Glucocorticoid-receptor activation in hypertrophied skeletal muscle

This investigation was undertaken 1) to determine whether the increased glucocorticoid-receptor binding activities, observed in hypertrophied plantaris muscles, are associated with a reduced ability to undergo receptor activation and 2) to examine whether glucocorticoid-receptor complexes in hypertrophied muscles undergo a shift in the relative distribution of the two thermally activated receptor forms (termed binder II and corticosteroid binder IB) to a distribution that is found in slow-twitch or heart muscle types. Plantaris muscles of female adrenalectomized rats, enlarged by surgical removal of synergists, were 60% heavier and had higher glucocorticoid cytosol binding (125 +/- 14 vs. 79 +/- 8 fmol/mg protein) than these muscles of controls. Activation, which was quantitated by the ability of the steroid-receptor complex to bind to DNA, was similar in overloaded and control muscles (57 +/- 2 vs. 62 +/- 4%). Diethylaminoethyl-cellulose chromatography of activated receptors showed approximately 16% of the radioactivity appearing as binder II and 38% as binder IB in both hypertrophied and control muscles. These results show that although enlarged plantaris muscles are undergoing certain fast- to slow-twitch biochemical transformations, the activated glucocorticoid-receptor distribution does not shift to that observed in slow fibers.