On the Role of Calcium Ions in the Regulation of Glycogenolysis in Mouse Brain Cortical Slices

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; alpha-1,4-glucan:orthophosphate glucosyltransferase) induced by noradrenaline and by depolarization. Activation of phosphorylase by 100 microM noradrenaline is mediated by beta-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 mM K+ or 100 microM 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 half-maximal activation of phosphorylase occurs at about 12 mM 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.

Effect of Topical Adenosine Deaminase Treatment on the Functional Hyperemic and Hypoxic Responses of Cerebrocortical Microcirculation

“…This result shows that the pial membrane and the brain cortex are permeable to adenosine deaminase; hence, after a prolonged period of superfusion, the tissue adeno sine deaminase concentration may not be much smaller than in the superfusate fluid. Concerning the penetrability of adenosine deaminase into the brain, our results correspond to the findings of Ververken et al (1982) who showed on 0.25-mm thick brain slices that adenosine deaminase, even after a 5-min period of incubation, inhibits com pletely the production of cyclic AMP evoked by 10-7 mol/ml adenosine. Vnder our experimental conditions, 95% of the optical signals are derived from the upper 0.2-mm-thick layers of the brain cortex (D6ra, 1984b).…”

“…Furthermore, because 20-min pretreatment of the brain cortices with 0.025 Vlml adenosine deaminase inhibited al most completely the 10-7 mol/ml adenosine-in duced cortical vasodilatation, it may be assumed that the adenosine deaminase concentration in the tissue after prolonged treatment is not much lower than that in the superfusate fluid. This assumption agrees with the experimental findings of Ververken et al (1982) and the calculations of Proctor and Duling (1982). The latter investigators, using the equation described for a one-dimensional diffusion process and a sheet of tissue thickness of 0.2 mm, calculated that after a 10-min equilibration period, the tissue adenosine deaminase concentration should be at least 90% that of the superfusate fluid.…”

“…For this purpose, topical treatment of the brain cortex with adenosine de aminase seemed to be an appropriate experimental tool. Adenosine deaminase potently deaminates adenosine to the nonvasoactive inosine, and, ac cording to the findings of Ververken et al (1982), readily penetrates isolated brain slices. It was ex pected that if the adenosine concentration in the vicinity of the pial and intracortical vessels of the resting, hypoxic, and activated brain were in a va soactive range, adenosine deaminase treatment would produce vasoconstriction in the resting brain and would distinctly inhibit the vasodilative effect of hypoxia and brain activation.…”

In vitro ischemia promotes calcium influx and intracellular calcium release in hippocampal astrocytes

“…Astroglial [Ca2+], signals also might trigger the release of auxiliary metabolites from astroglial glycogen stores (Phelps, 1972). Elevations in external [K'10 can trigger glyco-genolysis in both brain slices (Ververken et al, 1982;Hof et al, 1988) and cultured astrocytes (Cambray-Deakin et al, 1988). Loading of these stores can reduce neuronal death from hypoglycemia in neuron-astrocyte cocultures (Swanson and Choi, 1993) and possibly ischemic neuronal death in vivo (Swanson et al, 1990), whereas inhibition of glial glycogen synthesis or glucose uptake can potentiate hypoxic neuronal death in vitro (Virgin et al, 1991;Tombaugh et al, 1992).…”

Astrocytic Adrenoceptors: A Major Drug Target in Neurological and Psychiatric Disorders?

“…The most direct indication may be the occurrence of glycogenolysis, an astrocyte-specific process, which has been demonstrated not only during one-trial aversive learning in day-old chicks [242][243] but also during odor learning in rats pups [342]. However, glycogenolysis in brain tissue and in cultured astrocytes can be evoked not only by β-or α 2 -adrenergic stimulation but also by an elevation of the extracellular concentration of K + [343][344][345].…”