Primary heart cell cultures of embryonic guinea pigs and the neonatal rat were established and incubated with purified diphtheria toxin. The rat heart cell cultures were refractory to the effects of the toxin; protein synthesis proceeded normally as measured by the incorporation of tritiated leucine into cell proteins; beating heart cells continued to contract; and the cell monolayers remained intact after exposure to the toxin for periods as long as 72 hr. These findings are compatible with the species resistance of the rat to diphtheria toxin. The guinea pig heart cell cultures were found to be extremely sensitive to the toxin. Protein synthesis was inhibited by approximately 50% after incubation with small quantities of toxin for 3 hr. Increasing the concentration of the length of exposure to the toxin did not increase this inhibition significantly. In addition, diphtheria toxin exerted a true cytopathic effect on the guinea pig heart cells. Monolayers were destroyed when incubated with the toxin for 2 to 3 days.

The results show that the heart cells reflect species resistance or sensitivity to diphtheria toxin in the absence of neural or endocrine influences and suggest further that the toxin exerts a direct toxicity to muscle cells of the heart. It is not yet possible to explain in biochemical terms why the toxin seems to act specifically on cardiac tissues.

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