Dopamine receptor D1 mediates the inhibition of dopamine on the distal colonic motility.

The motility of distal colon could be inhibited by dopamine (DA), yet, the involved receptor is controversial according to the published reports. The goal of present study was to investigate DA receptor(s) mediated inhibition of DA on the colonic motility in rat. The contraction of isolated colon strips was assessed through isometric force transducer. The expressions of DA receptors in distal colon were detected through immunofluorescence and Western blot. DA concentration in colonic smooth muscle was measured by liquid chromatography/mass spectrometry. The results showed that DA inhibited the spontaneous motility of distal colon in a dose-dependent manner with EC50 8.3 μM. Tetrodotoxin increased colonic contractive frequency, but failed to affect the inhibition of DA on the colonic motility. Pretreatment with SCH-23390, an antagonist of dopaminergic receptor D1, shifted the dose-response curve to the right with EC50 of DA 37 μM. However, blocking dopaminergic receptor D2 with sulpiride, had no effect. The immunoreactivity of D1 and D2 were detected in the distal colon including myenteric plexus and smooth muscle. Acute cold-restraint stress (CRS) enhanced spontaneous contraction of rat distal colon, which was more sensitive to DA compared with control. Moreover, DA content and D1 expression in smooth muscle layer were increased under CRS condition. In conclusion, D1 in the smooth muscle is mediated DA inhibition on the spontaneous contraction of rat distal colon. The increased DA content and D1 receptor expression in the smooth muscle layer could be a compensatory effect under CRS condition to balance the enhanced colonic motility.