Cardiac chemoreceptors: pharmacological curiosities or physiological tools?

Area of Science:

• Cardiovascular Physiology
• Neuroscience
• Autonomic Nervous System

Background:

• Acute central hypovolemia triggers complex physiological responses.
• Cardiogenic chemoreflexes are hypothesized to play a role in regulating hemodynamics during hypovolemia.
• The involvement of specific neural pathways, including opioid receptors and afferent signals, remains unclear.

Purpose of the Study:

• To investigate the physiological role of cardiogenic chemoreflexes in acute central hypovolemia in rabbits.
• To determine if a brain-stem delta-opioid receptor mechanism mediates the sympathoinhibitory phase of hypovolemia.
• To assess the contribution of myocardial and epicardial afferents to the hemodynamic response.

Main Methods:

• Unanesthetized rabbits were used to study hemodynamic responses.
• Pharmacological blockade of delta-opioid receptors was achieved via fourth ventricular injection of ICI 174864.
• Specific afferent pathways were stimulated or blocked using phenylbiguanide, nicotine, MDL 72222, and mecamylamine HCl.

Main Results:

• Blockade of delta-opioid receptors did not affect reflex hemodynamic responses to left atrial phenylbiguanide or intrapericardial nicotine.
• Intravenous 5-HT3 receptor antagonist MDL 72222 had no impact on the hypovolemic response.
• Intrapericardial nicotinic ganglionic cholinoceptor antagonist mecamylamine HCl did not alter the hemodynamic response to acute central hypovolemia.

Conclusions:

• Phenylbiguanide-sensitive myocardial afferents and nicotine-sensitive epicardial afferents are not involved in the hypovolemic response in rabbits.
• The reflex effects of stimulating these afferents do not rely on a brain-stem delta-opioid mechanism.
• These findings suggest alternative mechanisms mediate the sympathoinhibitory response to acute central hypovolemia.