The effect of verapamil on the gastrocnemius and soleus muscles of the cat in vivo

The effects of verapamil on the force of contraction of soleus and gastrocnemius muscles of the cat were examined in vivo. The rapid intra-arterial administration of verapamil (100-800 micrograms/kg) produced a dose-dependent increase in the contractile force of soleus and gastrocnemius muscles stimulated supramaximally via their nerves. Verapamil also potentiated twitch tension in d-tubocurarine-treated, directly stimulated soleus preparations. This agent had no effect, however, on the contractile force of 14- to 16-day chronically denervated, directly stimulated soleus muscle. Verapamil (400 microgram/kg, i.a.) also produced a significant increase in muscle action potential duration while having no effect on amplitudes, rise times or resting membrane potential. It is concluded that verapamil acts directly on innervated skeletal muscle to increase muscle force and it is suggested that the probable site of action is at the muscle membrane.