Arterial Baroreflex Control of Multi‐ and Single‐Unit Muscle Sympathetic Nerve Activity in Young Unmedicated Hypertensives
Authors: Incognito Anthony V , Samora Milena , Shepherd Andrew D , Cartafina Roberta A , Guimarães Gabriel MN , Daher Mauricio , Millar Philip J , Vianna Lauro C 
Published: June 2021

Pharmacologic investigations of sympathetic baroreflex function in patients with hypertension have reported a shift in operating points to higher pressures. Still controversial is whether baroreflex sensitivity (BRS) is preserved or reduced in the hypertensive state, with most prior work limited to patients >45 yr, commonly medicated, and hypertension status based on past guidelines (systolic and diastolic BP ≥140/90 mmHg). Whether sympathetic BRS is altered in young hypertensives has yet to be investigated. We tested 8 young to middle‐aged (age: 35 ± 8 yr; BMI: 29 ± 4 kg/m2), unmedicated male hypertensives (10 min supine BP 137 ± 4/77 ± 3 mmHg; characterized by new guidelines of BP ≥130/80 mmHg) and 8 age‐ (29 ± 7 yr, p=0.13) and BMI‐(29 ± 4 kg/m2, p=0.94) matched male normotensives (112 ± 5/64 ± 4 mmHg, both p<0.001). Beat‐to‐beat heart rate (electrocardiography) and BP (Finometer) and multi‐ and single‐unit muscle sympathetic nerve activity (MSNA; microneurography) were obtained during 3–10 min of supine rest preceding a modified‐Oxford protocol, which involved a bolus injection of 100–150 μg sodium nitroprusside followed by 100–150 μg phenylephrine hydrochloride 1 min afterwards. Resting heart rate (60 ± 9 vs 64 ± 12 bpm, p=0.43), multi‐unit MSNA burst frequency (32 ± 14 vs 35 ± 12 bursts/min, p=0.60), total MSNA (854 ± 542 vs 1262 ± 515 AU/min, p=0.14), and single‐unit MSNA spike frequency (22 ± 24 [n=26] vs 25 ± 28 [n=29] spikes/min, p=0.69) were similar between normotensives and hypertensives, although hypertensives had higher resting multi‐unit burst amplitude (32 ± 5 vs 39 ± 4 % largest burst, p=0.007). We assessed the likelihood of a MSNA burst (burst incidence) and spike (spike incidence), as well as burst amplitude (% largest burst), for each given 3 mmHg diastolic BP bin observed throughout the modified‐Oxford protocol. Using linear segments of the MSNA‐BP response curves, we quantified sympathetic BRS as the slope of a weighted linear regression. Strong (r>0.6) negative relationships were attained for multi‐unit MSNA in all 16 participants and in all MSNA single‐units (n=55). Multi‐ (−5.7 ± 2.0 vs −5.0 ± 1.6 bursts/100 heartbeats/mmHg, p=0.48; −3.9 ± 1.6 vs −4.0 ± 2.4 % largest burst/mmHg, p=0.96) and single‐unit (−2.8 ± 1.3 vs −2.5 spikes/100 heartbeats/mmHg, p=0.45) MSNA BRS were similar between normotensives and hypertensives, however, the multi‐unit MSNA‐BP response curve was shifted to a higher operating point in hypertensives (64 ± 6 vs 79 ± 6 mmHg eliciting 50 bursts/100 heartbeats, p<0.001). Our observations of increases in resting MSNA burst amplitude and preserved BRS suggest increased central sympathetic outflow independent of the arterial baroreflex in early‐stage hypertension.Support or Funding InformationNatural Sciences and Engineering Research Council of Canada (NSERC) Canada Graduate Scholarship, Michael Smith Foreign Study Supplement, and Discovery Grant (06019); Coordination for the Improvement of Higher Education Personnel (CAPES); Foundation for Research Support of the Federal District (FAPDF)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.