This study examines the influence of receptor expression level on signaling pathways activated via endothelin type A receptor (ET
AR) expressed in Chinese hamster ovary cells at 32,100 (ET
AR-high-CHO) and 893 (ET
AR-low-CHO) fmolmg protein
−1. Endothelin-1 (ET-1) elicited a sustained increase in intracellular Ca
2+ concentration ([Ca
2+]
i), which was dependent on G
q/11 protein, phospholipase C (PLC), Na
+/H
+ exchanger (NHE), and p38 mitogen–activated protein kinase (p38MAPK) in ET
AR-high-CHO, whereas the sustained [Ca
2+]
i increase was negligible in ET
AR-low-CHO. Functional study with Cytosensor
TM microphysiometer showed that ET-1 evoked an NHE1-mediated increase in extracellular acidification rate (ECAR) in ET
AR-high-CHO and ET
AR-low-CHO. In ET
AR-high-CHO, the ECAR response at 30 min after ET-1 stimulation was insensitive to G
q/11 and PLC inhibitors, but sensitive to the p38MAPK inhibitor. In ET
AR-low-CHO, the ECAR response at 30 min was sensitive to these inhibitors. Western blot analysis demonstrated that ET-1–induced p38MAPK phosphorylation in ET
AR-low-CHO but not in ET
AR-high-CHO was mediated via G
q/11 and PLC. The G
q/11/PLC-independent p38MAPK phosphorylation in ET
AR-high-CHO was suppressed by expression of the C terminus of G
α12 protein to disrupt receptor-G
12 protein coupling. These results provide evidence for multiple signaling pathways of ET
AR that were activated via at least the G
q/11/PLC/NHE, G
12/p38MAPK/NHE, and G
q/11/PLC/p38MAPK/NHE cascades in an expression level–dependent manner.
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