Intracellular sodium concentration in cultured cerebellar granule cells challenged with glutamate.
We monitored simultaneously the changes in the intracellular sodium concentration ([Na+]i) and intracellular calcium concentration ([Ca2+]i) in individual neurons from primary cultures of cerebellar granule cells loaded with sodium-binding benzofuran isophthalate and fluo-3. An application of glutamate (50 microM) in Mg(2+)-free medium containing 10 microM glycine evoked [Na+]i and [Ca2+]i increases that exceeded 60 mM and 1 microM, respectively. The kinetics of [Na+]i and [Ca2+]i decreases after the termination of the glutamate pulse were different. [Na+]i failed to decrease immediately after glutamate withdrawal and the delay in the onset of [Na+]i decrease after the glutamate pulse termination was proportional to the glutamate dose, the glutamate pulse duration, and the extent of [Ca2+]i elevation elicited by glutamate. The kinetics of [Ca2+]i decrease were biphasic, with the first phase occurring immediately after glutamate withdrawal and the second phase being correlated in time with a [Na+]i value lower than 15-20 mM. These results were interpreted to indicate that the glutamate-evoked calcium influx may lead to sodium homeostasis destabilization. The delay in the restoration of the sodium gradient may in turn prolong the neuronal exposure to toxic [Ca2+]i values, due to the decrease in the efficiency of the Na+/Ca2+ exchanger to extrude calcium. The glutamate effects on [Na+]i and [Ca2+]i were potentiated by glycine. Glycine (10 microM) added alone also evoked [Na+]i and [Ca2+]i increases; this effect was inhibited by a competitive inhibitor of the N-methyl-D-aspartate receptor, 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid, indicating an involvement of endogenous glutamate.
Kiedrowski
L
L
Fidia-Georgetown Institute for the Neurosciences, Georgetown University, Washington D.C. 20007.
Wroblewski
J T
JT
Costa
E
E
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D002531:Cerebellum / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D005971:Glutamates / Q000494:pharmacology*; D018698:Glutamic Acid; D005998:Glycine / Q000494:pharmacology; D006706:Homeostasis; D007700:Kinetics; D010879:Piperazines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors; D012964:Sodium / Q000378:metabolism*
D005971:Glutamates; D010879:Piperazines; D016194:Receptors, N-Methyl-D-Aspartate; D018698:Glutamic Acid; C050592:3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid; D012964:Sodium; D002118:Calcium; D005998:Glycine