Insignificant effects of plasma catecholamines on dynamic heart rate regulation by the cardiac sympathetic nerve

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Regulation
• Exercise Physiology

Background:

• Plasma catecholamines, including norepinephrine (NE) and epinephrine (Epi), rise significantly during strenuous exercise.
• The precise impact of these elevated catecholamine levels on the dynamic regulation of heart rate (HR) by the cardiac sympathetic nerve remains unclear.

Purpose of the Study:

• To investigate the influence of plasma catecholamines on the transfer function relating cardiac sympathetic nerve stimulation to heart rate.
• To determine if high physiological concentrations of NE and Epi interfere with dynamic HR regulation.

Main Methods:

• Utilized anesthetized rabbits for the study.
• Stimulated the right cardiac sympathetic nerve using a binary white noise signal while continuously measuring HR.
• Administered intravenous NE and Epi at varying doses (1 and 10 µg·kg⁻¹·min⁻¹) to assess their effects.

Main Results:

• Intravenous NE administration, increasing plasma NE 10-fold, did not significantly alter the dynamic gain of the sympathetic nerve-to-HR transfer function.
• Similarly, intravenous Epi administration, elevating plasma Epi 10-fold, showed no significant change in the dynamic gain of the transfer function.
• Calculated dynamic gains remained consistent across baseline and catecholamine-infused conditions for both NE and Epi.

Conclusions:

• Physiologically relevant high concentrations of plasma catecholamines do not impede the dynamic heart rate regulation mediated by the cardiac sympathetic nerve.
• The findings suggest that other regulatory mechanisms may be more critical for fine-tuning heart rate responses during conditions of high sympathetic drive and elevated circulating catecholamines.