ATP‐induced, sustained calcium signalling in cultured rat cortical astrocytes: evidence for a non‐capacitative, P2X7‐like‐mediated calcium entry

Abstract

The receptor mechanisms regulating the ATP-induced free cytosolic Ca 2+ concentration ([Ca 2+ ] i ) changes in cultured rat cortical type-1 astrocytes were analyzed using fura-2-based Ca 2+ imaging microscopy. Upon prolonged ATP challenge (1^100 W WM), astroglial cells displayed a biphasic [Ca 2+ ] i response consisting of an initial peak followed by a sustained elevation. Suramin and pyridoxalphosphate-6-azophenyl-2P P,4P P-disulfonic acid blocked both components, albeit to a di¡erent extent. By contrast, the selective P2X7 antagonist oxidized ATP irreversibly abrogated the sustained [Ca 2+ ] i signal without a¡ecting the transient phase. Finally, astrocyte challenge with the selective P2X7 agonist 3P P-O-(4-benzoyl)benzoyl-ATP evoked a sustained [Ca 2+ ] i elevation, which occluded that induced by ATP. We can conclude that in cultured cortical astrocytes the ATP-mediated sustained [Ca 2+ ] i rise does not implicate capacitative Ca 2+ entry but involves Ca 2+ in£ux through P2X7-like receptors. ß

Dynamics of a minimal neural model consisting of an astrocyte, a neuron, and an interneuron

“…How do these results change when the desensitization effect of ATP receptors are included in the model? In fact, the data reported in Nobile et al [10] clearly show that the P2X receptor desensitizes when exposed for several seconds to ATP. Thus, a very simple model describing the desensitization process of the ionotropic purinergic receptor was introduced in the calcium model.…”

“…In Di Garbo et al [36], it was shown that the above model is capable of describing, both qualitatively and quantitatively, the experimental results reported in [10]. In particular, it was shown that the model is able to reproduce the ATP-evoked biphasic calcium response (the transient and the sustained phase) in the astrocyte: the transient phase corresponds to the release of calcium from the ER store, while the sustained part is mainly determined by the influx of extracellular calcium mediated by the P2X purinoreceptors.…”

“…The G([ATP] ex ) function was obtained by fitting the experimental data reported in Nobile et al [10] which describe the amplitude of the sustained calcium response of the astrocyte against the extracellular ATP concentration. The ATP-evoked calcium response mediated by the activation of a metabotropic ATP receptor coupled to a G-protein, leading to the production of IP 3 , is described as follows.…”

“…3) is now defined as v ATP(P2X) = k ATP(P2X) G [ATP] ex D(t), where D(t) = C N 1 − e −(t−t * )/τ off e −(t−t * )/τoff is a solution of the equationD(t) + PḊ(t) + QD(t) = 0 with P = (2/τ off + 1/τ on ) and Q = (1/τ off + 1/τ on )/τ off ; moreover, τ on = (2 μMs)/[ATP] ex , τ off = 400 s and C N is a normalization constant such that D(t) ≤ 1. The constants τ off and τ on were chosen in order to reproduce qualitatively and quantitatively the experimental data reported in Nobile et al [10]. The corresponding simulation results describing the desensitization effects of the ionotropic purinergic receptor are shown in the bottom panels of Fig.…”

“…with an elevation of their intracellular calcium levels ([Ca 2+ ] i ) [3][4][5][6][7][8][9][10][11][12][13][14]. The [Ca 2+ ] i increase in these cells mediates the release of glutamate, ATP, and other neuroactive substances that are capable, by a feedback mechanism, of modulating synaptic communication between neurons [11][12][13][14].…”