Effect of a single bout of exhaustive exercise on integrated
baroreflex function after 16 days of head-down tilt.
Engelke, Keith A., Donald F. Doerr, and Victor A. Convertino.
Department of Physiology, University of Florida, Gainesville, FL
32610; NASA-Kennedy Space Center, FL 32899; NASA-Ames Research
Center, Moffett Field, CA 94035; Physiology Research Branch, Clinical
Sciences Division, Brooks Air Force Base, TX 78235
APStracts 2:0093R, 1995.
We tested the hypothesis that one bout of maximal exercise performed
24 h prior to re-ambulation from 16 days of 6 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 months.
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-sec Valsalva maneuver (VM) at a
controlled expiratory pressure of 30 mmHg. The ratio of change in HR
to change in MAP (_HR/_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 msec/mmHg; P = 0.05) as was _HR/_MAP during
phase II (-1.5 to -0.8 beats/mmHg; P = 0.002). After exercise,
isolated carotid baroreflex activity and phase II _HR/_MAP returned
to pre-HDT levels but remained attenuated in the control condition.
Phase IV _HR/_MAP was not altered by HDT or exercise. The late phase
II increase of MAP was 71% greater following exercise compared to
control (7 mmHg vs. 2 mmHg; P = 0.041). Heightened baroreflex
function was associated with maintenance of orthostatic tolerance
following 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.
Received 14 March 1994; accepted in final form 30 March 1995.
APS Manuscript Number R123-4.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 April 1995.