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

Abstract

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 beta1-adrenergic blockade (metoprolol, 0.15 +/- 0.003 mg kg(-1)) or parasympathetic blockade (glycopyrrolate, 13.6 +/- 1.5 microg 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)) ofthe 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 G(max). 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.

Figure 1. A summary of the CBR-HR baroreflex function curves

CBR-HR stimulus–response curves at rest (continuous line) and during isometric exercise (dashed line) under control (A), metoprolol (B) and glycopyrrolate (C) conditions. Symbols denote actual group data for all subjects (mean ± s.e.m.); THR, carotid sinus threshold pressure; SAT, carotid sinus saturation pressure; CP, centring point; OP, prestimulus operating point; ECSP, estimated carotid sinus pressure; HR, heart rate. Lines on panels A and B represent mean data fitted to the logistic function model. Lines on panel C represent the mean data fitted to linear regression.

Figure 2. A summary of the CBR-MAP baroreflex function curves

CBR-MAP stimulus–response curves at rest (continuous line) and during isometric exercise (dashed line) under control (A), metoprolol (B) and glycopyrrolate (C) conditions. ECSP, estimated carotid sinus pressure; MAP, mean arterial pressure. Lines on all panels represent mean data fitted to the logistic function model.