Effects of spaceflight and thyroid deficiency on hindlimb development. I. Muscle mass and IGF-I expression

Thyroid deficiency (TD) in neonatal rats causes reduced growth of skeletal muscle that is disproportionately greater than that for other tissues (G. R . Adams, S. A. McCue, IM. Zeng, and K. M. Baldwin. Am. J. Physiol. Regulato ry Integrative Camp. Physiol. 276: R954-R961, 1999). TD depresses plasma in sulin-like growth factor I (IGF-I) levels, suggesting a mechanism for this effect. We hypothesized that TD and exposure to spaceflight (SF) would inte ract to reduce skeletal muscle growth via a reduction in IGF-I levels. Neon atal rats were flown in space for 16 days. There was a similar, nonadditive reduction in the growth of the body (similar to 50%) and muscle weight (fa st muscles, similar to 60%) with either TD or SF. In the soleus muscle, eit her SF or TD alone resulted in growth reductions that were augmented by SF- TD interactions. There were strong correlations between 1) muscle mass and muscle IGF-I levels and 2) circulating IGF-I and body weight. These results indicate that either hypothyroidism or exposure to SF will limit the somat ic and muscle-specific growth of neonatal rats. The impact of these perturb ations on skeletal muscle growth is relatively greater than the effect on s omatic growth. The mechanisms by which either TD or SF impact growth appear to have a common pathway involving the control of plasma and muscle IGF-I concentrations.