Vol.:(0123456789) 1 3 Clinical Autonomic Research (2018) 28:593–595 https://doi.org/10.1007/s10286-018-0559-2 LETTER TO THE EDITOR Pharmacological assessment of the arterial barorefex in a young healthy obese male with extremely low baseline muscle sympathetic nerve activity Anthony V. Incognito 1  · Milena Samora 2  · Roberta A. Cartafna 3  · Gabriel M. N. Guimarães 3  · Mauricio Daher 4  · Philip J. Millar 1  · Lauro C. Vianna 2 Received: 17 July 2018 / Accepted: 13 August 2018 / Published online: 20 August 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Keywords Barorefex · Sympathetic nervous system · Blood pressure · Microneurography · Modifed-Oxford Abbreviations BMI Body mass index BRS Barorefex sensitivity DBP Diastolic blood pressure HR Heart rate MSNA Muscle sympathetic nerve activity SBP Systolic blood pressure Dear Editors, Multi-unit microneurographic recordings of muscle sym- pathetic nerve activity (MSNA) in humans have shown sympathetic impulses to present as pulse-synchronous bursts under barorefex control, increasing in response to spontaneous diastolic blood pressure (DBP) reductions and inhibited during elevations [1]. Studies commonly report large interindividual variability in baseline MSNA (42–84 bursts/100 heartbeats in upright sitting), which may in part be exacerbated by the supine posture (range increased to 10–83 bursts/100 heartbeats) [1]. The increase of low-end MSNA values when supine could be due to cardiopulmonary baroreceptor inhibition of MSNA via variable elevations in central venous pressure [1, 2] and/or from signifcant reduc- tions in the magnitude of spontaneous blood pressure fuc- tuations [1, 3], which would induce a more tonic loading of the arterial barorefex. Theoretically, if strong inhibitory mechanisms maintain baseline tonicity, we would expect some individuals to possess absent baseline MSNA. This has yet to be reported, potentially due to the mislabeling of individuals as failed microneurographic recordings. In this case report, testing followed written informed con- sent and study procedures were approved by the University of Brasilia institutional research committee in accordance with the Declaration of Helsinki. A young (32 years), obese (height 180 cm; weight 110 kg; BMI 34 kg/m 2 ), unmedi- cated male, with no previously diagnosed medical condi- tions, was assessed. Using the microneurography technique in the right common fbular nerve, we observed near-absent baseline MSNA. Usually, microneurographic experiments are terminated when MSNA bursts are absent at baseline, as this often indicates distant microelectrode placement to active postganglionic muscle sympathetic eferent fbres. Indeed, we changed the microelectrode recording site mul- tiple times during the 1 h search, repeatably fnding nerves directed towards skeletal muscle (indicative by auditory feedback during tapping/palpation of the tibialis anterior/ peroneal muscles and absent auditory feedback from light stroking of skin on the dorsal foot/lower shank) with no detectable spontaneous bursts before suspecting absent baseline activity. We began the experimental protocol after observing large bursts during an apnea following a maxi- mal expiratory efort (to evoke increases in MSNA) and no response to unexpected clapping (a startle maneuver which evokes skin sympathetic nerve activity). The protocol involved a 10 min baseline to ensure stabil- ity of the recording site, followed by an additional 10 min * Lauro C. Vianna lcvianna@unb.br 1 Department of Human Health and Nutritional Sciences, University of Guelph, Guelph, ON, Canada 2 NeuroVASQ-Integrative Physiology Laboratory, Faculty of Physical Education, University of Brasilia, Asa Norte, Brasília, DF 70910-900, Brazil 3 University Hospital, University of Brasilia, Brasilia, DF, Brazil 4 Institute of Cardiology of Federal District, Brasília, DF, Brazil