Thymic involution in the suspended rat model for weightlessness: decreased glucocorticoid receptor concentration

Hindlimb muscle atrophy, thymic involution and adrenal hypertrophy in rats during spaceflight can be simulated using suspension models. Skeletal muscle and thymus are sensitive to glucocorticoids (GC) and previous studies have demonstrated that muscle atrophy in suspended rats is associated with increased GC receptor concentration. The objectives were to confirm thymic involution during suspension, and determine if involution correlated with increased GC receptor concentration. Seven days of antiorthostatic (AO) suspension of rats produced a significant (P<0.001) reduction in thymic wet weight not associated with an alteration of % water content. GC receptor concentration (pmol/mg protein) decreased 20% (P<0.025) in thymus glands from 7 day AO suspended rats. Suspension, therefore, is associated with involution of the thymus but this is not dependent upon AO positioning. Thymus GC receptor concentrations were depressed in 7 day suspended rats, in contrast with previous observations on skeletal muscle, suggesting that different mechanisms may underlie these responses.