Evidence for the presence of functional protease-activated receptor-4 (PAR4) in the rat colon

Background and aims: Protease activated receptors (PARs) have been postulated to play a role during intestinal inflammation. The presence and role played by PAR4 in gastrointestinal functions have not been fully clarified. The aims of this study were: (i) to examine expression of PAR 4 in rat proximal colon; (ii) to determine the mechanical effects induced by PAR4 activation in longitudinal muscle; and (iii) to characterise the underlying mechanisms. Methods: PAR4 expression was determined by reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. Mechanical activity was recorded as changes in isometric tension. Results: A PCR product corresponding to the predicted size of the PAR4 signal was amplified from RNA prepared from the colon of rats, showing the presence of PAR4 in those tissues. Immunohistochemistry revealed that PAR4 protein was expressed on epithelial surfaces and submucosa. PAR4 activating peptides, GYPGKF-NH2 and AYPGKG-NH2, produced concentration dependent contractile effects on longitudinal muscle. Tetrodotoxin (TTX) or atropine significantly reduced the contractile responses to AYPGKG-NH2, and atropine after TTX did not cause any further reduction. NK1 receptor antagonist, SR140333, or NK2 receptor antagonist, SR48968, alone or in combination, produced a reduction in PAR4 induced contractile effect, and when coadministered with TTX abolished it. Capsaicin markedly reduced the contractions evoked by AYPGKG-NH2. Conclusions: The present results suggest that PAR4 is functionally expressed in rat colon and its activation induces contraction of the longitudinal muscle both through TTX sensitive release of acetylcholine and release of tachykinins, probably from sensory nerves. These actions may contribute to motility disturbances during intestinal trauma and inflammation.