On the possible role of potassium ions in the action of terbutaline on skeletal muscle contractions.

The soleus, a slow-contracting, and the extensor digitorum longus (EDL), a fast-contracting muscle from guinea-pig were prepared for isometric recording in vitro, and the effect of terbutaline, a beta-adrenoceptor agonist, was studied during various experimental conditions. In the soleus terbutaline caused an initial depression of the force of contraction followed by an increase. The degree of fusion of subtetanic contractions was reduced throughout the experiment. The EDL responded with an increased force only. The force of contraction of the two muscles increased with the K+-ion concentration of the medium up to 6 mM. Higher concentrations of K+ caused a depression which was partly prevented by terbutaline. When the contractions of the soleus muscle had been depressed by excess K+, depletion of K+ or by ouabain, terbutaline restored the twitch tension. It is suggested that the contractile machinery of the muscles is controlled by the ionic balance which in turn is changed following beta-adrenoceptor stimulation.