Characterization of functional endothelin receptors in the porcine myometrium

To characterize the endothelin (ET) receptor that mediates the contraction induced by ET-1 in the porcine myometrium, we carried out a contraction study, radioligand binding study and molecular study (reverse transcription polymerase chain reaction) for detection of ET receptor-coding genes (mRNA). ET-1 (1 nM-1 microM) caused a tetrodotoxin-insensitive contraction in both longitudinal and circular muscles, but the longitudinal muscle was more sensitive to ET-1 than was the circular muscle. On the other hand, ET-3 and sarafotoxin S6c were less effective to cause a contractile response. The contraction induced by ET-1 was markedly inhibited by BQ123 and FR139317, but BQ788 only slightly inhibited the response induced by ET-1. The radioligand binding study indicated the presence of a single class of 125I-ET-1 binding sites with the same Kd value in both muscle layers. However, Bmax in the longitudinal muscle (3252 fmol/mg protein) was significantly higher than that in the circular muscle (1883 fmol/mg protein). ET-1 and FR139317 inhibited the specific 125I-ET-1 binding completely, but ET-3, sarafotoxin S6c and BQ3020 only slightly inhibited the specific binding (inhibition, 10-20%), suggesting that ET(A) is the dominant ET receptor subtype in the porcine myometrium. The results of the molecular study indicated the expression of both ET(A) and ET(B) receptor-coding genes in the porcine myometrium. In conclusion, ET-1 causes contraction of the porcine myometrium through activation of the ET(A) receptor present on smooth muscle cells. There is a marked muscle layer-related difference (longitudinal muscle > circular muscle) in the ET-1-induced contraction and the ET(A) receptor concentration.