DIFFERENTIAL-EFFECTS OF ANESTHETICS ON IN-VIVO SKELETAL-MUSCLE CONTRACTILE FUNCTION IN THE MOUSE

The purpose of this study was to evaluate the effects of four anesthet ic regimens on in vivo contractile function of mouse ankle dorsiflexor muscles. The torque-frequency and torque-velocity relationships were determined for the following anesthetics: fentanyl-droperidol and diaz epam (F-d/d); ketamine and xylazine (K/x); pentobarbital sodium (Ps); and methoxyflurane (Mf). Mf, Ps, and F-d/d regimens resulted in compar able contractile responses at low doses, whereas K/x produced a relati ve depression in isometric contractile function as shown by a decrease in the torque-time integral at the 300-Hz stimulation frequency (-13. 9%; P < 0.05). Moreover, K/x caused a shift to the left. in the torque -frequency curve as indicated by increases in torque-time integrals at 25 and 50 Hz. Both Ps and F-d/d regimens exhibited dose-dependent eff ects during the isovelocity contractions. Ps significantly reduced wor k (-28.7%) and average power (-28.9%) at 800% at the high dose. In con trast, F-d/d anesthesia resulted in increases in peak torque (16-20%) and work (15-18%) output at all eccentric contraction velocities at th e high dose, whereas average power was increased only at -800 (17%) an d -1,000 degrees/s (17%). In conclusion, commonly used anesthetic regi mens can affect the contractile response in vivo; K/x and Ps yield sma ller torque outputs, whereas Mf and F-d/d consistently produce larger contractile responses. Mf and F-d/d are recommended for use in studyin g skeletal muscle function in mice in vivo.