DIFFERENT EFFECTS OF THROMBIN RECEPTOR ACTIVATION ON ENDOTHELIUM AND SMOOTH-MUSCLE CELLS OF HUMAN CORONARY-BYPASS VESSELS - IMPLICATIONS FOR VENOUS BYPASS GRAFT FAILURE

Background Thrombin is implicated in coronany bypass graft disease; it cleaves its receptor's extracellular N-terminal domain and unmasks a new N-terminus as a tethered ligand. We studied the effects of thrombi n receptor activation in human internal mammary artery (IMA) and saphe nous vein (SV). Methods and Results To study the effects of thrombin r eceptor activation on vasomotion, isolated blood vessels were suspende d for isometric tension recording, and the effects on cell proliferati on were studied in cultured smooth muscle cells (SMCs) of IMA and SV. Thrombin receptor expression in IMA and SV was analyzed by reverse tra nscription polymerase chain reaction and immunohistology. Receptor fun ction was studied by analyzing the activation of mitogen-activated pro tein kinase (p42(MAPK)). In IMA thrombin evoked endothelium-dependent relaxations (65+/-5%) that were mimicked by thrombin receptor agonist peptide (TRAP) and reduced by the thrombin inhibitors recombinant (r-) hirudin and o-Phe-Pro-Arg-chloromethyl ketone (PPACK) (P<.05). In SV thrombin caused contractions (36/-5% of 100 mmol/L KCl) that were inhi bited by r-hirudin or PPACK (P<.05) but nut mimicked by TRAP. In SMCs thrombin induced more pronounced [H-3]thymidine incorporation (inhibit ed by r-hirudin or PPACK) in SV than IMA (P<.05), bur activation of p4 2(MAPK) was similar in both vessels. TRAP induced weaker activation of p42(MAPK) than thrombin and did not stimulate [H-3]thymidine incorpor ation in SMCs of SV or IMA. Immunohistology and RT-PCR demonstrated th at the endothelium and SMCs of IMA and SV express thrombin receptor. C onclusions Functional thrombin receptors are present on endothelium an d SMCs of IMA and SV. Endothelial thrombin receptors mediate relaxatio n in IMA but not SV. Thrombin causes much more pronounced contraction and proliferation in SMCs of SV than IMA independent of tethered recep tors, suggesting other thrombin receptors exist. These differences of thrombin receptor activation in IMA and SV may be important in the dev elopment of and therapy for graft disease.