Renal sympathetic nerve activity during asphyxia in fetal sheep

Area of Science:

• Fetal Physiology
• Autonomic Nervous System Development
• Cardiovascular Adaptation

Background:

• The sympathetic nervous system (SNS) is vital for fetal adaptation to in utero stress like asphyxia.
• SNS responses mature late in gestation, but their role in asphyxia adaptation is not fully understood.

Purpose of the Study:

• To test if increased renal sympathetic nerve activity (RSNA) primarily causes decreased renal vascular conductance (RVC) during umbilical cord occlusion in preterm fetal sheep.
• To determine if near-term fetuses show a faster vasomotor response and greater RSNA increase compared to preterm fetuses.

Main Methods:

• Investigated fetal sheep at preterm (101 days) and near-term (119 days) stages.
• Induced complete umbilical cord occlusion to simulate asphyxia.
• Used surgical and chemical denervation to assess the causality of RSNA on RVC.

Main Results:

• All fetal sheep exhibited increased RSNA during occlusion; it was more sustained but not significantly greater in near-term fetuses.
• Near-term fetuses had a more rapid initial decrease in RVC that preceded the significant RSNA increase.
• Chemical denervation in near-term fetuses led to a drop in blood pressure, indicating critical nonrenal sympathetic roles.

Conclusions:

• RSNA increase is not the primary driver of RVC changes during fetal asphyxia, even at 0.7 gestation.
• By 0.8 gestation, sympathetic innervation of nonrenal vessels is essential for maintaining arterial blood pressure during acute asphyxia adaptation.