Novel role of the Ca2+-ATPase in NMDA-induced intracellular acidification

The mechanism involved in N-methyl-d-glucamine (NMDA)-induced Ca2+-dependent intracellular acidosis is not clear. In this study, we investigated in detail several possible mechanisms using cultured rat cerebellar granule cells and microfluorometry [fura 2-AM or 2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-AM]. When 100 μM NMDA or 40 mM KCl was added, a marked increase in the intracellular Ca2+ concentration ([Ca2+]i) and a decrease in the intracellular pH were seen. Acidosis was completely prevented by the use of Ca2+-free medium or 1,2-bis(2-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid-AM, suggesting that it resulted from an influx of extracellular Ca2+. The following four mechanisms that could conceivably have been involved were excluded: 1) Ca2+ displacement of intracellular H+ from common binding sites; 2) activation of an acid loader or inhibition of acid extruders; 3) overproduction of CO2 or lactate; and 4) collapse of the mitochondrial membrane potential due to Ca2+uptake, resulting in inhibition of cytosolic H+ uptake. However, NMDA/KCl-induced acidosis was largely prevented by glycolytic inhibitors (iodoacetate or deoxyglucose in glucose-free medium) or by inhibitors of the Ca2+-ATPase (i.e., Ca2+/H+exchanger), including La3+, orthovanadate, eosin B, or an extracellular pH of 8.5. Our results therefore suggest that Ca2+-ATPase is involved in NMDA-induced intracellular acidosis in granule cells. We also provide new evidence that NMDA-evoked intracellular acidosis probably serves as a negative feedback signal, probably with the acidification itself inhibiting the NMDA-induced [Ca2+]iincrease.