AbstractIsolated 3‐ and 5‐day chick embryo hearts contain sufficient endogenous substrates to maintain their pulsatile activity for several hours under aerobic conditions, and even after five hours in substrate‐free medium the rates are 40 to 50% of the original rates. Carbohydrate appears to be an important component of the endogenous substrates since 1 mM 2‐deoxyglucose causes rapid failure of rate, and glycolysis appears to be a major energy pathway since the rate is depressed only about 50% by 2‐hour's exposure to 10 mM fluoroacetate. In nitrogen the hearts rapidly become asystolic in the absence of added substrate. Recovery of the rate occurs if oxygen is reintroduced within one hour, but longer periods of anoxia result in progressively less recovery, especially with the 3‐day hearts which appear to be particularly susceptible to irreversible damage. With 5.55 mM glucose as substrate there is little decrease in the original aerobic heart rate during five hours, and the hearts can tolerate total anoxia for five hours with rates only slightly less than the aerobic rates. The hypothesis of a preferential pentose phosphate pathway of glucose catabolism in the very young chick embryo heart is discussed, but no direct evidence in support of its existence is revealed in
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