Cardiovascular responses to carotid sinus baroreceptor stimulation during moderate to severe exercise in man

Our objective was to assess the importance of arterial baroreflexes in maintaining vasoconstriction in active muscle during moderate to severe exercise. Eight subjects exercised for 8-15 min on a cycle ergometer at three levels (averages 94, 194, 261 W) requiring 40-88% of VO2 max. Four times during each exercise level pulsatile negative pressure (-50 mmHg) was applied over the carotid sinuses for 30 s; suction was applied at each ECG R-wave for 250-400 ms. Before and during each neck suction, femoral venous blood flow (FVBF) was measured by constant infusion thermal dilution. At 94 W neck suction significantly reduced blood pressure (BP) (15 mmHg) and heart rate (HR) (7 beats min-1), and raised leg vascular conductance (LVC) (11.4%) without changing FVBF. At 194 W, neck suction reduced BP (9 mmHg), HR (4 beats min-1) and FVBF (5.1%, 240 ml min-1), and raised LVC (5.2%). At 261 W, LVC was unchanged by neck suction, but BP and FVBF both fell (9 mmHg and 650 ml min-1 or 7.4%). We conclude that competing local vasodilation and sympathetic vasoconstriction control muscle blood flow during moderate exercise, and vasoconstrictor tone can be withdrawn by baroreceptor stimulation. High levels of vasoconstrictor outflow to muscle in severe exercise may not originate from baroreflexes.