Cardiac and vasomotor components of the carotid baroreflex control of aterial blood pressure during isometric exercise in humans

We sought to examine the importance of the cardiac component of the carotid baroreflex (CBR) in control of blood pressure during isometric exercise. Nine subjects performed 4 min of ischaemic isometric calf exercise at 20% of maximum voluntary contraction. Trials were repeated with beta(1)-adrenergic blockade (metoprolol, 0.15 +/- 0.003 mg kg(-1)) or parasympathetic blockade (glycopyrrolate, 13.6 +/- 1.5 mu g kg(-1)). CBR function was determined using rapid pulses of neck pressure and neck suction from +40 to - 80 mmHg, while heart rate (HR), mean arterial pressure (MAP) and changes in stroke volume (SV, Modelflow method) were measured. Metoprolol decreased and glycopyrrolate increased HR and cardiac output both at rest and during exercise (P <0.05), while resting and exercising blood pressure were unchanged. Glycopyrrolate reduced the maximal gain (G(max)) of the CBR-HR function curve (-0.58 +/- 0.10 to -0.06 +/- 0.01 beats min(-1) mmHg(-1), P <0.05), but had no effect on the G(max) of the CBR-MAP function curve. During isometric exercise the CBR-HR curve was shifted upward and rightward in the metoprolol and no drug conditions, while the control of HR was significantly attenuated with glycopyrrolate (P <0.05). Regardless of drug administration isometric exercise produced an upward and rightward resetting of the CBR control of MAP with no change in Gmax. Thus, despite marked reductions in CBR control of HR following parasympathetic blockade, CBR control of blood pressure was well maintained. These data suggest that alterations in vasomotor tone are the primary mechanism by which the CBR modulates blood pressure during low intensity isometric exercise.