The mechanisms of the action of hemolysin extracted from Vibrio parahaemolyticus in the S-A node and right atrium cells of rabbit were studied by means of the single sucrose gap and isometric tension recording methods. Hemolysin caused the membrane to depolarize reversibly without affecting the action potential generating mechanism. Lowering of [Na+]o inhibited membrane depolarization in the presence of hemolysin while the readmission of normal Tyrode solution induced depolarization. Tetrodotoxin (TTX) barely antagonized the depolarizing action of hemolysin but slowed the rate of development of depolarization. Therefore, this depolarization is considered to be primarily due to the increase in conductance to Na which TTX may not block. The dose-response relationship was obtained by measuring a change in membrane resistance. The concentration necessary to yield one-half of the maximum reduction of the membrane resistance was determined to be 7.5μg/ml.
Accumulation of Na within the cell may be responsible for an increase of twitch tension observed during the action of a low concentration of hemolysin. On the other hand, a higher concentration of hemolysin seemed to promote exchange of intracellular Na with extracellular Ca, especially when the Na concentration of the perfusing solution was reduced, and led to stronger contracture.
