Insignificant Effects of Plasma Catecholamines on Dynamic Heart Rate Regulation by the Cardiac Sympathetic Nerve

Although plasma catecholamines such as norepinephrine (NE) and epinephrine (Epi) increase during severe exercise, the effects of high levels of plasma catecholamines on dynamic heart rate (HR) regulation by the cardiac sympathetic nerve remains unknown. The aim of the present study was to examine the effects of plasma catecholamines on the transfer function from sympathetic nerve stimulation to HR. In anesthetized rabbits, we randomly stimulated the right cardiac sympathetic nerve according to a binary white noise signal while measuring HR. The effects of intravenous NE administration at 1 and 10 mugmiddotkg^-1middoth^-1 were examined in 6 rabbits. The effects of intravenous Epi administration at 1 and 10 mugmiddotkg^-1middoth^-1 were examined in different 6 rabbits. Although plasma NE increased 10 times as high as the baseline level during the NE administration at mugmiddotkg^-1middoth^-1 , dynamic gain of the transfer function was not changed significantly (7.1plusmn1.2, 6.9plusmn1.1, and 7.7plusmn1.1 beatsmiddotmin^-1middotHz^-1). Similarly, although plasma Epi increased 10 times as high as the baseline level during the Epi administration at 10 mugmiddotkg^-1middoth^-1, dynamic gain of the transfer function was not changed significantly (7.5plusmn0.8, 7.9plusmn0.8, and 7.6plusmn1.2 beatsmiddotmin^-1middotHz^-1). In conclusion, plasma catecholamines of physiologically-relevant high concentrations did not interfere with the dynamic HR regulation by the cardiac sympathetic nerve