To better understand the pathophysiological significance of high plasma norepinephrine (NE) concentration in regulating heart rate (HR), we examined the interactions between high plasma NE and dynamic vagal control of HR. In anesthetized rabbits with sinoaortic denervation and vagotomy, using a binary white noise sequence (0-10 Hz) for 10 min, we stimulated the right vagus and estimated the transfer function from vagal stimulation to HR response. The transfer function approximated a first-order low-pass filter with pure delay. Infusion of NE (100 μg·kg -1 ·h -1 iv) attenuated the dynamic gain from 6.2 ± 0.8 to 3.9 ± 1.2 beats·min -1 ·Hz -1 , (n = 7, P < 0.05) without affecting the corner frequency or pure delay. Simultaneous intravenous administration of phentolamine (1 mg·kg -1 ·h -1 ) and NE (100 μg·kg -1 ·h -1 ) abolished the inhibitory effect of NE on the dynamic gain (6.3 ± 0.8 vs. 6.4 ± 1.3 beats·min -1 ·Hz -1 , not significant, n = 7). The inhibitory effect of NE at infusion rates of 10, 50, and 100 μg·kg -1 ·h -1 on dynamic vagal control of HR was dose-dependent (n = 5). In conclusion, high plasma NE attenuated the dynamic HR response to vagal stimulation, probably via activation of α-adrenergic receptors on the preganglionic and/or postganglionic cardiac vagal nerve terminals.
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