Distinct roles of second and third intracellular loops of human endothelin receptors in the selective activation of G alpha s/G alpha i.

Endothelin-1 (ET-1) stimulated cAMP formation in Chinese hamster ovary cells stably expressing human wild-type ET(A) (CHO/hET(A) cells) and inhibited the formation in cells expressing human wild-type ETB (CHO/hETB cells), suggesting a selective coupling of hET(A) and hETB with G alpha s and G alpha i, respectively. To find out the receptor domain(s) that determined the selective coupling, a series of chimeric receptors between hET(A) and hETB were expressed on CHO cells and the effect of ET-1 on cAMP formation in each cell line was tested. hET(A) with the replacement of the second and/or third intracellular loop (ICLII and/or III) to the corresponding region(s) of hETB failed to transmit the stimulatory effect of ET-1. hETB with the replacement of ICLIII to the corresponding region of hET(A) failed to transmit the inhibitory effect of ET-1. A chimeric receptor with ICLII of hETB and with ICLIII of hET(A) failed to transmit either effect. These results indicated the roles of ICLII and III of hETR as major determinants of the selective coupling of hET(A)/hETB with G alpha s/G alpha i, respectively.