Efflux of 45calcium from cultured primary astrocytes: Effects of glutamate receptor agonists and antagonists

Abstract

The effects of high-K, glutamate and glutamate receptor agonists on the efflux of Ca 2+ were studied in cultured primary astrocytes, prepared from the brains of newborn rats. An increase in efflux of 45Ca 2+ produced by a large extracellular concentration of K + was effectively inhibited by verapamil, a blocker of voltage-gated Ca 2+ channels, suggesting that these cells have functional voltage-sensitive Ca 2+ channels. Glutamate and its agonists kainate, quisqualate and N- methyl- d-aspartate (NMDA) stimulated the efflux of preloaded 45Ca 2+, in a dose-dependent manner. The most effective agonist was quisqualate followed by glutamate, whereas kainate and NMDA were less potent. In the Mg-free medium, the response to NMDA was significantly increased. The quisqualate receptor agonist (RS)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) also stimulated the efflux, about equally effectively as quisqualate. The glutamate-, quisqualate- and kainate-induced efflux of 45Ca 2+ was significantly inhibited by l-glutamatediethylester (GDEE) and that of NMDA by dl-2-amino-5-phosphonopentanoic acid (AP5). The kainate-induced efflux was totally inhibited by verapamil, but that of glutamate only partially. No effect of verapamil was observed on the quisqualate-induced efflux of 45Ca 2+. The results imply that, in cultured astrocytes, Ca 2+ fluxes induced by glutamate agonists, occur partially through voltage-dependent Ca 2+ channels. The extensive release of 45Ca 2+ caused by quisqualate was mainly due to a release from internal stores.