Adrenaline stimulated glycogen breakdown in rat epitrochlearis muscles: fibre type specificity and relation to phosphorylase transformation.

The effect of adrenaline on glycogen breakdown in different skeletal muscle fibre types was investigated in the epitrochlearis muscle in vitro. Histochemical studies showed that adrenaline stimulated glycogen breakdown in all three major fibre types, with a higher absolute glycogen breakdown in type IIB fibres compared to type IIA and type I fibres. In biochemical studies we found that the glycogenolytic rate decreased during prolonged incubation with adrenaline, although the percentage phosphorylase in the a form and the concentration of glucose-6-phosphate (G-6-P) remained high. In the dose response studies we found an EC50 of 2.2 x 10(-8) M adrenaline for adrenaline stimulated glycogen breakdown, with an EC50 of 2.0 x 10(-7) M for adrenaline stimulated accumulation of G-6-P, excluding G-6-P as the key inhibitor of phosphorylase a activity.