Cardiopulmonary baroreceptors modulate carotid baroreflex control of heart rate during dynamic exercise in humans

Numerous studies have reported that, at rest, maximal carotid baroreflex gain is modulated by cardiopulmonary baroreceptors. The purpose of this study was to measure the maximal gain for carotid baroreflex control of heart rate (HR) and blood pressure [mean arterial pressure (MAP)] during dynamic exercise alone and when exercise was accompanied by two levels of cardiopulmonary baroreceptor unloading. Lower body negative pressure (LBNP) produced similar reductions in central venous pressure (CVP) at rest and during exercise. Baroreflex gain for HR at rest was not affected by low- level LBNP but was significantly increased by high-level LBNP [-0.31 ± 0.05 to -0.57 ± 0.23 beats · min^-1 · mmHg^- carotid sinus pressure (CSP), no LBNP vs. high-level LBNP, P < 0.05]. Exercise combined with low- and high- level LBNP resulted in reflex HR gains (-0.83 ± 0.44 and -0.83 ± 0.17 beats · min^- · mmHg^-1 CSP, low- vs. high-level LBNP, P < 0.05) that were significantly greater than the algebraic sum of the HR gain measured during exercise and LBNP alone. Neither exercise nor the two levels of LBNP significantly altered baroreflex gain for MAP. The slope relating HR gain to CVP at rest was significant (0.07 ± 0.02 beats · min^-1 · mmHg CSP^-1 · mmHg CVP^-1, P < 0.01) and increased 114% during exercise (P < 0.05). A similar analysis of MAP gain found no difference in slope between rest and exercise (P > 0.05). Together these data indicate 1) a tonic inhibitory interaction between cardiopulmonary baroreceptors and carotid baroreflex control of HR during exercise, and 2) a nonlinear relationship between deactivation of cardiopulmonary baroreceptors and changes in carotid baroreflex gain. This interaction may improve overall cardiovascular regulation when central blood volume and/or CVP is reduced, as has been reported during prolonged exercise in the heat.