Modulation by protease-activated receptors of the rat duodenal motility in vitro:: possible mechanisms underlying the evoked contraction and relaxation

1 The present study examined effects of agonist enzymes and receptor-activating peptides for protease-activated receptors (PARs) on duodenal motility in the rat, and also investigated possible mechanisms underlying the evoked responses. 2 Thrombin at 0.03-0.1 mu M and the PAR-1-activating peptide SFLLR-NH2 at 3-100 mu M or TFLLR-NH2 at 10-50 mu M produced a dual action, relaxation followed by contraction of the duodenal longitudinal muscle. The PAR-2-activating peptide SLIGRL-NH2 at 10-100 mM elicited only small contraction. Trypsin at 0.08 mu M induced small contraction, or relaxation followed by contraction, depending on preparations. The PAR-4-activating peptide GYPGKF-NH2 at 1000 mu M exhibited no effect. 3 The contractile responses of the duodenal strips to TFLLR-NH2 and to SLIGRL-NH2 were partially attenuated by the L-type calcium channel blocker nifedipine (1 mu M), the protein kinase C inhibitor GF109203X (1 mu M) and the tyrosine kinase inhibitor genistein (15 mu M), but were resistant to indomethacin (3 mu M) and tetrodotoxin (1-10 mu M). 4 The relaxation of the preparations exerted by TFLLR-NH2 was unaffected by indomethacin (3 mu M), propranolol (5 mu M), N-G-nitro-L-arginine methyl ester (100 mu M) and tetrodotoxin (1-10 mu M). This relaxation was resistant to either GF109203X (1 mu M) or genistein (15 mu M), but was, remarkably, attenuated by combined application of these two kinase inhibitors. 5 Apamin (0.1 mu M), an inhibitor of calcium-activated, small-conductance potassium channels, but not charybdotoxin (0.1 mu M), completely abolished the PAR-1-mediated duodenal relaxation, and significantly enhanced the PAR-1-mediated contraction. 6 These findings demonstrate that PAR-I plays a dual role, suppression and facilitation of smooth muscle motility in the rat duodenum, while PAR-2 plays a minor excitatory role in the muscle, and that PAR-4 is not involved in the duodenal tension modulation. The results also suggest that the contractile responses to PAR-1 and PAR-2 activation are mediated, in part, by activation of L-type calcium channels, protein kinase C and tyrosine kinase, and that the relaxation response to PAR-I activation occurs via activation of apamin-sensitive, but charybdotoxin-insensitive, potassium channels, in which both protein kinase C and tyrosine kinase might be involved synergistically.