Potentiation by thrombin of hyposmotic glutamate and taurine efflux from cultured astrocytes: signalling chains.

Activation of protein-activated receptor (PAR-1) by thrombin potentiates the hyposmotic efflux of (3)H-D-aspartate and (3)H-taurine from cultured cerebellar astrocytes. This effect is mediated by a thrombin-elicited increase in cytosolic Ca(2+) levels [Ca(2+)](i) and the activation of phosphoinositide-3-kinase (PI3K). These signalling pathways operate independently showing additive effects if prevented simultaneously. The contribution of the Ca(2+)-mediated pathway to thrombin-increased D-aspartate or taurine efflux, evaluated by the inhibitory effect of preventing [Ca(2+)](i) rise, was higher for D-aspartate (64% efflux decrease) than for taurine (40% decrease). The PI3K blocker decreased 48% and 36% D-aspartate and taurine efflux, respectively. Hyposmolarity increases phosphorylation of EGFR and c-src, but thrombin did not enhance this effect. Blockade of EGFR/src phosphorylation marginally reduced (11-14%) the hyposmolarity plus thrombin efflux of D-aspartate; taurine efflux was more sensitive to these blockers (18-26%). Since thrombin has no effect increasing EGFR/src phosphorylation in astrocytes, the contribution of this transactivation pathway may represent the inhibition of the hyposmotic efflux solely.