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Am J Physiol Heart Circ Physiol 295: H1855-H1866, 2008. First published August 29, 2008; doi:10.1152/ajpheart.522.2008
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Contrasting effects of presynaptic {alpha}2-adrenergic autoinhibition and pharmacologic augmentation of presynaptic inhibition on sympathetic heart rate control

Tadayoshi Miyamoto,1,2 Toru Kawada,2 Yusuke Yanagiya,2 Tsuyoshi Akiyama,3 Atsunori Kamiya,2 Masaki Mizuno,2 Hiroshi Takaki,2 Kenji Sunagawa,4 and Masaru Sugimachi2

1Department of Physical Therapy, Faculty of Health Sciences, Morinomiya University of Medical Sciences; and 2Department of Cardiovascular Dynamics, Advanced Medical Engineering Center, and 3Department of Cardiac Physiology, National Cardiovascular Center Research Institute, Osaka; and 4Department of Cardiovascular Medicine, Graduate School of Medical Sciences, Kyusyu University, Fukuoka, Japan

Submitted 16 May 2008 ; accepted in final form 19 August 2008

Presynaptic {alpha}2-adrenergic receptors are known to exert feedback inhibition on norepinephrine release from the sympathetic nerve terminals. To elucidate the dynamic characteristics of the inhibition, we stimulated the right cardiac sympathetic nerve according to a binary white noise signal while measuring heart rate (HR) in anesthetized rabbits (n = 6). We estimated the transfer function from cardiac sympathetic nerve stimulation to HR and the corresponding step response of HR, with and without the blockade of presynaptic inhibition by yohimbine (1 mg/kg followed by 0.1 mg·kg–1·h–1 iv). We also examined the effect of the {alpha}2-adrenergic receptor agonist clonidine (0.3 and 1.5 mg·kg–1·h–1 iv) in different rabbits (n = 5). Yohimbine increased the maximum step response (from 7.2 ± 0.8 to 12.2 ± 1.7 beats/min, means ± SE, P < 0.05) without significantly affecting the initial slope (0.93 ± 0.23 vs. 0.94 ± 0.22 beats·min–1·s–1). Higher dose but not lower dose clonidine significantly decreased the maximum step response (from 6.3 ± 0.8 to 6.8 ± 1.0 and 2.8 ± 0.5 beats/min, P < 0.05) and also reduced the initial slope (from 0.56 ± 0.07 to 0.51 ± 0.04 and 0.22 ± 0.06 beats·min–1·s–1, P < 0.05). Our findings indicate that presynaptic {alpha}2-adrenergic autoinhibition limits the maximum response without significantly compromising the rapidity of effector response. In contrast, pharmacologic augmentation of the presynaptic inhibition not only attenuates the maximum response but also results in a sluggish effector response.

systems analysis; transfer function; {alpha}-adrenergic blockade; rabbits


Address for reprint requests and other correspondence: T. Miyamoto, Dept. of Physical Therapy, Faculty of Health Sciences, Morinomiya Univ. of Medical Sciences, Osaka 559-8611, Japan (e-mail: miyamoto{at}morinomiya-u.ac.jp)






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