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Arterial baroreflex control of muscle sympathetic nerve activity in the transition from rest to steady-state dynamic exercise in humans

¹Department of Integrative Physiology, University of North Texas Health Science Center, Fort Worth, Texas; ²Department of Medical Pharmacology and Physiology, ³Dalton Cardiovascular Research Center, University of Missouri, Columbia; and ⁴Harry S. Truman Memorial Veterans Hospital, Department of Veterans Affairs Medical Center, Columbia, Missouri

Submitted 18 June 2007 ; accepted in final form 1 August 2007

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 (O_2peak). Specifically, DBP-MSNA relations were examined in eight subjects (25 ± 1 yr) at the start of unloaded arm cycling and then during the initial and a later period of arm cycling once the 50% O_2peak work rate was achieved. Heart rate and arterial blood pressure were progressively increased throughout exercise. Although resting MSNA [16 ± 2 burst/min; 181 ± 36 arbitrary units (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) periods 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% O_2peak 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 reflex sensitivity.

arterial blood pressure; sympathetic nervous system; arm cycling; exercise onset

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