Sensitization of the endothelin-A receptor (ETA) occurs during HSC transdifferentiation, but the underlying mechanisms remained unclear. Sensitization of ETA was studied in quiescent and activated hepatic stellate cells (HSC) at the levels of receptor phosphorylation, localization, endothelin (ET)-1-induced Ca2+ signals, and cell contraction. The endothelin-1 (ET-1) concentrations required to obtain an ETA-mediated Ca2+ signal in 50% of HSC cultured for 1 to 2 or 10 days were approximately 1.2 and 0.012 nmol/L, respectively. This transdifferentiation-dependent sensitization of ETA was accompanied by receptor translocation to the plasma membrane. Cyclic AMP rapidly desensitized ETA in activated HSC and shifted their ET-1 responsiveness from picomolar to nanomolar concentrations with respect to Ca2+ signals and HSC contraction. ETA desensitization also occurred in response to prostaglandin E2, adenosine, or ETB stimulation. Desensitization by cAMP in activated HSC was accompanied by an increased Ser/Thr phosphorylation of ETA and their rapid internalization. Quiescent HSC exhibited Ser/Thr phosphorylation of the ETA protein, which was not affected by cAMP. In conclusion, the ETA response in HSC is regulated by protein kinase A (PKA)-dependent receptor phosphorylation and internalization. This may explain the transdifferentiation-dependent sensitization of HSC towards ET-1 and its reversal by cAMP and ETB activation.