Volume regulation and the efflux of amino acids from cells in incubated slices of rat cerebral cortex. II. Dependence on Ca 2+ ions

Abstract

The efflux of γ-amino isobutyric acid (GABA) and l-glutamate from pre-loaded cells in rat cerebral cortical slices has been studied during interventions designed to affect the availability of intracellular Ca 2+ during hyposmotic swelling and membrane depolarization due to raised extracellular K +. Calmodulin-dependent acceleration of amino acid efflux in hyposmotic media, with cell swelling less than would be predicted on the basis of perfect osmometric behaviour (see Ref. [1]), was unaffected by Ca-ionophore in the presence of external Ca 2+ or by the omission of external Ca 2+, but was suppressed by pre-exposure of slices to thapsigargin (2 μM), which is reported to deplete cytosolic Ca 2+, and by TMB-8 (0.5 mM), which blocks release of Ca 2+ from internal stores. TMB-8 also led to significant cell swelling. The effects of TMB-8 were reversed by Ca-ionophore. Raised external K + (54 mM) led to accelerated amino acid efflux which required calmodulin activation and was blocked by (i) omission of external Ca 2+, (ii) the voltage-sensitive Ca 2+ channel blocker nifedipine (1 μM), (iii) the anion transport inhibitor DIDS (25 μM for GABA, 100 μM for l-glutamate, see Ref. [1]), and (iv) the -SH group acetylator N-ethylmaleimide. TMB-8 was without effect in high K + media. These results suggest that while enhanced amino acids efflux probably occurs through the same population of Ca/calmodulin-dependent, DIDS-sensitive pathways following hyposmotic shock or membrane depolarization, the source of Ca 2+ ions required for the activation of these pathways may depend upon which of these acceleratory stimuli is applied.