Expression of Thrombin Receptors in Human Atherosclerotic Coronary Arteries Leads to an Exaggerated Vasoconstrictory Response In Vitro

Alpha-thrombin can alter vascular tone by proteolytic cleavage of its cell-surface receptor, which exposes a tethered peptide sequence, Ser-Phe-Leu-Leu-Arg-Asn (SFLLRN) that activates the receptor. We investigated the effects of increasing severity of coronary atherosclerosis on SFLLRN-induced responses on 165 human coronary artery rings isolated fresh from 15 patients who underwent cardiac transplantation. In 40 coronary rings with minimal intimal proliferation, addition of 0.001-5 microM SFLLRN resulted in a dose- and endothelium-dependent relaxation reaching a maximum of -87.0 +/- 2.3% (mean +/- SEM) and median inhibitory concentration (IC50) of 0.1 microM. Increasing severity of atherosclerotic lesion, as determined by morphometric quantification of intimal thickening under light microscopy, resulted in graded decreases in both sensitivity and magnitude of the observed relaxation. The maximal relaxations in coronary arteries with mild and moderate intimal proliferation were -76.7 +/- 3.5% (mean +/- SEM of 41 rings) and -63.6 +/- 6.4% (mean +/- SEM of 22 rings), respectively. In the 21 coronary rings with severe intimal proliferation, no significant SFLLRN-induced relaxation was noted. Mechanical disruption of intimal endothelium abolished the SFLLRN-induced relaxation observed in the minimal to mild intimally thickened arteries, whereas in arteries with moderate and severe intimal thickening, a significant SFLLRN-induced contraction (19 +/- 10% and 43 +/- 7%, respectively) was observed. Similar endothelium-dependent relaxations in minimal atherosclerotic and endothelium-independent contraction in severe atherosclerotic coronary arteries were also observed with alpha-thrombin. These findings confirm a recent in situ hybridization and immunohistochemistry study reporting localization of cloned thrombin receptors only in endothelium of "normal appearing" human abdominal aortae and induced expression of thrombin receptors in intimal/medial regions of the atherosclerotic vessels and further demonstrate that similar expression of thrombin receptors in human atherosclerotic coronary arteries leads to an unmasking of a marked vasoconstrictory response.