Contractile responses of rat fast-twitch and slow-twitch muscles to glucocorticoid treatment.

In an attempt to determine whether chronic glucocorticoid treatment affects speed- and strength-related properties of fast-twitch muscle and slow-twitch muscle, in-situ contractile properties of soleus and gastrocnemius muscles were measured in rats after 14 days on a regiment of triamcinolone acetonide injected at a dosage of 2 mg/kg/day. No effects on contractile speed or strength were noted for either muscle. The time to peak tension in soleus muscle was significantly longer in the treated group (43.0 +/- 1.0 msec) than in the control group (38.7 +/- 0.7 msec); however, this was attributed to a prolongation of "active state" duration, as suggested by the fact that treated muscles also showed significantly elevated twitch-tetanus ratios. The treatment-induced enhancement of the specific twitch tension was more pronounced in the gastrocnemius muscles (198% +/- 19% of control) than in the soleus muscles (153% +/- 9% of control). The difference in response to steroid treatment may reflect structural and functional differences in fast- and slow-muscle membrane systems.