ET_A receptor antagonists inhibit intimal smooth muscle cell proliferation in human vessels

We have determined the ability of the endothelin (ET)_A receptor antagonist, PD156707 (CI 1020), to inhibit intimal proliferation in human saphenous veins maintained in organ culture. After 28 days in culture, veins exposed to 1µM PD156707 exhibited a significant reduction in intima to intima-plus-media ratio (I:I+M ratio) (0.14±0.02, n = 15) and an increase in lumen area (3.1±0.8mm²) compared with veins cultured without the antagonist (I:I+M, 0.29±0.02; lumen area, 2.5±0.7mm²; n = 23) but were not significantly different from pre-cultured controls (I:I+M, 0.15±0.02; lumen area, 4.4±1.2mm²; n = 17) (Dunn's test for non-parametric multiple comparisons: α<0.05). In organ bath experiments, ET-1 and 5-hydroxytryptamine constricted pre-cultured control vessels with pD₂ values (where pD₂ is defined as the negative logarithm of the molar EC₅₀ value of an agonist) of 8.9±0.4 and 7.0±0.4 (n = 3) and E_max (efficacy) values of 86±3% and 71±13% (compared with constriction induced by KCl) respectively. There was no difference in the responsiveness of veins cultured for 14 days to either agonist, indicating that the vessels maintained in organ culture remain viable. Crucially, vein segments cultured with 1µM PD156707 (a concentration that antagonized ET-1 responses in pre-cultured control vessels) contracted to ET-1 with a potency comparable to that obtained in vessels cultured in the absence of the antagonist (pD₂ = 8.9±0.4 and 8.0±0.6 respectively, n = 3) confirming that PD156707 was not toxic to the tissue at the concentration used. In conclusion we have shown that the ET_A-selective antagonist, PD156707, completely blocked intimal hyperplasia in human saphenous veins in organ culture, suggesting that ET_A antagonists may be beneficial in preventing or delaying saphenous vein graft disease in patients receiving bypass grafts for coronary artery disease.