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Received February 4, 2005
Revised February 24, 2005
Accepted after revision May 2, 2005
1 University of North Texas Health Science Center
2 University of Birmingham
3 University of Copenhagen
* To whom correspondence should be addressed. E-mail: sogoh{at}hsc.unt.edu.
A combination of sympathoexcitation and vagal withdrawal increases heart rate (HR) during exercise, however, their specific contribution to the arterial baroreflex sensitivity remains unclear. Eight subjects performed 25 min bouts of exercise at a HR of 90, 120, and 150 beats/min, respectively, with and without metoprolol (0.16 ± 0.01 mg/kg) or glycopyrrolate (12.6 ± 1.6 µg/kg; mean ± SE). Carotid baroreflex (CBR) function was determined using 5-s pulses of neck pressure (NP) and neck suction (NS) from +40 to -80 Torr, while transfer function gain (GTF) was calculated to assess the linear dynamic relationship between mean arterial pressure and HR. Spontaneous baroreflex sensitivity (SBR) was evaluated as the slope of sequences of three consecutive beats in which systolic blood pressure and the R-R interval of the ECG either increased or decreased, in a linear fashion. The 
-1 adrenergic blockade decreased and vagal cardiac blockade increased HR both at rest and during exercise (P<0.05). The gain at the operating point of the modeled reflex function curve (GOP) obtained using NP and NS decreased with workload independent of -1 adrenergic blockade. In contrast, vagal blockade decreased GOP from -0.40 ± 0.04 to -0.06 ± 0.01 bpm/mmHg at rest (P<0.05). Furthermore, as workload increased both GOP and SBR and GOP and GTF were correlated (P<0.001), suggesting that the two dynamic methods applied to evaluate ABR function provide the same information as the modeled GOP. These findings suggest that during exercise the reduction of arterial baroreceptor reflex sensitivity at the operating point was a result of vagal withdrawal rather than an increase in sympathetic activity.
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