Effect of oxytocin on contraction of rabbit proximal colon in vitro

AIM: To investigate the effects of oxytocin (OT) on isolated rabbit proximal colon and its mechanism. METHODS: Both longitudinal muscle (LM) and circular muscle (CM) were suspended in a tissue chamber containing 5 mL Krebs solution (37 °C), bubbled continuously with 950 mL·L-1 O2 and 50 mL·L-1 CO2. Isometric spontaneous contractile responses to oxytocin or other drugs were recorded in circular and longitudinal muscle strips. RESULTS: OT (0.1 U·L-1) failed to elicit significant effects on the contractile activity of proximal colonic smooth muscle strips (P > 0.05). OT (1 to 10 U·L-1) decreased the mean contractile amplitude and the contractile frequency of CM and LM. Hexamethonium (10 μmol·L-1) partly blocked the inhibition of oxytocin (1 U·L-1) on the contractile frenquency of CM. Nω-nitro-L-arginine-methylester (L-NAME, 1 μmol·L-1), progesterone (32 μmol·L-1) and estrogen (2.6 μmol·L-1) had no effects on OT-induced responses. CONCLUSION: OT inhibits the motility of proximal colon in rabbits. The action is partly relevant with N receptor, but irrelevant with that of NO, progesterone or estrogen.