Baroreceptor function in man following peripheral alpha 1- and alpha 2-adrenoceptor stimulation

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Pharmacology

Background:

• Baroreceptor reflex is crucial for blood pressure regulation.
• Alpha-adrenergic agonists modulate cardiovascular function.
• The role of pre-synaptic alpha-adrenoreceptors in baroreceptor function requires clarification.

Purpose of the Study:

• To compare baroreceptor sensitivity to methoxamine and alpha-methyl-noradrenaline.
• To investigate the effect of propranolol on baroreceptor-mediated responses.
• To assess the physiological role of pre-synaptic alpha-adrenoreceptors in human baroreceptor modulation.

Main Methods:

• Six healthy males received methoxamine and alpha-methyl-noradrenaline infusions.
• Valsalva's Manoeuvre was performed before and after propranolol (80 mg) or placebo administration.
• Baroreceptor sensitivity was calculated as the change in R-R interval per mmHg systolic blood pressure change.

Main Results:

• Baroreceptor sensitivity was lower with alpha-methyl-noradrenaline compared to methoxamine (p<0.05).
• Propranolol eliminated differences in baroreceptor-mediated bradycardia and shifted sensitivity lines leftward (p<0.05).
• Propranolol increased baroreceptor sensitivity during Valsalva's Manoeuvre release.

Conclusions:

• The reduced bradycardia with alpha-methyl-noradrenaline does not support a physiological role for pre-synaptic alpha-adrenoreceptors in modulating human baroreceptor function.
• Alpha-methyl-noradrenaline's beta 1-agonist activity may explain its differential effect on baroreceptor sensitivity.
• Propranolol significantly alters baroreceptor reflex responses to adrenergic agonists.