Bainbridge reflex in conscious, unrestrained, and tranquilized baboons

Area of Science:

• Physiology
• Cardiovascular Research
• Primate Studies

Background:

• The Bainbridge reflex is a mechanism that increases heart rate in response to increased venous return.
• Its existence and characteristics in larger primates, particularly with lower spontaneous heart rates, require further investigation.

Purpose of the Study:

• To investigate the presence and characteristics of the Bainbridge reflex in conscious, unrestrained baboons.
• To determine the neural pathways involved in mediating the heart rate response to volume loading in this species.

Main Methods:

• Volume loading was performed in baboons instrumented with miniature left ventricular pressure and diameter gauges.
• Heart rate responses were measured during saline infusions in both tranquilized and conscious states.
• Pharmacological interventions including cholinergic blockade (atropine), ganglionic blockade (hexamethonium), vagotomy, and combined blockades were employed.

Main Results:

• Volume loading significantly increased cardiac rate in tranquilized and conscious baboons (32-41 beats/min increase, P < 0.01).
• The heart rate increase in response to volume loading was abolished after cholinergic blockade, ganglionic blockade, vagotomy, or combined blockades.
• Volume loading did not alter heart rate in smaller baboons with higher spontaneous heart rates.

Conclusions:

• The Bainbridge reflex is present in larger primates with relatively low spontaneous heart rates.
• The reflex is mediated by vagal and/or sympathetic neural pathways, suggesting a complex autonomic control mechanism.