Cardiovascular response to stress: baroreflex resetting and hemodynamics

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Regulation
• Stress Response

Background:

• Borderline hypertensive rats (BHR) are a model for studying blood pressure regulation.
• Baroreflex resetting is a key mechanism in adapting to stress.
• The role of the baroreflex in maintaining blood pressure during stress with compromised cardiac output is not fully understood.

Purpose of the Study:

• To test if baroreflex resetting prevents a fall in blood pressure (BP) when cardiac output (CO) is reduced during air-jet stress.
• To investigate the role of sinoaortic baroreceptors in the pressor response to stress.
• To determine if the arterial baroreflex is necessary for initiating or maintaining the pressor response during stress.

Main Methods:

• Eight-week-old BHR were instrumented for measuring CO, arterial pressure, and heart rate (HR).
• Sinoaortic baroreceptors were either denervated (SAD) or left intact.
• Baroreflex function was assessed using phenylephrine and sodium nitroprusside injections during baseline and air-jet stress.
• Metoprolol was administered to assess its effects on BP, CO, and HR during stress.

Main Results:

• Air-jet stress shifted the baroreflex midpoint to higher BP levels, indicating resetting.
• Metoprolol administration during stress reduced HR and restored CO to prestress levels without altering the pressor response.
• In SAD BHR, air-jet stress elevated BP similarly to intact rats.
• Metoprolol administration to SAD rats during stress paradoxically increased BP.

Conclusions:

• The sinoaortic baroreflex is reset during air-jet stress and integrates reflex changes in BP.
• The arterial baroreflex is not required for the initiation or maintenance of the pressor response during stress.
• The arterial baroreflex does not prevent a fall in BP when CO is compromised during stress.