On the Role of Calcium Ions in the Regulation of Glycogenolysis in Mouse Brain Cortical Slices |
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Authors: | D. Ververken P. Van Veldhoven C. Proost H. Carton H. De Wulf |
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Affiliation: | Lahoruforiuni voor Neurochernie utzd Afdding Biochernie, Campus Gashisberg, Kafholieke Universitrit Leuvrn, Belgium |
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Abstract: | Abstract: Using mouse brain cortical slices, we investigated the relative roles of cyclic AMP and of calcium ions as the intracellular messengers for the activation of glycogen phosphorylase (EC 2.4.1.1; α-1,4-glucan:orthophosphate glucosyltransferase) induced by noradrenaline and by depolarization. Activation of phosphorylase by 100 μM noradrenaline is mediated by β-adrenergic receptors and does not require the copresence of adenosine. The role of the concomitant small increase in cyclic AMP is questioned. Short-term treatment with EGTA or LaCl3 abolishes the noradrenaline activation of phosphorylase, pointing to a critical role of extracellular calcium. Depolarization by 25 m M K+ or 100 μ M veratridine produces a rapid and large (fourfold) activation of phosphorylase. Only veratridine increases the cyclic AMP levels; exogenous adenosine deaminase essentially blocks this cyclic AMP accumulation but not the phosphorylase activation. A halfmaximal activation of phosphorylase occurs at about 12 m M K+. Addition of EGTA or LaCl3, reduces the effect of both depolarizations to a slight and transient activation of phosphorylase. These results indicate that activation of glycogen phosphorylase by K+ or veratridine occurs by a cyclic AMP-independent and calcium-dependent mechanism. The calcium dependency of brain phosphorylase kinase renders this kinase the prime target enzyme for regulation of glycogenolysis by calcium ions. |
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Keywords: | Glycogen phosphorylase Cyclic AMP Calcium Noradrenaline Depolarization Brain cortex |
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