Date of Award

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Harold Silverman

Abstract

To determine the pathways used for glycogen synthesis in chronically active and normal muscle, muscles from C57B1/6J dy$\sp{\rm 2J}/$dy$\sp{\rm 2J}$(dy$\sp{\rm 2J}$) mice were evaluated biochemically and histochemically. The gastrocnemius muscle (GAST) of the dy$\sp{\rm 2J}$ mouse, a chronically active muscle, contained twice as much glycogen and lactate as control GAST. The dy$\sp{\rm 2J}$ triceps muscle (TRI), which is not chronically active, showed no elevation in glycogen content. An intraperitoneal injection of $\sp{14}$C-lactate resulted in increased incorporation of $\sp{14}$C into glycogen by dy$\sp{\rm 2J}$ compared to control GAST. Both normal and dy$\sp{\rm 2J}$ GAST incorporated $\sp{14}$C into glycogen in an in situ preparation, indicating direct glycogen synthesis from lactate. Autoradiography revealed that high glycogen containing muscle fibers in the dy$\sp{\rm 2J}$ GAST have the highest capacity for glyconeogenesis. Glycogen synthase (GS), malic enzyme (ME) and phosphoenolpyruvate carboxykinase (PEPCK) are all elevated in dy$\sp{\rm 2J}$ GAST, but not in dy$\sp{\rm 2J}$ TRI compared to normal controls. High glycogen fibers in the dy$\sp{\rm 2J}$ had higher activities of GS and ME than any other fibers. The variation in glycogen content along the length of single muscle fibers increased with increasing glycogen content. However, the variation was low enough in all fiber types to allow for a single histochemical section to be a good predictor of glycogen in that fiber. Glucose uptake and glycogen synthesis from glucose (glycogenesis) were elevated in chronically active muscles, in vivo and in vitro. The diaphragm muscle had the highest rates of glucose uptake and glycogenesis. Insulin stimulation of glucose uptake and glycogenesis were enhanced in chronically active muscles from dy$\sp{\rm 2J}$ mice. Specific inhibitors of PEPCK inhibited glyconeogenesis in skeletal muscle, demonstrating the involvement of PEPCK. Previous contractile activity had no effect on glyconeogenic rates. Glyconeogenic rates were linearly dependent on substrate concentration and had a pH optimum of 6.6. Normal and chronically active muscles utilize both lactate and glucose for glycogen synthesis. Chronically active muscles store increased amounts of glycogen. The increased glycogen may provide the chronically active muscle with some additional resistance to fatigue.

Pages

190

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