Hearts isolated from 11- and 12-day rat embryos were incubated in a simple salt solution to which was added the metabolic inhibitors iodoacetate, malonate, 2, 4-dinitrophenol or trypan blue. Comparisons were made between the two age-groups and it was observed that both 11- and 12-day hearts exhibited depressed contractile activity with the glycolytic inhibitor iodoacetate. Malonate did not depress contraction rate in the younger hearts but significantly depressed the rate in the older hearts. A greater inhibitory action on the older hearts as compared with the younger hearts was also produced by 2, 4-dinitrophenol. These results were interpreted as further evidence of a shift in dependence on pathways other than glycolysis between days 11 and 12 of development.

The teratogenic agent trypan blue was shown to depress contraction rate when added to the incubation medium. This effect could be reversed by adding glycolytic intermediates such as fructose-1, 6-diphosphate or alpha glycerophosphate. These results were suggestive of a direct inhibitory effect of trypan blue either on glucose uptake or its complete catabolism.

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