BED REST SUPPRESSES BIOASSAYABLE GROWTH-HORMONE RELEASE IN RESPONSE TCS MUSCLE-ACTIVITY

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. Mu scle activity consisted of a series of unilateral isometric plantar fl exions, including 4 maximal voluntary contractions (MVCs), 48 contract ions 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 aft er muscle activity to measure plasma growth hormone by radioimmunoassa y (IGH) and by bioassay (BGH) of tibia epiphyseal cartilage growth in hypophysectomized rats. Plasma IGH was unchanged by muscle activity be fore, during, or after bed rest. Before bed rest, muscle activity incr eased (P < 0.05) BGH by 66% at -13 or -12 days (2,146 +/- 192 to 3,565 +/- 197 mu g/l) and by 92% at -8 or -7 days (2,162 +/- 159 to 4,161 /- 204 mu g/l). After 2 or 3 days of bed rest, there was no response o f 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 con centration of BGH was significantly lower after than before muscle act ivity (2,594 +/- 211 to 2,085 +/- 109 mu g/l). After completion of bed rest, muscle activity increased BGH by 31% at 2 or 3 days (1,807 +/- 117 to 2,379 +/- 473 mu g/l; P < 0.05), and by 10 or 11 days the BGH r esponse was similar to that before bed rest (1,881 +/- 75 to 4,160 +/- 315 mu g/l; P < 0.05). These data demonstrate that the ambulatory sta te 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.