Hormonal regulation of the rate of the glycogen/glucose‐1‐phosphate cycle in skeletal muscle

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Abstract
A method is presented which allows simultaneous estimation of rates of glycogen synthesis and glycogenolysis in an isolated incubated skeletal muscle, thus allowing measurement of the glycogen/glucose‐1‐phosphate substrate cycling rate. In the absence of hormonal additions, the measured rates of glycogen synthesis and breakdown were similar [respectively, 0.54 ± 0.12 (8) and 0.74 ± 0.10 (8) μmol glucosyl equiv. h−1 (g tissue)−1]. Incremental addition of insulin stimulated glycogen synthesis up to threefold and reduced glycogenolysis by about sevenfold; the half‐maximally effective concentration of insulin on both processes was about 100 μU/ml (0.7 nM). Incremental addition of adrenaline (in the presence of 1 mU insulin/ml) caused a dramatic increase in the glycogenolytic rate (about 15‐fold), but a much less marked inhibition of glycogen synthetic rate. In addition to hormonal manipulation of the muscle preparation in vitro, the effects of cold exposure, the hyperthyroid state, a single exercise bout and exercise‐training of animals in vivo on the rates of glycogen synthesis and breakdown in the isolated incubated muscle preparation have been investigated. Significant changes in measured glycogen synthesis, breakdown and glycogen/glucose‐1‐phosphate cycling have been observed, both under basal conditions and in response to hormonal additions in vitro. The results are discussed with respect to the possible physiological importance of this substrate cycle.