Endothelin receptors in human coronary artery and aorta

1 ETA and ETB-selective and non-selective ligands were used to define the endothelin receptors in the media (vascular smooth muscle layer) of human aorta and coronary artery. Saturation experiments with iodinated endothelin-1 (ET-1), endothelin-2 and sarafotoxin 6b (S6b) identified high affinity binding sites in aorta (K-D [I-125]-ET-1 0.33+/-0.02 nM (n=9), K-D [I-125]-ET-2 1.04+/-0.23 nM (n=5), K-D [I-125]-S6b 0.15+/-0.01 nM (n=9 +/-s.e.mean)) and coronary artery (K-D [I-125)-ET-1 0.43+/-0.10 nM, K-D [I-125]-ET-2 0.71+/-0.17 nM, K-D [I-125]-S6b 0.27+/-0.03 nM (n=3 +/-s.e.mean)). Hill coefficients (n(H)) approached unity in each case. 2 No specific binding was detectable with [I-125]-ET-3 (4 pM-4 nM) in aorta. Unlabelled ET-3 competed monophasically with [I-125]-ET-1 in aorta (K-D, 8.21+/-1.62 nM, compared to unlabelled ET-1 KD, 0.60+/-0.20 nM) (n=3 +/-s.e.mean). In coronary artery, the K-D and B-max values calculated from [I-125]-ET-3 saturation experiments were 2.13+/-1.39 nM and 20.6+/-12.9 fmol mg(-1) protein, respectively (n=3 +/-s.e.mean). 3 ETA antagonists competed monophasically for [I-125]-ET-1 (100 pM) binding sites with nanomolar or subnanomolar affinity in the aorta (K-D BQ123, 0.47+/-0.13 nM; K-D FR139317, 0.40+/-0.10 nM; K-D PD151242, 2.09+/-0.48 nM) and coronary artery (KD FR139317, 0.41+/-0.13 nM; K-D PD151242, 3.60+/-0.74 nM) (n=3 +/-s.e.mean). However, two site fits were preferred on analysis of competition experiments with ETB-selective agonists versus [I-125]-ET-1 in coronary artery (BQ3020: KDETA 0.96+/-0.14 mu M, K-D ETB 1.34+/-1.08 nM and sarafotoxin 6c: K-D ETA 1.15+/-0.14 mu M, K-D ETB 1.77+/-0.72 nM) (n=3 +/-s.e.mean). The selectivity of the agonists for ETB receptors (700 fold) was lower than reported in other species. 4 Sarafotoxin 6b (2 pM-2 mu M) completely inhibited [I-125]-ET-1 (100 pM) binding in aorta (KD 1.36+/-0.22 nM) (n=3 +/-s.e.mean). The non-peptide compounds Ro462005 and bosentan, competed with [I-125]-ET-1 binding in coronary artery with KD values of 0.19+/-0.04 mu M and 2.94+/-0.95 nM, respectively (n=3 +/-s.e.mean). 5 Inhibition of [I-125]-ET-2 and [I-125]-S6b binding by FR139317 was similar to the inhibition of [I-125]-ET-1 binding in both arteries, being monophasic with K,values in the same range. 6 ETA receptors in coronary artery media were detected by [I-125]-PD151242 (K-D 0.23+/-0.04 nM, B-max 10.1+/-1.2 fmol mg(-1) protein) (n=3 +/-s.e.mean). [I-125]-BQ3020, an ETB-selective radioligand, indicated the presence of a smaller population of ETB receptors in this tissue (K-D 0.60+/-0.31 nM, B-max 4.5+/-2.1 fmol mg(-1) protein) (n=3 +/-s.e.mean). 7 Autoradiography with [I-125]-PD151242 and [I-125]-BQ3020 confirmed the predominance of ET, receptors in the media of both arteries. 8 The results of this study indicate that ETA receptors predominate in the vascular smooth muscle of human cardiac arteries, with a small and variable population of ETB receptors detectable in the coronary artery.