THE ROLE OF NA-ABDOMINIS TO CA-FREE RINGER SOLUTION WITH NA(2)EDTA( CHANNELS IN TWITCH GENERATION DURING EXPOSURE OF THE FROG RECTUS)

The aim of the study was to investigate whether Na+ channels play a ro le in the twitch component of the response of the isolated frog rectus abdominis to Ca2+-free Ringer solution with 0.2 mM Na(2)EDTA 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 n ot 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 1 0(-5) M) and quinidine (10(-6) - 2 X 10(-5) M) inhibited maximal twitc h amplitude in a concentration dependent manner. These findings strong ly suggest that activation of tetrodotoxin sensitive Na+ channel may p lay a primary role at twitch generation during exposure of the frog re ctus abdominis to Ca2+-free Ringer solution with Na-2 EDTA.