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This Article Full Text Full Text (PDF) All Versions of this Article: 295/5/H1855    most recent 522.2008v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Miyamoto, T. Articles by Sugimachi, M. Search for Related Content PubMed PubMed Citation Articles by Miyamoto, T. Articles by Sugimachi, M.

Contrasting effects of presynaptic ₂-adrenergic autoinhibition and pharmacologic augmentation of presynaptic inhibition on sympathetic heart rate control

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 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 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 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; -adrenergic blockade; rabbits