Autonomic nervous system influence on arterial baroreflex control of heart rate during exercise in humans

A combination of sympathoexcitation and vagal withdrawal increases heart rate (HR) during exercise, however, their specific contribution to arterial baroreflex sensitivity remains unclear. Eight subjects performed 25 min bouts of exercise at a HR of 90, 120, and 150 beats min(-1), respectively, with and without metoprolol (0.16 +/- 0.01 mg kg(-1); mean +/- S.E.M.) or glycopyrrolate (12.6 +/- 1.6 mu g kg(-1)). 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 (G(TF)) 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 beta-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 modelled reflex function curve (G(OP)) obtained using NP and NS decreased with workload independent of beta-1 adrenergic blockade. In contrast, vagal blockade decreased G(OP) from -0.40 +/- 0.04 to -0.06 +/- 0.01 beats min(-1) mm Hg-1 at rest (P < 0.05). Furthermore, as workload increased both G(OP) and SBR, and G(OP) and G(TF) were correlated (P < 0.001), suggesting that the two dynamic methods applied to evaluate arterial baroreflex (ABR) function provide the same information as the modelled G(OP). 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.