Forearm vascular responses to baroreceptor unloading at the onset of dynamic exercise

To determine the extent to which reflexes accompanying muscular exercise (associated with central command) interact with cardiopulmonary (CP) baroreceptor-mediated reflexes controlling forearm vascular resistance (FVR), we examined the forearm vasoconstrictor response at the onset of dynamic exercise, with and without CP baroreflex unloading, in 10 physically active men. CP baroreceptors were unloaded by application of lower body negative pressure (LBNP) at rest and during five 4-min bouts of supine exercise at 25 and 32°C. Exercise intensities were 10 (essentially no load) and 100 W, and LBNP was applied at -10, -20, -30, and -40 mmHg during rest and at -20 and - 40 mmHg during exercise. Resting FVR was 33.0 ± 3.2 and 14.0 ± 2.7 resistance units, and cardiac stroke volume (SV) was 117 ± 7 and 126 ± 9 ml/beat at 25 and 32°C, respectively. We found a linear relationship between the increase in FVR and decrease in SV during LBNP; the slope of the relationship was significantly lower at 32°C (FVR = 51.7 - 0.29SV) than at 25°C (FVR = 123 - 0.79SV). At the onset of 100-W exercise without LBNP, FVR increased significantly to 50.2 ± 9.0 and 21.2 ± 3.2 units at 25 and 32°C, respectively, whereas SV was unchanged. Application of -40-mmHg LBNP reduced SV significantly to 68 ± 5 and 71 ± 6 ml/beat and increased FVR significantly to 89.0 ± 11.3 and 36.3 ± 7.6 units at 25 and 32°C, respectively. At the onset of 100-W exercise during -40-mmHg LBNP, SV increased significantly to 91 ± 8 and 98 ± 6 ml/beat and FVR decreased to 71.5 ± 8.9 and 32.5 ± 5.1 units at 2 min and 65.6 ± 6.5 and 26.1 + 3.7 units at 3 min of exercise at 25 and 32°C, respectively. During exercise the linear relationship between FVR and SV was similar to that at rest, except for the 1st min of 100-W exercise, at which time the FVR response per unit change in SV was augmented. These data illustrate that the forearm vasoconstrictor response at the onset of dynamic exercise is modulated by the unloading of CP baroreceptors. In addition, a positive interaction between the reflexes accompanying muscular exercise and CP stimulation appeared to occur within the 1st min of moderate exercise.