Protease-activated receptor (PAR)-1 and PAR-2 are expressed on gastrointestinal smooth muscle, but knowledge of their functionality is limited. The aim of this study was to determine if PAR-1 and PAR-2 mediate gastrointestinal smooth muscle relaxation and to clarify the underlying mechanisms.

Responses to PAR activation using the serine proteases thrombin and trypsin and the peptide agonists for PAR-1 and PAR-2, SFLLRN-NH₂ and SLIGRL-NH₂, respectively, were investigated in submaximally contracted longitudinal strips of mouse gastric fundus and guinea pig taenia coli.

In mouse gastric fundus, both thrombin and trypsin caused relaxations followed by contractions. SFLLRN-NH₂ and SLIGRL-NH₂ caused similar biphasic responses, the relaxation components of which were eliminated by apamin or ryanodine. For SFLLRN-NH₂, apamin and ryanodine revealed contractions. Nifedipine inhibited both relaxations and contractions to each peptide. In guinea-pig taenia coli, thrombin but not trypsin caused relaxation, whereas SFLLRN-NH₂ and SLIGRL-NH₂ caused concentration-dependent relaxations that were eliminated by apamin but were unaffected by ryanodine.

The mouse gastric fundus and guinea pig taenia coli contain functional PAR-1 and PAR-2 that mediate relaxations via ryanodine-sensitive and -insensitive activation of small-conductance, Ca²⁺-activated K⁺ channels. We propose that smooth muscle PARs act as sensors for inflammatory signals in gut and respond by inhibiting gut motility during peritoneal infections or tissue damage. GASTROENTEROLOGY 1999;116:586-592