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Vol. 83, Issue 6, 2086-2090, December 1997
1 Department of Physiological
Science and 3 Brain Research
Institute,
Received 24 January 1997; accepted in final form 29 July 1997.
McCall, G. E., C. Goulet, R. E. Grindeland, J. A. Hodgson,
A. J. Bigbee, and V. R. Edgerton. Bed rest suppresses bioassayable growth hormone release in response to muscle activity.
J. Appl. Physiol. 83(6):
2086-2090, 1997.
humans; isometric exercise; endocrine regulation; plantar flexion
Hormonal responses to muscle activity were
studied in eight men before (
13 or 12 and 8 or 7 days), during (2 or 3, 8 or 9, and 13 or 14 days) and after (+2 or +3 and +10 or +11 days) 17 days of bed rest. Muscle activity consisted of a series of unilateral isometric plantar flexions, including 4 maximal voluntary contractions (MVCs), 48 contractions at
30% MVC, and 12 contractions at 80% MVC, all performed at a 4:1-s
work-to-rest ratio. Blood was collected before and immediately after
muscle activity to measure plasma growth hormone by radioimmunoassay (IGH) and by bioassay (BGH) of tibia epiphyseal cartilage growth in
hypophysectomized rats. Plasma IGH was unchanged by muscle activity
before, during, or after bed rest. Before bed rest, muscle activity
increased (P < 0.05) BGH by 66% at
13 or 12 days (2,146 ± 192 to 3,565 ± 197 µg/l)
and by 92% at 8 or 7 days (2,162 ± 159 to 4,161 ± 204 µg/l). After 2 or 3 days of bed rest, there was no response
of BGH to the muscle activity, a pattern that persisted through 8 or 9 days of bed rest. However, after 13 or 14 days of bed rest, plasma
concentration of BGH was significantly lower after than before muscle
activity (2,594 ± 211 to 2,085 ± 109 µg/l). After completion
of bed rest, muscle activity increased BGH by 31% at 2 or 3 days
(1,807 ± 117 to 2,379 ± 473 µg/l;
P < 0.05), and by 10 or 11 days the
BGH response was similar to that before bed rest (1,881 ± 75 to
4,160 ± 315 µg/l; P < 0.05). These data demonstrate that the ambulatory state of an individual can
have a major impact on the release of BGH, but not IGH, in response to
a single bout of muscle activity.
0161-7567/97 $5.00
Copyright © 1997 the American Physiological Society
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