Bed rest suppresses bioassayable growth hormone release in response to muscle activity

G. E. McCall 1 , C. Goulet 1 , R. E. Grindeland 2 , J. A. Hodgson 1 , 3 , A. J. Bigbee 2 , and V. R. Edgerton 1 , 3 1 Department of Physiological Science and 3 Brain Research Institute, University of California, Los Angeles 90095; and 2 National Aeronautics and Space Administration Ames Research Center, Moffett Field, California 94035 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. 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