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Studies on the mechanisms of phosphorylase activation Harwood, James Percival


The effects of epinephrine and electrical stimulation on the activation of glycogen phosphorylase were studied in isolated rat diaphragm and frog sartorius, and rat gastrocnemius in vivo. The resting ratio of phosphorylase, as expressed by the ratio of phosphorylase a to total phosphorylase (-AMP/+AMP ratio) was found to be low of the order of 0.05 in diaphragm and sartorius. In rat gastrocnemius this value was high at 0.26. As measured by the ratio of activity at pH 6.8 to that at pH 8.2 (pH 6.8/8.2 ratio), phosphorylase kinase was essentially in its inactive form under conditions of no stimulation. On treatment with epinephrine, phosphorylase was activated, and there was a significant increase in the activity ratio for phosphorylase kinase, up to 10-fold, indicating that conversion of nonactivated kinase to its activated form had occurred. Epinephrine also produced marked increases, up to 15-fold, in the tissue levels of adenosine 31, 51-monophosphate (cyclic AMP). When muscle was induced to contract by electrical stimulation, phosphorylase was markedly and rapidly activated. In contrast to the effect of epinephrine, electrical stimulation produced no conversion of phosphorylase kinase to its activated form as measured by the pH 6.8/8.2 ratio. This was found for both direct and neural stimulation, at various frequencies, for different times of stimulation, in vitro and in vivo, and in two species. No increase in the tissue levels of cyclic AMP were detected on electrical stimulation. It was concluded that the mechanism of activation of phosphorylase during electrical stimulation is basically different from that produced by adrenergic amines. The data strongly suggests that during muscle contraction phosphorylase is activated by a mechanism which does not involve conversion of phosphorylase kinase to its activated form. In further work, the relationship between phosphorylase activation and muscle contraction was studied. It was found that for any given frequency of stimulation, phosphorylase was activated within 2 sec to a particular ratio for that frequency. On further stimulation, the ratio did not increase. When the temperature was lowered, the steady state phosphorylase ratio for a given frequency was lowered, but activation still occurred rapidly. In experiments in which calcium was removed from the medium by using chelating agents, a correlation was demonstrated between phosphorylase activation and contractile tension. From these results it appears that the mechanism of phosphorylase activation is closely coupled to the contractile mechanism. It is proposed that calcium ion, which is important in excitation-contraction coupling and tension development, is responsible for phosphorylase activation. It is further suggested that calcium ion released into the myoplasm may act with nonactivated phosphorylase kinase to catalyse the conversion of phosphorylase b to phosphorylase a.

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