Glycogen mobilization and work in the rat heart

¹ Department of Physiology, Boston University School of Medicine, Boston, Massachusetts

Glycogen mobilization was studied in isolated rat heart trabeculae carneae stimulated to contract isometrically. Glycogen content of stimulated and nonstimulated muscle preparations was determined, as was glucose content of the fluid bathing the muscles. Isometric twitch curves were integrated over periods of stimulation and the results, in milligram seconds of tension time, used as an index of work performance. Relationships between work performance, 48-hr fasting, hypophysectomy, and mobilization of glycogen were investigated. The amount of glycogen mobilized by the isolated tissue was correlated with initial glycogen content but not with work performance. There was no correlation between initial glycogen content and work done by the tissue. These experiments indicate that the isolated cardiac muscle preparation relies on substrate(s) other than carbohydrate as an immediate source of energy. Heart preparations from 48-hr fasted and from hypophysectomized animals showed significantly greater work performance than did preparations from normal animals, but depletion of glycogen stores again appeared to be related to initial glycogen content and not to work done.