Anesthetics can alter subsequent in vitro assessment of contractility in slow and fast skeletal muscles of rat.

Anesthetic agents can interfere with measurement of skeletal muscle contractility in vivo or in situ. Data obtained in vitro are however believed to be unaffected by such drugs. Our objective was to compare in vitro contractile measurements of fast- and slow-twitch muscles dissected from rats anesthetized with pentobarbital sodium (PS, 50 mg/kg ip) or with a mixture of ketamine and xylazine (KX, 87. 5:12.5 mg/kg ip). The soleus (Sol) and extensor digitorum longus (EDL) muscles were precisely dissected 10 and 20 min after induction of anesthesia and equilibrated for 20 min in vitro before measuring contractile properties. All data obtained from PS rats were comparable with published values obtained under similar conditions. In EDL, maximum tetanic tension (Po) in KX rats was significantly decreased at both times compared with that in PS muscles. In the Sol, only the muscles exposed for 20 min to KX showed a decreased Po. These results clearly emphasize the need for investigators assessing skeletal muscle contractility in vitro to take into account the type of anesthetics used and the time of in vivo exposition to the drug.