Enhanced expression of contractile endothelin ETB receptors in rat coronary artery after organ culture

Endothelin-1 is a potent vasoconstrictor mediating its effects via two receptor subtypes, the endothelin type A (ETA) preferentially situated on smooth muscle cells, mediating vasoconstriction and endothelin type B (ETB) mainly located on endothelial cells, mediating vasodilatation. In cardiovascular disease and in organ culture in vitro, endothelin ETB receptors are up-regulated on smooth muscle cells. The objectives of the present study were to characterise the endothelin receptor-induced vasoconstriction and quantify the endothelin receptor mRNA levels and immunoreactivity in fresh and cultured rat coronary arteries. We demonstrate that endothelin-1 induces strong and equal concentration-dependent contractions in fresh and cultured segments from the left anterior descending coronary artery. Sarafotoxin 6c, an endothelin ETB receptor agonist, had negligible effect in fresh arteries but produced significant vasoconstriction after organ culture. The endothelin ETB receptor mRNA level and the receptor protein immunoreactivity were increased, whereas the level of endothelin ETA receptor mRNA was down-regulated but not its receptor protein immunoreactivity after organ culture. Pharmacological inhibition of endothelium-derived dilatory mediators did not influence endothelin ETA or ETB receptor-mediated vasoconstriction in fresh segments. In cultured arteries, inhibition of endothelial vasodilators potentiated the effect of sarafotoxin 6c. In conclusion, endothelin ETB receptor stimulation in cultured coronary arteries elicits vasoconstriction. This is likely not related to endothelial dysfunction with putative loss of its vasodilator components, but rather explained by the up-regulation of contractile endothelin ETB receptors on smooth muscle cells.