. In both groups, basal NE levels were similar and doubled in response to hypotension. In mature fetuses, PRA basal levels were 6.89 +/- 1.80 ng.ml-1.h-1 and increased two- to threefold with hypotension. In immature fetuses, PRA basal levels were 2.42 +/- 0.86 ng.ml-1.h-1 and did not change with hypotension. No changes were observed with DW infusion in either group. Arterial blood gases were normal and remained unchanged. We conclude that in the lamb fetus, NE responses to hypotension are present before and are independent of the development of PRA responses and that before 0.72 gestation there is a functional deficit in the renin-angiotensin system.