Biomedical subjects

J Cordier

Publications and source records attributed to J Cordier.

18 recordsLinked to original sources

PubMed1992

Synergistic regulation of cytosolic Ca2+ concentration by somatostatin and alpha 1-adrenergic agonists in mouse astrocytes.

The effects of somatostatin and alpha 1-adrenergic receptor agonists on cytosolic Ca2+ in striatal astrocytes from the embryonic mouse in primary culture have been investigated by microfluorimetry. Methoxamine or somatostatin 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 since it was not observed in the absence of external Ca2+. Voltage-independent Ca2+ channels contribute to this process. Indeed, voltage-operated calcium channels are not involved since neither dihydropyridines nor La3+ were effective in suppressing the sustained cytosolic Ca2+ elevation. Moreover, depolarization by 50 mM KCl, which was ineffective alone, suppressed the effect of somatostatin observed in the presence of the alpha 1 agonist, methoxamine. The implication of arachidonic acid in the observed potentiation is suggested by the following observations: 1) arachidonic acid induced a sustained elevation of cytosolic Ca2+ similar to that evoked by the co-application of methoxamine and somatostatin; 2) the addition of ETYA, an inactive and non-metabolizable analogue of arachidonic acid suppressed the calcium plateau produced by the agonists. In addition, direct activation of PKC by an exogeneous diacylglycerol analogue allowed somatostatin alone to evoke a sustained elevation of cytosolic Ca2+. Therefore, methoxamine through the successive activation of PLC and PKC could allow a lipase, probably PLA2, to be stimulated by somatostatin. Since arachidonic acid has already been shown to trigger the opening of K+ channels and the formation of inositol phosphates, somatostatin, through the arachidonic acid-mediated hyperpolarization could increase the Ca2+ driving force and thus improve Ca2+ influx through the inositol phosphate gated channels.

5,8,11,14-Eicosatetraynoic Acid↗

PubMed1992

2-Chloroadenosine potentiates the alpha 1-adrenergic activation of phospholipase C through a mechanism involving arachidonic acid and glutamate in striatal astrocytes.

In cultured striatal astrocytes, 2-chloroadenosine, an adenosine analog resistant to adenosine deaminase, although inactive alone, markedly potentiated the activation of phospholipase C induced by methoxamine, an alpha 1-adrenergic agonist. This effect was suppressed by antagonists of either A1 adenosine or alpha 1-adrenergic receptors. An influx of calcium and two distinct G-proteins are involved in this phenomenon since the potentiating effect of 2-chloradenosine was suppressed in the absence of external calcium or when cells were pretreated with pertussis toxin. In addition, arachidonic acid is likely involved in this potentiating effect. This was shown first by examining the effects of inhibitors of phospholipase A2 or arachidonic metabolism, then by examining the action of arachidonic acid on the production of inositol phosphates in either the presence or absence of methoxamine, and finally by measuring the release of arachidonic acid. The sequential activation of phospholipase C and of protein kinase C is required for the 2-chloroadenosine-induced activation of phospholipase A2 since 2-chloroadenosine markedly stimulated phospholipase C activity in the absence of methoxamine when protein kinase C was activated by a diacylglycerol analog. Finally, the enhancing effect of 2-chloroadenosine on the methoxamine-evoked response seems to result from an inhibition of glutamate reuptake into astrocytes by arachidonic acid. Indeed, the potentiating effect of 2-chloroadenosine was suppressed when external glutamate was removed enzymatically and mimicked by either selective inhibitors of the glutamate reuptake process or direct application of glutamate.

2-Chloroadenosine↗

PubMed1991

Somatostatin potentiates the alpha 1-adrenergic activation of phospholipase C in striatal astrocytes through a mechanism involving arachidonic acid and glutamate.

As previously shown with adenosine, somatostatin, which is ineffective alone, enhanced the alpha 1-adrenergic-agonist-stimulated production of inositol phosphates in cultured striatal astrocytes. This effect was suppressed in cells pretreated with pertussis toxin. It required external calcium and was selectively antagonized by both mepacrine, an inhibitor of phospholipase A2, and 5,8,11,14-eicosatetraynoic acid, a nonmetabolizable analog of arachidonic acid. In addition, a long-lasting elevation of cytosolic calcium and a release of arachidonic acid were observed only under the combined stimulation of somatostatin and alpha 1-adrenergic receptors. Arachidonic acid could in turn inhibit glutamate uptake into astrocytes, and the resulting external accumulation of glutamate could account for the somatostatin-evoked amplification of the alpha 1-adrenergic-agonist-stimulated hydrolysis of inositol-phospholipids. The effect of somatostatin was indeed reproduced by glutamate or glutamate uptake inhibitors and suppressed by enzymatic removal of external glutamate. Thus, astrocytes may contribute to long-term plasticity events in glutamatergic synapses through regulation of external glutamate levels.

5,8,11,14-Eicosatetraynoic Acid↗

PubMed1991

Adrenergic regulation of intercellular communications between cultured striatal astrocytes from the mouse.

The permeability of gap junctions in cultured striatal astrocytes was investigated by the scrape-loading/dyetransfer technique. Prolonged application of norepinephrine (NE) (10 microM) reduced by half the extent of dye (Lucifer yellow) spread. This effect was linked to the activation of alpha 1-adrenergic receptors since it was mimicked by methoxamine and antagonized by prazosin. The adenosine agonist 2-chloroadenosine (10 microM), which potentiates the NE-evoked activation of phospholipase C (PLC) in striatal astrocytes, also potentiated the NE-evoked closure of gap junctions, the effect being as important as that observed with the uncoupling agent octanol. Measurements of inositol phospholipid turnover performed in identical experimental conditions revealed a close relationship between the extent of PLC activation and the magnitude of the uncoupling process. The effect of NE was mimicked by both phorbol ester and arachidonic acid, suggesting that biochemical events linked to PLC stimulation such as protein kinase C activation and/or eicosanoid production are likely involved in the NE-induced uncoupling. In addition, in the presence of a cAMP phosphodiesterase inhibitor, the stimulation of beta-adrenergic receptors by isoproterenol (10 microM) led to a large increase in cAMP accumulation correlated with an extension of dye diffusion. This observation suggests that junctional permeability could also be controlled by a cAMP-dependent mechanism. Altogether these results indicate that intercellular communication between cultured astrocytes can be regulated by different second messenger pathways as a result of the action of neurotransmitters on their receptors.

Animals↗

PubMed1991

Gap junctions in cultured astrocytes: single-channel currents and characterization of channel-forming protein.

Currents from gap junction channels were recorded from pairs of astrocytes in primary culture using the double whole-cell recording technique. In weakly coupled pairs, single-channel events could be resolved without pharmacological uncoupling treatment. Under these conditions, unitary conductance was 56 +/- 7 pS, and except for multiples of this value, no other level of conductance was observed consistently. To characterize the type of junctional protein constituting astrocyte gap junction channels, immunological and biochemical experiments were carried out on the same material. Specific cDNA probes for three connexins identified in mammals (Cx26, Cx32, and Cx43) showed that only Cx43 mRNA was expressed in cultured astrocytes. The presence of Cx43 protein in cultured astrocytes was demonstrated by immunoblotting, immunofluorescence, and immunogold labeling using anti-peptide antibodies specific to Cx43. These results strongly suggest that gap junctions in astrocytes have a 50-60 pS unitary conductance associated with channels composed of Cx43 protein.

Animals↗

PubMed1978