An ATP-driven primary transport system operative for L-glutamate or L-aspartate in Streptococcus mutans is, through the entire pH range from 5.5 to 8.5, specifically stimulated by extracellular potassium ions. The stimulation by potassium ions observed in the low pH range between 5.5 and 7 has been interpreted to be due to potassium ion-dependent regulation of the intracellular pH (the first mechanism). In the high pH range from 7 to 8.5, on the other hand, the present study demonstrates that potassium stimulation is essentially not associated with such intracellular pH regulation. This conclusion is based on our observation that potassium stimulation in the high pH range is insensitive to a proton conductor, carbonyl cyanide-p-trifluoromethoxy-phenyl-hydrazone. Since none of the other monovalent cations, including sodium, rubidium, ammonium, and Tris ions, could replace potassium ions in significantly stimulating glutamate transport, it is most likely that the influx of potassium ions specifically cancels the membrane potential derived by movement of glutamate with the net negative charges across a membrane and thus facilitates transport (the second mechanism). The second mechanism appears to be operative even in a low pH range, in addition to the first mechanism.
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