Gastrointestinal functions of proteinase-activated receptors

Proteinase-activated receptors (PARs) are a family of G-protein-coupled-seven-trans-membrane-domain receptors, consisting of four family members. PARs, especially PAR-1, a thrombin receptor, and PAR-2, a receptor for trypsin, tryptase and coagulation factors VIIa and Xa, are abundantly distributed throughout the gastrointestinal tract. PAR-2, but not other PARs, induces salivary and pancreatic exocrine secretion. Both PAR-2 and PAR-1 play protective roles in the gastric mucosa, modulating a variety of gastric functions. However, the mechanisms underlying the mucosal protection caused by PAR-2 and PAR-1 are entirely different. In the intestinal mucosa, PAR-2 appears to play a dual role, being pro- and anti-inflammatory. PAR-1, PAR-2 and also PAR-4 modulate the motility of the smooth muscle in the gastrointestinal tract including the esophageal muscularis mucosae, producing contraction and/or relaxation upon activation. Thus, PARs, especially PAR-1 and PAR-2, play extensive roles in modulating the gastrointestinal functions.