Signal Transduction Pathways Involved in the Mechanical Responses to Protease-Activated Receptors in Rat Colon

Abstract

Recording simultaneously in vitro the changes of endoluminal pressure (index of circular muscle activity) and isometric tension (index of longitudinal muscle activity), we examined the mechanisms responsible for the apamin-sensitive relaxant and contractile responses induced by protease-activated receptor (PAR)-1 and PAR-2 activating peptides, SFLLRN-NH₂ and SLIGRL-NH₂, respectively, in rat colon. In the circular muscle, the inhibitory effects of SFLLRN-NH₂ and SLIGRL-NH₂ were significantly reduced by ryanodine, an inhibitor of Ca²⁺release from the sarcoplasmic reticulum, but unaffected by 1-[6-[[17β-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122), a phospholipase C (PLC) inhibitor, 3-[1-[3-(dimethylaminopropyl]-1H-indol-3-yl]-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione monohydrochloride (GF109203X), a protein kinase C (PKC) inhibitor, or genistein, a tyrosine kinase inhibitor. In the longitudinal muscle, the contractile responses to SFLLRN-NH₂ and SLIGRL-NH₂ were significantly reduced by nifedipine, an L-type calcium channel blocker, ryanodine, GF109203X, genistein, and abolished by U73122. The effects of genistein were additive with GF109203X but not with nifedipine. In the longitudinal muscle, the relaxant responses to the highest concentrations of SFLLRN-NH₂ and SLIGRL-NH₂were abolished by nifedipine, reduced by genistein, and unaffected by ryanodine or GF109203X. In conclusion, influx of extracellular Ca²⁺ through L-type voltage-dependent channels or release of Ca²⁺ from intracellular stores are determining for the opening of the apamin-sensitive K⁺ channels responsible for longitudinal muscle relaxation or circular muscle inhibitory response, respectively, in rat colon. The longitudinal muscle contraction is mediated by activation of PLC; PKC and tyrosine kinase are involved in the cascade process, playing a parallel role. Indeed, tyrosine kinase and L-type Ca²⁺ channels would act sequentially. The influx of Ca²⁺ in turn would cause release of Ca²⁺from sarcoplasmic reticulum.