Effects of acute exercise on attenuated vagal baroreflex function during bed rest.
We measured carotid baroreceptor-cardiac reflex responses in six healthy men, 24 h before and 24 h after a bout of leg exercise during 6 degrees head-down bed rest to determine if depressed vagal baroreflex function associated with exposure to microgravity environments could be reversed by a single exposure to acute intense exercise. Baroreflex responses were measured before bed rest and on day 7 of bed rest. An exercise bout consisting of dynamic and isometric actions of the quadriceps at graded speeds and resistances was performed on day 8 of bed rest and measurements of baroreflex response were repeated 24 h later. Vagally-mediated cardiac responses were provoked with ramped neck pressure-suction sequences comprising pressure elevations to +40 mm Hg, followed by serial, R-wave triggered 15 mm Hg reductions, to -65 mm Hg. Baroreceptor stimulus-cardiac response relationships were derived by plotting each R-R interval as a function of systolic pressure less the neck chamber pressure applied during the interval. Compared with pre-bed rest baseline measurements, 7 d of bed rest decreased the gain (maximum slope) of the baroreflex stimulus-response relationship by 16.8 +/- 3.4% (p < 0.05). On day 9 of bed rest, 24 h after exercise, the maximum slope of the baroreflex stimulus-response relationship was increased (p < 0.05) by 10.7 +/- 3.7% above pre-bed rest levels and 34.3 +/- 7.9% above bed rest day 7. Our data verify that vagally-mediated baroreflex function is depressed by exposure to simulated microgravity and demonstrate that this effect can be acutely reversed by exposure to a single bout of intense exercise.
Convertino
V A
VA
Biomedical Operations and Research Office, NASA, Kennedy Space Center, FL 32899.
Doerr
D F
DF
Guëll
A
A
Marini
J F
JF
D000328:Adult; D000337:Aerospace Medicine; D001510:Bed Rest / Q000009:adverse effects*; D001794:Blood Pressure / Q000502:physiology*; D002346:Carotid Sinus / Q000502:physiology*; D015444:Exercise / Q000502:physiology*; D006801:Humans; D008297:Male; D011311:Pressoreceptors / Q000502:physiology*; D014630:Vagus Nerve / Q000502:physiology