Spatial and temporal differing control of sympathetic activities during hemorrhage

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Regulation

Background:

• Hemorrhagic hypotension triggers complex autonomic responses.
• Understanding the differential regulation of sympathetic nerve activity during blood loss is crucial.

Purpose of the Study:

• To investigate the reflex control of regional sympathetic nerve activity during sustained hemorrhagic hypotension.
• To elucidate the roles of arterial baroreceptors and vagal afferents in modulating sympathetic responses to hemorrhage.

Main Methods:

• Simultaneous multifiber recordings of cardiac, renal, hepatic, adrenal, and splenic sympathetic nerve activities in dogs.
• Sustained hemorrhagic hypotension to a mean blood pressure of 50 mmHg.
• Experiments involved intact baroreceptors, cervical vagotomy, and denervation of carotid sinus and aortic nerves.

Main Results:

• Intact baroreceptors initially increased all measured sympathetic activities, with renal nerve activity showing subsequent inhibition.
• Vagotomy abolished sympathoinhibition in renal nerves and reversed heart rate decrease, enhancing hepatic nerve activity.
• Denervation of systemic baroreceptors eliminated significant sympathetic nerve activity changes during hemorrhage.

Conclusions:

• Early sympathetic response to hemorrhage is sympathoexcitatory, mediated by arterial baroreceptor and cardiopulmonary receptor unloading.
• Vagal afferents participate in inhibiting this sympathoexcitation during sustained hypotension, with organ-specific modulation.
• Systemic baroreceptor denervation abolishes reflex sympathetic responses to hemorrhage.