Uptake into glial cells helps to terminate glutamate's neurotransmitter action and to keep its extracellular concentration, [Glu]o, below neurotoxic levels. The accumulative power of the uptake carrier stems from its transport of inorganic ions such as sodium (into the cell) and potassium (out of the cell). There is controversy over whether the carrier also transports a proton (or pH-changing anion). Here we show that the carrier generates an alkalinization outside and an acidification inside glial cells, and transports anions out of the cells, suggesting that there is a carrier cycle in which two Na+ accompany each glutamate anion into the cell, while one K+ and one OH- (or HCO3-) are transported out. This stoichiometry predicts a minimum [Glu]o of 0.6 microM normally (tonically activating presynaptic autoreceptors and post-synaptic NMDA receptors), and 370 microM during brain anoxia (high enough to kill neurons). Transport of OH-/HCO3- on the uptake carrier generates significant pH changes, and may provide a mechanism for neuron-glial interaction.

A combination of microspectrofluorimetry and single cell voltage-clamp was used to examine the response to ATP of cultured neurons from rat dorsal root ganglia. ATP activated an inward current and a rise in internal calcium concentration that was dependent on the external calcium concentration and on the magnitude of the ATP-induced current response. The response was not affected by prerelease of internal calcium stores with caffeine. The rise in internal calcium was increased at hyperpolarized membrane potentials as the calcium driving force was increased. These results demonstrate that the ATP-gated channels in these cells can admit a significant amount of calcium in a physiological calcium gradient. This alternative calcium entry pathway could provide an internal calcium signal that is spatially distinct to that generated by voltage-gated calcium entry.

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SUMMARY1. The effect of sulphur-containing analogues of glutamate and aspartate on the membrane current of glial cells was studied by whole-cell clamping Muller cells isolated from the salamander retina.2. L-Cysteic acid (CA), L-cysteinesulphinic acid (CSA), L-homocysteic acid (HCA), L-homocysteinesulphinic acid (HCSA) and S-sulpho-L-cysteine (SC) all evoked an inward membrane current that was large at negative potentials, and was smaller (but did not reverse) at more positive potentials up to + 30 mV.3. Removal of external sodium ions abolished the amino acid-evoked currents. Whole-cell clamping with pipettes containing no potassium led to a rapid suppression of the currents, that did not occur when potassium was included in the pipette.4. The dependence of the currents on sulphur-containing amino acid concentration obeyed first-order Michaelis-Menten kinetics. The current evoked by co-application of L-glutamate and a sulphur-containing analogue was smaller than the sum of the currents produced by glutamate alone and by the sulphur analogue alone.5. These data are consistent with the sulphur amino acid-evoked current being caused by uptake on the electrogenic glutamate uptake carrier, which co-transports an excess of Na+ ions into the cell, and counter-transports one K+ ion out of the cell.

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