Hyposmotically induced amino acid release from the rat cerebral cortex: role of phospholipases and protein kinases.

In an evaluation of the contribution of swelling-induced amino acid release, through the regulatory volume decrease (RVD) process, to cerebral ischemic injury, studies of the role of phospholipases and protein kinases in the response to hyposmotic stress were undertaken using an in vivo rat cortical cup model. Hyposmotic stress induced significant releases of aspartate, glutamate, glycine, phosphoethanolamine, taurine and GABA from the rat cerebral cortex. Taurine release was most affected, exhibiting a greater than 9-fold increase during the hyposmotic stimulus. The phospholipase A2 (PLA2) inhibitors 4-bromophenacyl bromide (1 microM) and 7,7-dimethyleicosadienoic acid (5 microM) had no significant effects on hyposmotically induced amino acid release. AACOCF3 (50 microM), an inhibitor of cytosolic PLA2 decreased taurine release to 84% of DMSO controls. The release of the other amino acids was not affected. The phospholipase C inhibitor U73122 (5 microM) had no significant effects on amino acid release. The protein kinase C (PKC) inhibitor chelerythrine (5 microM) significantly reduced hyposmotically induced taurine release to 72% of saline controls but had no significant effects on the other amino acids. Stimulation of PKC with phorbol 12-myristate, 13-acetate (10 microM) did not significantly change taurine, glutamate, glycine or phosphethanolamine release. The releases of aspartate and GABA were enhanced 2 to 3 fold. Phorbol 12,13-didecanoate (10 microM), another potent stimulator of PKC, significantly increased taurine release to 122% of DMSO controls. The releases of aspartate, glutamate and glycine were enhanced 2.5 to 3.5 fold. Similarly, stimulation of protein kinase A with forskolin (100 microM) significantly increased taurine, aspartate, and glycine release 1.5- to 2-fold compared to DMSO controls. In summary, phospholipases may play a minor role in volume regulation. These studies also support the hypothesis that protein kinases play a modulatory role in the RVD response. The results show that although RVD may play a role, additional mechanisms, including phospholipase activation, must be involved in the ischemia-evoked release of excitotoxic amino acids.