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1 Integrativr Physiology, University of North Texas Health Science Center, Fort Worth, Texas, United States
2 Medical Pharmacology & Physiology, University of Missouri-Columbia, Columbia, Missouri, United States
3 Integrative Physiology, Univ North Texas Health Science Center, United States
4 Department of Medical Pharmacology and Physiology, University of Missouri, Columbia, Missouri, United States
* To whom correspondence should be addressed. E-mail: sogoh{at}hsc.unt.edu.
We sought to investigate arterial baroreflex (ABR) control of muscle sympathetic nerve activity (MSNA) in the transition from rest to steady-state dynamic exercise. This was accomplished by assessing the relationship between spontaneous variations in diastolic blood pressure (DBP) and MSNA at rest and during the time course of reaching steady-state arm cycling at 50% peak oxygen uptake (VO2peak). Specifically, DBP-MSNA relations were examined in 8 subjects (25±1 yrs) at the start of unloaded arm cycling, and then during the initial and a later period of arm cycling once the 50% VO2peak work rate was achieved. Heart rate and arterial blood pressure were progressively increased throughout exercise. Although MSNA at rest (16±2 burst/min; 181±36 (au) total activity) was unchanged during unloaded cycling, MSNA burst frequency and total activity were significantly elevated during the initial (27±4 burst/min; 367±76 (au); P<0.05) and later (36±7 burst/min; 444±91 (au); P<0.05) period of exercise. The relationships between DBP and burst incidence, burst strength, and total MSNA, were progressively shifted rightward from unloaded to the initial to the later period of 50% VO2peak arm cycling without any changes in the slopes of the linear regressions (i.e., ABR sensitivity). Thus, a continuous and dynamic resetting of the ABR control of MSNA occurred during the transition from rest to steady-state dynamic exercise. These findings indicate that the ABR control of MSNA was well maintained throughout dynamic exercise in humans progressively being reset to operate around the exercise-induced elevations in blood pressure and MSNA without any changes in sensitivity.
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