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J Appl Physiol 83: 1522-1530, 1997;
8750-7587/97 $5.00
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Vol. 83, Issue 5, 1522-1530, 1997

Growth hormone, IGF-I, and exercise effects on non-weightbearing fast muscles of hypophysectomized rats

Elena J. Grossman1,2, Richard E. Grindeland1, Roland R. Roy3, Robert J. Talmadge2,3, Juliann Evans1, and V. Reggie Edgerton2,3

1 Life Science Division, National Aeronautics and Space Administration-Ames Research Center, Moffett Field 94035; and 2 Department of Physiological Science and 3 Brain Research Institute, University of California, Los Angeles, California 90095

Received 3 March 1997; accepted in final form 14 July 1997.

Grossman, Elena J., Richard E. Grindeland, Roland R. Roy, Robert J. Talmadge, Juliann Evans, and V. Reggie Edgerton. Growth hormone, IGF-I, and exercise effects on non-weight-bearing fast muscles of hypophysectomized rats. J. Appl. Physiol. 83(5): 1522-1530, 1997.---The effects of growth hormone (GH) or insulin-like growth factor I (IGF-I) with or without exercise (ladder climbing) in countering the effects of unweighting on fast muscles of hypophysectomized rats during 10 days of hindlimb suspension were determined. Compared with untreated suspended rats, muscle weights were 16-29% larger in GH-treated and 5-15% larger in IGF-I-treated suspended rats. Exercise alone had no effect on muscle weights. Compared with ambulatory control, the medial gastrocnemius weight in suspended, exercised rats was larger after GH treatment and maintained with IGF-I treatment. The combination of GH or IGF-I plus exercise in suspended rats resulted in an increase in the size of each predominant fiber type, i.e., types I, I+IIa and IIa+IIx, in the medial gastrocnemius compared with untreated suspended rats. Normal ambulation or exercise during suspension increased the proportion of fibers expressing embryonic myosin heavy chain in hypophysectomized rats. The phenotype of the medial gastrocnemius was minimally affected by GH, IGF-I, and/or exercise. These results show that there is an IGF-I, as well as a GH, and exercise interactive effect in maintaining medial gastrocnemius fiber size in suspended hypophysectomized rats.

growth factors; hindlimb suspension; medial gastrocnemius; myosin heavy chain; fiber cross-sectional area; insulin-like growth factor I

0161-7567/97 $5.00 Copyright © 1997 the American Physiological Society




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