Summary
Serine proteinases are involved in several physiological processes and elicit profound
cellular effects in a variety of tissues. Besides the thrombin receptor a second receptor,
activated by trypsin, the proteinase-activated receptor 2 (PAR-2), was cloned and
characterized. Both enzymes generate a new extracellular N-terminus by limited proteolytic
cleavage which functions as tethered ligand to activate the receptor. Synthetic peptides
corresponding to the sequences of the newly generated N-terminus are able to mimic
the effects of the enzymes.
In porcine pulmonary arteries trypsin and the receptor-derived peptide SLIGRL elicited
an endothelium-dependent transient relaxation of PGF2α-precontracted vessels. The EC50 values for trypsin and SLIGRL amounted to 1.1 ± 0.2 nM and 5.4 ± 0.6¼M, respectively.
Trypsin and SLIGRL caused a homologous desensitization but thrombin and the thrombin
receptor-activating peptide SFLLRN were still able to elicit pronounced relaxant effects.
The trypsin- and SLIGRL-induced relaxant responses were markedly diminished after
blockade of the nitric oxide synthesis by NG-nitro-L-arginine methyl ester (200 ¼M) and were absent in endothelium-denuded vessels.
Indomethacin and hirudin did not influence the relaxant effects. The effect of trypsin
but not that of SLIGRL was blocked by the proteinase inhibitor aprotinin suggesting
that only proteolytically active trypsin activates the receptor. Benzamidine derivatives
of the 3-amidinophenylalanine type with different affinity for trypsin and thrombin
inhibited the vascular effects of trypsin (IC50 0.007-0.7 ¼M) correlating with its antitrypsin activity.
The data suggest that the vascular effects of trypsin and SLIGRL are mediated through
activation of PAR-2 which differs from the thrombin receptor.
