Abstract
The effect of tetrodotoxin on the heart muscle of Ciona intestinalis, a tunicate, was examined since tetrodotoxin is known to abolish the sodium spikes in various excitable tissues by suppressing the sodium conductance. Tetrodotoxin did not influence the shape of action potential of ciona heart even at the concentration of 10-4g/ml, which is about 10, 000 times that effective for the suppression of rate of rise of action potential in frog heart.
