Unproductive cleavage and the inactivation of protease‐activated receptor‐1 by trypsin in vascular endothelial cells

Using fura‐2 fluorometry of [Ca2+]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. Once stimulated with thrombin, the subsequent application of trypsin induced a [Ca2+]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. The response to PAR1‐AP (SFLLRNP) was significantly (P