Spaceflight suppresses exercise-induced release of bioassayable
growth hormone.
McCall, G. E., C. Goulet, R. R. Roy, R. E. Grindeland, G. I. Boorman,
A. J. Bigbee, J. A. Hodgson, M. C. Greenisen and V. R. Edgerton,.
Dept. of Physiological Science1 and Brain Research Institute2,
Univ. of California, Los Angeles, CA 90095, NASA Ames Research
Center3, Moffett Field, CA 94035, and NASA Johnson Space Center4,
Houston, TX 77058.
APStracts 6:0238A, 1999.
We have reported that bed rest suppressed the release of bioassayable
growth hormone (BGH) which normally occurs after an acute bout of
unilateral plantarflexor exercise (15). In the present study, the
effects of spaceflight on the hormonal responses to this exercise
protocol were examined. Four male astronauts on the NASA Shuttle
Transport System (STS-78) mission completed the exercise protocol
before, during, and after a 17-d spaceflight. The maximal voluntary
contraction (MVC) torque output at the onset of exercise was similar
on all test days. Before spaceflight, plasma BGH increased 114-168%
from pre- to postexercise. During spaceflight and after two days
recovery at 1 g the BGH response to exercise was absent. After 4 d of
recovery this response was restored. Plasma concentrations of
immunoassayable GH were similar at all time points. The preexercise
plasma immunoassayable IGF-I levels were elevated after 12 or 13 d of
microgravity, and an 7% postexercise IGF-I increase was independent
of this spaceflight effect. The suppression of the BGH response to
exercise during spaceflight indicates that some minimum level of
chronic neuromuscular activity/loading is necessary to maintain a
normal exercise-induced BGH release. Moreover, these results suggest
that there is a muscle afferent-pituitary axis that can modulate BGH
release.
Received 8 February 1999; accepted in final form 20 May 1999.
APS Manuscript Number A086-9.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 14 June 1999