Using a microphysiometer with synchronized valve switching, we investigated real-time acid extrusion from CHO cells, in which human alpha-1a adrenoceptor (AR) is stably expressed, in response to noradrenaline (NA). The time course of the extracellular acidification rate after stimulation had two phases: in the first phase, it transiently reached a rate several times greater than the base rate with a peak at around 10 s, and in the second, it reached to 2 times the base rate and reached a plateau in 2 min. Both phases showed concentration-dependent increase of acidification rates in response to NA, but had distinct pEC
50 values: 5.6 for the transient phase and 7.2 for the steady phase. HOE642, an inhibitor of Na
+-H
+ exchanger (NHE) 1, inhibited the acid extrusion response in a concentration-dependent manner. HOE642 had high pIC
50 values (7.3) for inhibition of the transient phase response. In contrast, it revealed the presence of two components in the steady phase response: one had high pIC
50 values (8.2) and the other had low pIC
50 values (6.0). As Ca
2+ was depleted, the transient phase disappeared, while the steady phase was not affected. These results suggest that alpha-1a AR drives two acid extrusion sytems in CHO cells upon stimulation: one elicits the transient response that is largely mediated by a HOE642-sensitive and Ca
2+-dependent NHE, presumably NHE1, and the other induces the steady acid extrusion that is mediated by NHE1 and another NHE that has low sensitivity to HOE642.
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