Effects of pH on the uncoupled, coupled and pre-steady-state currents at the amino acid transporter KAAT1 expressed in Xenopus oocytes.

The effects of pH on the different kinds of currents occurring at the lepidopteran amino acid cotransporter KAAT1 have been examined using heterologous expression in Xenopus oocytes and voltage clamp. Acidic pH (5.5-4.5) caused a slight depression of the uncoupled current and a complete inhibition of the coupled and transient currents, in the presence of either Na+ or K+, the two ions physiologically relevant to the transport process. Conversely, at alkaline pH (9) no statistically significant effects could be observed on uncoupled, coupled and transient currents compared to the effects at pH 7.6. These effects of pH indicate that operation of the transporter is maximal in the physiologically alkaline native environment. The dose-response curves for the inhibition of coupled and transient currents were similar, with respective pKa values of 6. 29 +/- 0.05 and 6.40 +/- 0.03 and respective Hill coefficient values (nH) of 0.93 +/- 0.07 and 1.08 +/- 0.08, suggesting that the two effects can be explained by a single proton binding to the same site in the transporter.