The Role of Na⁺ Channels in Twitch Generation During Exposure of the Frog Rectus Abdominis to Ca-Free Ringer Solution with Na₂EDTA

 The aim of the study was to investigate whether Na⁺ channels play a role in the twitch component of the response of the isolated frog rectus abdominis to Ca²⁺-free Ringer solution with 0.2 mM Na₂EDTA by using tetrodotoxin and some other well known drugs that exhibit a blocking action on Na⁺ channels. In the presence of 5 x 10^-7 M tetrodotoxin, the twitch component, measured isotonically, disappeared. Although 10^-7 M d-tubocurarine was found to be ineffective, a complete blockage of twitch amplitude was observed at 5 x 10^-6 M concentration of the drug. The inhibitory action of d-tubocurarine on twitch response was not antagonized by 10^-6 and 10^-5 M carbachol. Propranolol (10^-6 - 10^-5 M), lidocaine (2 x 10^-6 - 10^-5 M), quinine (10^-6 - 2 x 10^-5 M) and quinidine (10^-6 - 2 x 10^-5 M) inhibited maximal twitch amplitude in a concentration dependent manner. These findings strongly suggest that activation of tetrodotoxin sensitive Na⁺ channel may play a primary role at twitch generation during exposure of the frog rectus abdominis to Ca²⁺-free Ringer solution with Na₂ EDTA.