Pulsating mammalian myocardial cells were found to be highly susceptible in tissue culture to rapid destruction by streptolysin O. Cessation of beating occurred almost immediately, followed within minutes by multiple cell membrane bleb formation. Parallel with these changes, the cytoplasm became intensely granular and the nuclear membrane apparently thickened when viewed by phase microscopy. At the ultrastructural level, the cell membrane blebs were found to contain relatively small numbers of granular fragments. The endoplasmic reticulum of damaged heart cells was quite swollen, and its contents were considerably condensed. The myofibers were not strikingly altered, but cytoplasmic and mitochondria vacuoles were rather abundant.

Cardiac endothelial, kidney epithelial, and fibroblast cells were also susceptible to lysis by this toxin, but the reactions occurred more slowly or bleb formation was less evident. An antiserotonin drug known to be protective against streptolysin-O in vivo (UML-491), did not protect against killing of cardiac cells at the tissue culture level. Serotonin could not be detected in the culture fluid after lysis of cardiac cells by streptolysin O.

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