Control of glycogen utilization and glucose uptake in the anaerobic turtle heart

¹ Department of Physiology, Harvard University Medical School, Boston, Massachusetts

Glucose uptake and lactate production of isolated, perfused turtle (Pseudemys scripta) hearts were measured under anaerobic conditions. At low pressure-volume workloads the hearts used glycogen stores rather than exogenous glucose to provide hexose substrate for glycolysis. Glycogenolysis increased in proportion to increased workload up to a critical maximum value for each heart. Increase of workload beyond this critical point was associated with utilization of exogenous glucose or, in the absence of glucose, with mechanical failure of the heart. Glucose-U-C¹⁴ was quantitatively (98%) converted into lactate, and neither dilution of exogenous glucose specific activity nor incorporation of label into glycogen or fatty acids occurred. The maximal rate of glycogenolysis was found to be a linear function of initial glycogen concentration. It is concluded that the input of hexose units into glycolysis is regulated jointly by hexokinase and phosphorylase; however, the known properties of these two enzymes will not fully account for all the features of this regulation.

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