A SINGLE BOUT OF EXHAUSTIVE EXERCISE AFFECTS INTEGRATED BAROREFLEX FUNCTION AFTER 16 DAYS OF HEAD-DOWN TILT

We tested the hypothesis that one bout of maximal exercise performed 24 h before reambulation from 16 days of 6 degrees head-down tilt (HDT) could increase integrated baroreflex sensitivity. Isolated carotid-cardiac and integrated baroreflex function was assessed in seven subjects before and after two periods of HDT separated by 11 mo. On the last day of one HDT period, subjects performed a single bout of maximal cycle ergometry (exercise). Subjects did not exercise after the other HDT period (control). Carotid-cardiac baroreflex sensitivity was evaluated using a neck collar device. Integrated baroreflex function was assessed by recording heart rate (HR) and blood pressure (MAP) during a 15-s Valsalva maneuver (VM) at a controlled expiratory pressure of 30 mmHg. The ratio of change in HR to change in MAP (Delta HR/Delta MAP) during phases II and IV of the VM was used as an index of cardiac baroreflex sensitivity. Baroreflex-mediated vasoconstriction was assessed by measuring the late phase II rise in MAP. Following HDT, carotid-cardiac baroreflex sensitivity was reduced (2.8 to 2.0 ms/mmHg; P = 0.05) as was Delta HR/Delta MAP during phase II (-1.5 to -0.8 beats/mmHg; P = 0.002). After exercise, isolated carotid baroreflex activity and phase II Delta HR/Delta MAP returned to pre-HDT levels but remained attenuated in the control condition. Phase IV Delta HR/Delta<MAP was not altered by HDT or exercise. The late phase II increase of MAP was 71% greater after exercise compared with control (7 vs. 2 mmHg; P = 0.041). Heightened baroreflex function was associated with maintenance of orthostatic tolerance after HDT in the exercise condition. These observations suggest that global baroreflex control of cardiac chronotropic and vasoconstrictive responses, which are diminished by HDT, are enhanced in the 24-h period after completion of a single bout of maximal exercise.