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 stu dy, radioligand binding study and molecular study (reverse transcription po lymerase chain reaction) for detection of ET receptor-coding genes (mRNA). ET-1 (1 nM-1 mu M) caused a tetrodotoxin-insensitive contraction in both lo ngitudinal and circular muscles, but the longitudinal muscle was more sensi tive to ET-1 than was the circular muscle. On the other hand, ET-3 and sara fotoxin S6c were Less effective to cause a contractile response. The contra ction induced by ET-1 was markedly inhibited by BQ123 and FR139317, but BQ7 88 only slightly inhibited the response induced by ET-1. The radioligand bi nding study indicated the presence of a single class of T-125-ET-1 binding sites with the same K-d value in both muscle layers. However, Bmax in the l ongitudinal muscle (3252 fmol/mg protein) was significantly higher than tha t in the circular muscle (1883 fmol/mg protein). ET-1 and FR139317 inhibite d the specific I-125-ET-1 binding completely, but ET-3, sarafotoxin S6c and BQ3020 only slightly inhibited the specific binding (inhibition, 10-20%), suggesting that ETA is the dominant ET receptor subtype in the porcine myom etrium. The results of the molecular study indicated the expression of both ETA and ETB receptor-coding genes in the porcine myometrium. In conclusion , ET-1 causes contraction of the porcine myometrium through activation of t he ETA receptor present on smooth muscle cells. There is a marked muscle la yer-related difference (longitudinal muscle > circular muscle) in the ET-1i nduced contraction and the ETA receptor concentration. (C) 2000 Elsevier Sc ience Inc. All rights reserved.