Volume regulation and the efflux of amino acids from cells in incubated slices of rat cerebral cortex. II. Dependence on Ca2+ ions.
The efflux of gamma-aminoisobutyric 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 Ca2+ 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 Ca2+ or by the omission of external Ca2+, but was suppressed by pre-exposure of slices to thapsigargin (2 microM), which is reported to deplete cytosolic Ca2+, and by TMB-8 (0.5 mM), which blocks release of Ca2+ 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 Ca2+, (ii) the voltage-sensitive Ca2+ channel blocker nifedipine (1 microM), (iii) the anion transport inhibitor DIDS (25 microM for GABA, 100 microM 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 Ca2+ ions required for the activation of these pathways may depend upon which of these acceleratory stimuli is applied.
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Department of Cell Physiology and Pharmacology, University of Leicester, UK.
D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D048429:Cell Size; D002540:Cerebral Cortex / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D005033:Ethylmaleimide / Q000494:pharmacology; D005707:Gallic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D018698:Glutamic Acid / Q000494:pharmacology; D066298:In Vitro Techniques; D009543:Nifedipine / Q000494:pharmacology; D009994:Osmolar Concentration; D051381:Rats; D019284:Thapsigargin / Q000494:pharmacology; D014268:Trifluoperazine / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D000596:Amino Acids; D014268:Trifluoperazine; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; C006014:8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate; D005707:Gallic Acid; D019284:Thapsigargin; D009543:Nifedipine; D005033:Ethylmaleimide; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; D002118:Calcium