Effects of the enantiomers of R,S‐salbutamol on incompletely fused tetanic contractions of slow‐ and fast‐twitch skeletal muscles of the guinea‐pig

The effects of racemic R,S‐salbutamol, and its individual enantiomers have been studied on incompletely fused (sub‐tetanic) contractile responses of fast‐ and slow‐contracting isolated skeletal muscles of the guinea‐pig. R,S‐salbutamol (2–4 μM) decreased the peak force of sub‐tetani in the slow‐contracting soleus muscle and increased the peak force of sub‐tetani in the fast‐contracting peroneus longus muscle. It also increased the force of the first twitch of sub‐tetani in both muscles. The decrease in the peak force of sub‐tetani in the soleus muscle was due to defusion of the individual twitches caused by a shortening of their time course. The effects of 4 μM of the racemate on both fast‐ and slow‐contracting muscles were mimicked by 2 μM R‐salbutamol (levalbuterol). However, 2 μM S‐salbutamol was devoid of activity in both muscles. We concluded that all the effects of R,S‐salbutamol on guinea‐pig skeletal muscles are due to the activity of the R‐enantiomer. Thus there is a common enantiomeric profile for the skeletal muscle and bronchorelaxant activity of the compound. British Journal of Pharmacology (1998) 123, 558–564; doi:10.1038/sj.bjp.0701627