The inhibitory effects of oxytocin on distal colonic contractile activity in rabbits are enhanced by ovarian steroids

Aims: To study the effects of oxytocin on isolated rabbit distal colon and the regulation of ovarian steroids by its action. Methods: Muscle strips parallel to either the circular or the longitudinal fibres were excised and suspended in tissue chambers containing 5 mL Krebs solution (37 °C) and bubbled continuously with 95% O₂ and 5% CO₂. The effects of oxytocin on isometric spontaneous contractile responses were recorded. The effects of atosiban, tetrodotoxin, Mg²⁺, progesterone and oestradiol on the oxytocin‐induced response were also examined. Results: Oxytocin (1, 10 and 100 nmol L^?1) dose dependently decreased the area under the contraction curve of distal colonic smooth muscle strips. The oxytocin receptor antagonist atosiban blocked the oxytocin (10 nmol L^?1)‐caused responses in a dose‐dependent manner. Tetrodotoxin (10 μmol L^?1) had no effect on the oxytocin‐induced response. Mg²⁺‐free Krebs solution attenuated the oxytocin‐induced response, but oestradiol (0.1 μmol L^?1) or progesterone (0.1 μmol L^?1) increased the oxytocin‐induced response. Conclusion: These results suggest that oxytocin inhibits the contractile motility of the distal colon, which is regulated by Mg²⁺ and ovarian steroids.