Methods: Pulpitis was induced in the upper incisor of Wistar rats by applying LPS for 6-24 h. The level of Mrp4 mRNA expression was determined in the LPS-inflamed and normal pulps by using real-time PCR. The protein expression for Mrp4 was analyzed with double immunofluorescence staining using an anti-Mrp4 antibody and CD31 (an endothelial cell marker). The amount of PGE2 released from inflamed pulp explants, in the presence or absence of dipyridamole (an Mrp4 inhibitor), was assessed by an enzyme-linked immunosorbent assay.
Results: The level of Mrp4 mRNA expression in the inflamed pulps was significantly higher at 6-12 h after the LPS application, compared with that in the normal pulp (P < 0.01 at 6 h and P < 0.05 at 12 h). Double immunofluorescent staining revealed that Mrp4-immunoreactivity was colocalized in CD31-expressing endothelial cells. Moreover, the Mrp4 inhibitor caused a 51.8% decrease in the amount of PGE2 released from the LPS-inflamed pulps (P < 0.01 at 24h).
Conclusions: In the LPS-inflamed pulp, Mrp4 mRNA was upregulated and immunolocalized in endothelial cells. The significant decrease of PGE2-release by the Mrp4 inhibitor suggests that this transporter contributes to the transport of PGE2 in the transmembrane efflux pathway.