Sensitization of the endothelin-A receptor (ET(A)) occurs during HSC transdifferentiation, but the underlying mechanisms remained unclear. Sensitization of ET(A) was studied in quiescent and activated hepatic stellate cells (HSC) at the levels of receptor phosphorylation, localization, endothelin (ET)-1-induced Ca(2+) signals, and cell contraction. The endothelin-1 (ET-1) concentrations required to obtain an ET(A)-mediated Ca(2+) 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 ET(A) was accompanied by receptor translocation to the plasma membrane. Cyclic AMP rapidly desensitized ET(A) in activated HSC and shifted their ET-1 responsiveness from picomolar to nanomolar concentrations with respect to Ca(2+) signals and HSC contraction. ET(A) desensitization also occurred in response to prostaglandin E(2), adenosine, or ET(B) stimulation. Desensitization by cAMP in activated HSC was accompanied by an increased Ser/Thr phosphorylation of ET(A) and their rapid internalization. Quiescent HSC exhibited Ser/Thr phosphorylation of the ET(A) protein, which was not affected by cAMP. In conclusion, the ET(A) 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 ET(B) activation.