1. Using fura-2 fluorometry of [Ca 2+] i in response to thrombin, trypsin and protease-activated receptor activating peptides (PAR-APs), we determined whether trypsin cleaves protease-activated receptor 1 (PAR1 ) and activates it in the endothelial cells of the porcine aortic valves and human umbilical vein. 2. Once stimulated with thrombin, the subsequent application of trypsin induced a [Ca 2+] i elevation similar to that obtained without the preceding stimulation with thrombin in the valvular endothelial cells. However, the preceding stimulation with trypsin abolished the subsequent response to thrombin, but not to bradykinin or substance P. 3. The response to PAR1-AP (SFLLRNP) was significantly (P < 0.05) reduced by the preceding stimulation with thrombin and PAR1-AP in the valvular endothelial cells, while, importantly, it remained unaffected by the preceding stimulation with either trypsin or PAR2-AP (SLIGRL). The response to PAR2-AP was reduced by the preceding stimulation with trypsin and PAP2-AP. PAR1-AP attenuated the subsequent responses not only to thrombin and PAR1-AP but also to trypsin and PAR2-AP, while PAR2-AP specifically attenuated the subsequent responses to trypsin and PAR2-AP. 4. In human umbilical vein endothelial cells, a higher affinity PAR1-AP (haPAR1-AP) (Ala-pF-Arg-Cha-HArg-Tyr-NH 2) specifically attenuated the responses to thrombin but not trypsin. On the other hand, the response to haPAR1-AP was significantly (P < 0.05) attenuated by the preceding stimulation with thrombin but not trypsin. 5. In conclusion, trypsin cleaved PAR1 but did not activate it in the endothelial cells. Moreover, the trypsin-cleaved PAR1 was no longer responsive to thrombin.
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