Synergistic Regulation of Cytosolic Ca²⁺ Concentration by Adenosine and α1‐Adrenergic Agonists in Mouse Striatal Astrocytes

Abstract

Adenosine has a broad array of actions on neurons but astrocytes also possess adenosine receptors. We have previously shown that adenosine, by acting on astrocytes in the striatum, can modulate neuronal responses mediated by receptors coupled to phospholipase C through an astrocyto - neuronal interaction. In addition, adenosine was found to potentiate the alpha1-adrenergic production of inositol phosphates in astrocytes. The mechanism involved in this potentiation was further investigated by examining the effects of adenosine and alpha1-adrenergic receptor agonists on cytosolic Ca2+ in cultured striatal astrocytes from the embryonic mouse in primary culture. When used alone, methoxamine, a selective agonist of alpha-adrenergic receptors or 2-chloroadenosine, a stable analogue of adenosine, induced a transitory increase in cytosolic Ca2+, but their combined addition led to a sustained increase in cytosolic Ca2+, which seems to be due to a Ca2+ influx, because it was not observed in the absence of external Ca2+. Voltage independent Ca2+ channels contribute to this process and different blockers of voltage-operated calcium channels, such as dihydropyridines, phenylalkylamines, La3+ or Co2+ were ineffective in suppressing the sustained cytosolic Ca2+ elevation. Three observations suggest the implication of arachidonic acid in the observed potentiation: (i) arachidonic acid induced a sustained elevation of cytosolic Ca2+ similar to that evoked by the coapplication of methoxamine and 2-chloroadenosine; (ii) the addition of arachidonic acid during the calcic plateau produced by the combined application of the agonists did not increase further cytosolic Ca2+ levels; (iii) in the presence of methoxamine, 2-chloroadenosine induced a release of arachidonic acid. The stimulation of phospholipase C and the resulting activation of protein kinase C induced by methoxamine seem to be required for the potentiating effect of 2-chloroadenosine on cytosolic Ca2+. In fact, the direct activation of protein kinase C by an exogenous diacylglycerol analogue mimicked the effect of methoxamine because, in this condition, 2-chloroadenosine alone evoked a sustained elevation of cytosolic Ca2+. Therefore, methoxamine, through the successive activation of phospholipase C and protein kinase C, could allow a lipase, probably phospholipase A2, to be stimulated by 2-chloroadenosine. Arachidonic acid has already been shown to trigger the opening of K+ channels and the formation of inositol phosphates in other cell types. Therefore, in striatal astrocytes, 2-chloroadenosine, through an arachidonic acid-mediated hyperpolarization, could increase the Ca2+ driving force and thus improve Ca2+ influx through inositol phosphate-gated channels. This hypothesis is further supported by the suppressing effect of a 50 mM KCI-induced depolarization on the long lasting elevation of cytosolic Ca2+ seen in the combined presence of 2-chloroadenosine and methoxamine.

Antagonism by riluzole of entry of calcium evoked by NMDA and veratridine in rat cultured granule cells: evidence for a dual mechanism of action

“…This study extends the use of the technique from previous applications in, for example pituitary cells (Mollard et al, 1989), cultured rat spinal neurones (Schieren & MacDermot, 1988) and astrocytes (Delumeau et al, 1991 by Simpson et al (1993), who have demonstrated that the rise in intracellular free calcium levels evoked by NMDA receptor activation is essentially due to calcium entry. Moreover, Frandsen et al (1989) demonstrated that glutamic acid directly stimulated 45Ca2" entry into cultured cortical neurones.…”

“…This is in agreement with our previous work on the release of D-[3H]-aspartate from cerebellar granule cells, where, again, the inhibitory effect of riluzole was abolished by pertussis toxin . These data suggest that the interaction between riluzole and excitatory amino acid-mediated events may involve activation of a pertussis toxin-sensitive G-protein, perhaps by directly activating a coupled receptor (such an interaction has been observed between adenosine and o-adrenoceptors on astrocytes by Delumeau et al (1991)). …”

“…Cytosolic calcium levels were monitored with the fluorescent dye indo-l (Grynkiewicz et al, 1985;Delumeau et al, 1991).…”

Interleukin-1 Enhances the ATP-Evoked Release of Arachidonic Acid from Mouse Astrocytes

“…Incubation media were recovered and centrifuged for 5 min at 200 ϫ g to eliminate nonadherent cells, and the radioactivity was estimated in the supernatant. HPLC analysis, performed as described previously (Delumeau et al, 1991), indicated that Ͼ95% of the radioactivity is recovered in a peak having the same retention time as authentic AA.…”

A Genistein-Sensitive Na+/Ca2+ Exchange Is Responsible for the Resting [Ca2+]i and Most of the Ca2+ Plasma Membrane Fluxes in Stimulated Rat Cerebellar Type 1 Astrocytes

“…There is evidence indicating that the relative importance of the Na + /Ca 2+ exchanger in different types of glial cells is highly variable [5][6][7][8][9][10][11][12][13]. Furthermore, its role has sometimes not been conclusively proved.…”