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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.
Ca<sup>2+</sup>-free medium with Na<sub>2</sub> EDTA
isolated frog rectus abdominis
Na<sup>+</sup> channel blocking activity
Acta Medica Okayama
Okayama University Medical School
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