Endothelin receptor expression and pharmacology in human saphenous vein graft

Abstract

1. We have investigated the expression and pharmacology of endothelin (ET) receptors in human aortocoronary saphenous vein grafts. 2. Subtype-selective ligands were used to autoradiographically identify ET(A) ([125I]-PD151242) and ET(B)([125I]-BQ3020) receptors. In graft saphenous vein ETA receptors predominated in the media, with few ET(B) receptors identified. Neither subtype was detected in the thickened neointima. 3. The ratio of medial ET(A):ET(B) receptors was 75%: 25% in both graft and control saphenous vein. 4. ET-1 contracted control (EC50 2.9 nM) and graft (EC50 4.5 nM) saphenous vein more potently than diseased coronary artery (EC50 25.5 nM). 5. In all three blood vessels ET-1 was 100 times more potent than ET-3 and three times more potent than sarafotoxin 6b (S6b). Little or no response was obtained in any vessel with the ET(B) agonist sarafotoxin 6c (S6c). 6. The ET(A) antagonist PD156707 (100 nM) blocked ET-1 responses in all three vessels with pKb values of approximately 8.0. 7. For individual graft veins the EC50 value for ET-1 and 'age' of graft in years showed a significant negative correlation. 8. In conclusion there is no alteration in ET receptor expression in the media of saphenous veins grafted into the coronary circulation compared to control veins. ETA receptors predominantly mediate the vasoconstrictor response to ET-1 in graft vein, with no apparent up-regulation of ET(B) receptors. The sensitivity of the graft vein to ET-1 increased with graft 'age', suggesting that these vessels may be particularly vulnerable to the increased plasma ET levels that are detected in patients with cardiovascular disease.

Figure 1

Colour-coded images of the distribution of ET receptors in transverse sections of two adjacent retrieved aortocoronary saphenous vein grafts. (A) Localization of specific ET_A binding with [¹²⁵I]-PD151242 (0.1 nM) and (B) specific ET_B binding with [¹²⁵I]-BQ3020 (0.3 nM). These images were computer generated by digitally subtracting the autoradiographic image of the non-specific binding, defined by inclusion of the respective unlabelled peptide (1 μM), from the total binding image for each radioligand. ¹²⁵I standards were co-exposed with the tissue sections and receptor density was colour-coded by interpolation from the resulting standards curve. (C) A section stained with haematoxylin and eosin. The graft on the left has a thickened intimal layer whilst the graft on the right is completely occluded.

Figure 2

Cumulative concentration-response curves to (a) ET-1 and (b) 5-HT in saphenous vein graft (SVgraft), control saphenous vein (SV) and atherosclerotic coronary artery (CA). Agonist responses are expressed as a percentage of the maximal contraction to KCl (50 mM) and are the means±s.e.mean (n=3–15).

Figure 3

Cumulative concentration-response curves to ET receptor agonists in (a) saphenous vein graft, (b) control saphenous vein and (c) atherosclerotic coronary artery. Agonist responses are expressed as a percentage of the maximal contraction to KCl (50 mM) and are the means±s.e.mean (n=3–15).

Figure 4

Antagonism of ET-1 by 100 nM PD156707 in (a) saphenous vein graft, (b) control saphenous vein and (c) control coronary artery. ET-1 responses are expressed as a percentage of the response to KCl (50 mM) and are the means±s.e.mean (n=5–7).

Figure 5

Influence of graft age (years) on sensitivity of grafts from individual patients (n=8) to ET-1 (EC₅₀ nM). A significant correlation (Pearson; r=−0.82; P<0.02) was obtained.